NSTest4/MCAT

Which of the following is most likely true of the 5'-UTR region of the FAM83H gene? A. It is transcribed, but is typically not translated or is only partially translated. B. It is always both transcribed and translated. C. It is neither transcribed nor translated. D. It is cleaved from the pre-mRNA transcript as part of a post-transcriptional modification.

A is correct. The 5'-untranslated region (5′-UTR) is the region of mRNA that is directly upstream from the initiation codon. This region is important for the regulation of translation of a transcript; from this information alone, we know that the 5'-UTR must be transcribed (eliminating choice C). However, as it is the "untranslated" region, we can conclude that this region is not translated or only partially translated into a protein (eliminating B). B: The 5'-UTR is not always translated. In fact, it typically is not translated, hence its name. C: The 5'-UTR is transcribed. D: Since we know this region influences translation, it must still be present in the mature mRNA formed as a result of post-transcriptional modification.

According to previous studies, what demographic factor would best account for the results of the phone survey? A. There was a high percentage of elderly female participants. B. Average income increased after the venues opened. C. Property crime decreased after the venues opened. D. There were high non-response rates of participants.

A is correct. The passage states that being elderly and being a female are risk factors for perceiving more crime than is actually occurring, which Table 1 shows was the case for 19% of respondents. Although this is not a majority, Figure 1 shows that crime did not actually increase; therefore, if elderly people and women are at higher risk of over-perceiving crime levels, a disproportionate representation of elderly women among the respondents could explain this finding.

Acetylation

Acetylation promotes transcription by attaching acetyl groups to lysine residues on histones, making them less positively-charged and causing a looser wrapping pattern that allows transcription factors to access the genome more easily.

Antibiotic Resistance

Antibiotic resistance is a major problem in modern medicine, and the genes responsible for antibiotic resistance are often found on plasmids (non-chromosomal bits of circular prokaryotic DNA) and spread through conjugation, which can be thought of as the bacterial equivalent of sexual reproduction. (However, conjugation is actually not a method of reproduction at all because it does not produce new bacterial cells. Instead, it is a form of gene transfer.) Conjugation involves the transfer of a plasmid through a bridge that is created when a sex pilus on one bacterium (often known as F+, which refers to the presence of the fertility factor, or as male) attaches to another bacterium (generally known as F−). During this process, the fertility factor itself is duplicated and transferred, converting the F− cell into an F+ cell. Another important context of antibiotic use is in genetic engineering techniques in which recombinant DNA is added to plasmids and introduced into bacteria (very commonly E. coli). Not all bacteria in a sample will take up the plasmids, so researchers must isolate those that have done so. For this purpose, plasmids used for genetic engineering generally contain antibiotic resistance genes, such that treatment with an antibiotic can kill off E. coli cells that did not take up any plasmids and select for those that did.

Assuming no alternative splicing occurs, approximately how many base pairs will be in a mature mRNA produced from the FAM83H gene? A. 1221 B. 1441 C. 1939 D. 2079

B is correct. A mature mRNA consists of a 5'-cap (one base pair), the 5' UTR, coding regions, the 3' UTR, and a poly(A) tail. Therefore, the length of a mature mRNA using the numbers given in Table 1 is: 1 + 130 + 300 + 250 + 150 + 180 + 200 + 90 + 140 = 1441. A: This answer choice neglects to include the 5'-UTR and the 3'-UTR. These regions have a significant influence on translation, so they certainly must be transcribed and end up in the mature mRNA product. Note also that these regions are distinct from the coding regions, since the UTRs typically do not end up in the final protein product.

Which value of the G-ratio would correspond to a complete lack of myelination? A. 2.0 B. 1.0 C. 0.0 D. -1.0

B is correct. If there was no myelination, the diameter of the myelinated region surrounding the axon would be exactly equal to the diameter of the axon itself, resulting in a G-ratio of 1.0. A: This would imply that the diameter of the axon is thicker than the diameter of the myelinated region surrounding it, which is not possible. C: Since the G-ratio is defined as the diameter of the axon divided by the diameter of the myelinated region surrounding it, a G-ratio approaching zero would correspond to either an infinitely thin axon or an infinitely thick myelination area. Values of 0.0 would never be reached in a real physical system. D: Diameters do not have negative values (even if we were to use vectors, diameter of a circular structure is not a vector), so this value is not physically possible.

If the researchers attempted to identify universal emotional signs of stress in the faces of subjects from several different countries and cultures, which of the following emotions would be LEAST likely be studied? A. Anger B. Shame C. Fear D. Happiness

B is correct. In order to study a universal emotion and its relationship to signs of stress, the researchers would need to use any of the "universally expressed" emotions. These include anger, disgust, fear, surprise, happiness, sadness, and contempt. Shame is not considered a universal emotion and would not help in looking for universal signs of emotional stress.

Some individuals believe that the identity shifts are attributed to power dynamics between medical students and physicians, where medical students learn professionalism through obediently following orders of those superior to them. Which individual would most support this claim? A. Albert Bandura B. Stanley Milgram C. Abraham Maslow D. B.F. Skinner

B is correct. Stanley Milgram's major contribution to the fields of psychology and sociology was his obedience experiment, where he demonstrated that participants would follow the orders of a superior if instructed to go against their conscience. While the question does not explicitly say that the medical students are going against their consciences, the element of obedience is clearly stated in the question stem, making choice B the best answer. Milgram's shock experiment

If a group of individuals diagnosed with personality disorder were found to believe that their socially destructive actions should have yielded neutral or positive social results with those around them, it would best support which personality theory? A. Humanistic perspective B. Social cognitive perspective C. Behaviorist perspective D. Biological perspective

B is correct. The correct answer must explain the behavior of the subjects in terms of their expectations of others. The social cognitive perspective is based upon expectations of others.

2,3-BPG in fetus

Because this positive charged lysine is replaced with a neutral serine residue in fetal hemoglobin, the 2,3-BPG will not be able to bind as well to the center pocket.

A psychosis arising from an advanced stage of syphilis, in which the disease attacks brain cells, is called: A. Korsakoff's syndrome. B. delirium tremens. C. schizotypal personality disorder. D. general paresis.

D is correct. General paresis, also known as general paralysis of the insane or paralytic dementia, is a neuropsychiatric disorder affecting the brain, caused by late-stage syphilis. A: Korsakoff's syndrome is a neurological disorder caused by a lack of thiamine (vitamin B1) in the brain. Its onset is linked to chronic alcohol abuse or severe malnutrition, or both. B: Delirium tremens is an acute episode of delirium that is usually caused by withdrawal from alcohol. C: Schizotypal personality disorder is a personality disorder characterized by a need for social isolation, anxiety in social situations, odd behavior and thinking, and often unconventional beliefs.

A particular type of bird makes its home inside the saguaro cactus by burrowing a hole in the side of the cactus. Such holes do not harm the cactus, although the cactus derives no benefit from the presence of the birds. This is an example of which of the following? A. Parasitism B. Competition C. Mutualism D. Commensalism

D is correct. This question asks us to understand the difference between different types of relationships that exist between species. When one species benefits but the other species is neither helped nor harmed, that is commensalism. That best characterizes the relationship in the question. A: Parasitism benefits one species at the cost of harm to another. B: Competition is when two species compete for access to a limited resource. C: Mutualism requires that both species benefit.

epitope

The specific site on an antigen that is recognized by an antibody is known as the epitope.

At the top, the normal force

can be zero but the gravitational force cannot.

might help...

height needed to stay on track at top of loop 5r/2.... might can use v = sqrt(gh)?? velocity for bernouille's equation: v = sq rt (2gh) and for PE and KE problems

acetyl group

methyl with a carbonyl In organic chemistry, acetyl is a moiety, the acyl with chemical formula CH3CO. It is sometimes represented by the symbol Ac.

Interaction variables

when you combine 2 independent variables have effects that, when combined with the effects of other variables, are not additive. In other words, two variables may impact the dependent variable much more when presented together than one would expect if the effects of each variable alone were added.

ideal buffer should have a pKa

within 1 pH unit of the expected experimental conditions BUFFERS: In the body and in the lab, it is sometimes desirable to avoid the large shifts in pH that can result from the addition of acid or base to an existing solution. A buffer is a solution that resists changes in pH upon addition of acid or base. While buffers cannot protect against addition of large amounts of acid/base, they are highly effective at maintaining pH when small to moderate quantities are added. A buffer must contain either a weak acid and its conjugate base or a weak base and its conjugate acid. The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log[A−]/[HA], where HA refers to a generic weak acid and A− refers to its conjugate weak base. The basic idea of a buffer is that if we add a small to moderate amount of a strong acid or base to the buffered solution, it will entirely protonate or deprotonate some of the weak acid/base. However, since the Henderson-Hasselbalch equation refers to the log of the [A−]/[HA] ratio, doing so will only minimally affect the pH. The bicarbonate buffer system (CO2 + H2O ⇌ H2CO3 ⇌ HCO3− + H+) plays a major role in maintaining the pH of blood within the narrow range of roughly 7.35-7.45 that is compatible with health. It also encodes a key relationship for understanding the importance of respiration in the context of human metabolism: through Le Châtelier's principle, increasing the amount of CO2 in the blood (as occurs as a result of cellular respiration) pushes the bicarbonate reaction to the right, increasing the amount of H+ present in the blood and thereby reducing its pH. Thus, it is essential for the body to be able to breathe out CO2 and breathe in O2.

Which of the following best explains the differences in adhesion between CA and AA macrophages seen in Figure 2? A. CA macrophages have higher integrin levels than AA macrophages. B. CA macrophages have lower integrin levels than AA macrophages. C. CA macrophages have higher cadherin levels than AA macrophages. D. CA macrophages have lower cadherin levels than AA macrophages.

A is correct. Integrins are transmembrane receptors that modulate cell-to-extracellular matrix interactions. Specifically, these proteins often attach the cell to collagen and fibronectin fibers. Figure 2 shows that CA macrophages have significantly higher adhesion than AA cells on collagen and fibronectin, but not on plastic. This evidence suggests that the difference in adhesion is caused by higher integrin levels in the CA macrophages. B: This result would contradict the greater CA adhesion shown in Figure 2. C, D: Cadherins are transmembrane proteins which play a primary role in cell-to-cell adhesion (remember that C stands for cell-to-cell), forming adherens junctions to bind cells within tissues together. They would not play a role in the cell-to-extracellular matrix interactions required to bind the macrophages to collagen or fibronectin. Content Foundations: Structural Proteins The most common function of proteins in the body is not enzymatic, but structural. Structural proteins are fibrous proteins that have an elongated shape and provide structural support for cells and organ tissues. The first type of fibrous proteins are keratins, which form the skin, hair, and nails. Keratins are classified as soft or hard according to their sulfur content (i.e. the relative number of cysteine residues in their polypeptide chains). The low-sulfur keratins of the skin are much more flexible than the high-S, hard keratins. The second type of fibrous proteins you must know are the actin and myosin proteins of muscle tissue. Actin and myosin interact to form cross-linkages that allow the sliding of the filaments over each other in muscle contraction, which takes place through the contraction and relaxation of the sarcomere, the fundamental unit of all muscle fibers. When muscle contracts, the actin and myosin filaments slide over each other and the H-zone (myosin-only region), Z-lines (sarcomere boundaries), and I-band (actin-only region) all shrink, while the A-band (the entire myosin region) remains the same size. The opposite occurs upon muscle relaxation. A third type of structural protein you should know for test day is collagen, which is found in tendons, forms connective ligaments within the body, and gives extra support to the skin. Collagen is a triple helix formed by three proteins that wrap around one another. Many collagen molecules are cross-linked together in the extracellular space to form collagen fibrils to provide structural support for the cell. Elastin polypeptide chains are cross-linked together to form flexible, elastic fibers that give stretched tissues flexibility and the ability to recoil spontaneously as soon as the stretching force is relaxed.

Behaviorists would assert that self-defeating behaviors are maintained by immediate reinforcement in the form of: A. relief from anxiety. B. defending the ego. C. negating one's self-image. D. increasing existential anxiety.

A is correct. People are more likely to behave in a self-defeating or destructive manner either when they perceive a threat or when they have low self-esteem. These people are more likely to be susceptible to having anxiety and emotional distress, which are problems that are usually directly related to a less favorable self-appraisal. Self-defeating or self-destructive behaviors often manifest due to an inability to handle either anxiety or stress which results from a lack of self-confidence. Thus engaging in these behaviors offers an immediate relief from anxiety which then reinforces that behavior. B: Self-defeating behaviors are those that sabotage the self. This is very different from ego defense mechanisms, which include projection, rationalization, regression, etc. and which serve to protect the ego. C: This question asks about reinforcement, or a good/pleasant response to a behavior that causes us to continue performing that behavior. Negating (nullifying or canceling out) characteristics of one's self-image is not pleasant or likely to reinforce any behavior. D: Similarly to choice C, increasing existential anxiety is an unpleasant experience and would not serve as reinforcement. ("Existential" means "related to one's own existence," although that is not necessary to know here.)

According to Figure 2, which chromosome has the lowest ratio of coding DNA to non-coding DNA? A. 18 B. 11 C. 1 D. Y

A is correct. The lowest genetic density would result from the chromosome with the lowest ratio of genes to base pairs. We can obtain this information from the data in Figure 2: Image Chromosome 18: 300 genes / 75 bp = 4 Chromosome 11: 2700 genes / 140 bp = 19 Chromosome 1: 4300 genes / 250 bp = 17 Chromosome Y: 500 genes / 50 bp = 10 Thus, chromosome 18 has the lowest gene density, meaning that it has the lowest ratio of coding to non-coding base pairs. B: Chromosome 11 has the highest ratio of coding to non-coding DNA of the choices given. C: From Figure 2, it is evident that chromosome 1 has a higher coding-to-non-coding ratio than chromosome 18. D: The Y chromosome is notable for being significantly smaller and having fewer genes than the X chromosome. However, Figure 2 shows that it still has a higher coding-to-non-coding ratio than chromosome 18.

In humans, traits A and B are autosomal and located on separate chromosomes. Trait A is dominant over a, and trait B is codominant with b. If a man with a genotype of aabb has children with a woman with a genotype of AABB, what proportion of their children would be expected to be homozygous for either trait? A. 0 B. 1/4 C. 1/2 D. 0 for girls, 1/2 for boys

A is correct. The question tells us that the traits are autosomal, so we know there will be no difference between boys and girls and we can eliminate choice D. Then consider each trait separately. If you cross AA and aa, all the children will be Aa. Similarly, if you cross BB and bb, then all of the children will be Bb. Thus, none of the children will be homozygous. B, C: These non-zero answers are impossible given that 100% of the children will be heterozygous for both traits (0% will be homozygous). D: Since these traits are autosomal, we would not expect a different proportion of homozygotes for females vs. males.

Acid-base buffers

Acid-base buffers confer resistance to a change in the pH of a solution when hydrogen ions or hydroxide ions are added or removed to solution. An acid-base buffer typically consists of a weak acid and its conjugate base. The most important buffer to know for the MCAT is the bicarbonate buffer system, which is shown below. H2O (aq) + CO2 (g) ⇌ H2CO3 (aq) ⇌ H+ (aq) + HCO3− (aq) Carbonic acid (H2CO3) has the conjugate base of HCO3−. Buffers work because the concentrations of the weak acid and its salt are large compared to the number of protons or hydroxide ions added or removed. When protons are added to the solution from an external source, some of the bicarbonate in the buffer is converted to carbonic acid, using up the protons added; when hydroxide ions are added to the solution, protons are dissociated from some of the carbonic acid in the buffer, converting it to bicarbonate and replacing the protons lost. Buffers resist pH changes best when the pH values are at or near the pKa value for the acid/base used, because that is when the conjugate acid and base have equal concentrations. Optimal buffering occurs when the pH is within approximately 1 pH unit from the pKa value of the system. The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log [conjugate base] / [acid]. On Test Day, you may see the bicarbonate buffer concept tested from a chemical, biochemical, or biological perspective. For example, hyperventilation (rapid deep breathing) results in excess CO2 being expelled from the blood, causing the pH to rise. In response, the buffer needs to release more H+ to lower the pH back to physiological norms. An additional fact to be aware of is that other mechanisms in the body are also used to regulate pH, since carbonic acid works best at a pH below physiological conditions, because its pKa1 (pKa1 = 6.3, pKa2 = 10.3) is much lower than the normal pH of blood (7.4).

Antibodies and Antigens

Antibodies let the body know when it needs to mobilize the immune response. To do this, they must recognize substances/cells that need to be eliminated and be recognized by other components of the immune system. The term "antigen" is used to refer to what antibodies recognize. There is no specific structural property that defines an antigen, although they often happen to be macromolecules (especially proteins) expressed on the surface of a cell or a viral envelope/capsule. However, external substances like pollen can also serve as antigens, causing pollen allergies. The structure of an antibody provides a bridge between these two functions. Antibodies have a Y-shaped structure consisting of two heavy chains and two light chains that are linked by disulfide bonds. Five classes of antibodies exist, classified according to the details of their heavy chains: immunoglobulin (Ig) A, IgD, IgE, IgG, and IgM. The "top" ends of the Y-shaped structure (that is, the part with both the heavy and the light chains) have a hypervariable antigen-recognizing area, and the rest of the antibody structure can be recognized by other cells of the immune system. The specific site on an antigen that is recognized by an antibody is known as the epitope. Extensive random recombination of the antigen-recognizing area of the antibody (also known as the paratope) allows the generation of antibodies that recognize potentially infinitely many types of antigens. Antibodies are used in the adaptive immune system, most notably by B cells. Antigen-antibody interactions are also used in many biotechnology-related applications, most notably western blotting, in which antibodies are used to visualize proteins of interest after gel electrophoresis.

Stimulus motives are: A. needs that are learned, such as the need for power or for achievement. B. innate but not necessary for survival. C. not innate but necessary for survival. D. innate and necessary for survival.

B is correct. A stimulus motive is defined as a motive that appears to be unlearned but causes an increase in stimulation, such as curiosity. These motives are not necessary for survival. A: Stimulus motives are not learned. C, D: Stimulus motives are not necessary for survival. For example, imagine a cat who lives inside an apartment. One of the cat's stimulus motives is the desire to explore his environment (for example, walk around the apartment, jump inside cupboards or cabinets, etc.). This is not necessary for survival — the cat could easily survive simply by sitting in one place and moving only to acquire food or water. Instead, this motive can be thought of as primarily for the cat's stimulation or entertainment.

To compare the percent of people experiencing crime in an area with the percent of people perceiving crime to be a problem, researchers could use what type of analysis? A. A paired samples t-test B. An independent samples t-test C. Linear regression D. A Pearson correlation coefficient

B is correct. An independent samples t-test is conducted when researchers wish to compare mean values of two groups. A: This test would be used if the results came from the same participants. C: Linear regression is used to predict scores from independent variables. D: A Pearson correlation coefficient is calculated to compare the association between two variables.

Participant 1 rides a fourth roller coaster as shown below. What is the minimum ramp height H if the ride at the top of the loop maintains the minimum speed needed to stay on the track throughout the loop? A. 10 m B. 20 m C. 30 m D. 35 m

B is correct. At the top of the loop, the gravitational and the normal forces (if any) point downward toward the center of the loop. Therefore, the net force causing centripetal acceleration is the sum of the gravitational force and the normal force. When the centripetal force is the minimum amount needed for the ride to stay on track, the normal force is zero. Note: At the top, the normal force can be zero but the gravitational force cannot. Let's designate the initial height where the ride starts as H. At the very beginning, the energy of the ride is the gravitational potential energy. Therefore, Ei = mgH. At the top of the loop, the ride includes both gravitational and potential energy. Therefore, Ef = mgH + ½ mv^2 = mg (2r) + ½ mv^2. The net acceleration at top (centripetal acceleration) = v^2/r. Since we concluded that the normal force at the top is zero, the net acceleration at the top is the gravitational acceleration, g. Therefore, v^2 = gr. Substituting this equation into the 'Ef' equation, we obtain the following: Ef = 2mgr + ½ mgr = (5/2) mgr To obey conservation of energy, Ei = Ef. Therefore: mgh = (5/2) mgr. We conclude that H is 5r/2. Since r is 8 m, H is 5(8 m)/2 = 20 m. A, C, D: These answers result from miscalculation.

By blocking norepinephrine reuptake, duloxetine could cause which of the following side effects? A. Increased frequency of urination B. High blood pressure C. Reduced heart rate D. Heartburn

B is correct. Blocking norepinephrine reuptake would potentiate its effects. Since norepinephrine is a mediator of the sympathetic nervous system response (commonly known as the fight-or-flight response), it may lead to side effects associated with sympathetic activation. High blood pressure, as a consequence of the fight-or-flight response, is a possible outcome.

Based on the passage, CR-3 is most likely to be: A. an immunoglobulin. B. a receptor involved in cell recognition. C. a transcription factor. D. a receptor tyrosine kinase.

B is correct. The passage states that CA macrophage phagocytosis can be inhibited by blocking CR-3. Since this inhibition involves antibodies, we know that CR-3 must "live" in the membrane and face the exterior, meaning that it is likely to be some kind of receptor. Phagocytosis in macrophages occurs after the macrophage has identified its target via receptors that recognize common structural motifs in pathogens or other target molecules (in this case, myelin). A: This is tempting, but the final paragraph states that the phagocytosis was inhibited by anti-CR3 antibodies (immunoglobulins), not that CR3 is an antibody itself. Note that macrophages do not produce antibodies. C: Phagocytosis is not under the control of a single transcription factor. D: Receptor tyrosine kinases are commonly found as hormone receptors; they do not play a direct role in macrophage phagocytosis.

Based on Figure 2, CA macrophages, in comparison to AA macrophages, will most likely: I. be less motile. II. be found in greater number in the blood vessels. III. have higher levels of dynein. A. I only B. I and II only C. II and III only D. I, II, and III

B is correct. With Roman numeral questions, we want to be sure to work efficiently. Let's being with Roman numeral I. Figure 2 shows that CA macrophages move slower (i.e. are less motile) than AA cells, making RN I correct. For statement II, the test-makers expect you to be familiar with the differentiation of major blood cell types, such as macrophages. Macrophages found in the blood are derived from monocytes that originate in the bone marrow, while macrophages in the tissues may also be derived from the embryonic yolk-sac. The MCAT will not expect you to make this differentiation without passage information, so we need to look again at Figure 2. CA macrophages have a much higher affinity for collagen. Collagen is a primary component in connective tissue, such as that found in the tendons, cartilage, and blood vessels. Thus CA cells are likely to be found in greater number in the blood vessels. Figure 2 also shows that CA cells have lower motility. Thus, if they migrate to and travel via the circulatory system, they would not likely migrate as far or as fast as AA cells. III: Dynein (shown below) is a motor protein that moves cellular "cargo" along microtubule "train tracks." It has a minimal effect on the overall motility of the cell, so we would not be justified in making a prediction one way or the other.

allele (genotype frequency) different from phenotype frequency

Bb and bb -------> p+q = 1; p^2 + 2pq + q^2 = 1 75% - b 25% - B but 50/50 phenotype if 9% have bb, take sq root of .09 = .3 = 30% so brown would be 70% if p = .3 b/c p + q = 1 but phenotype would be 91% for brown but homozygous brown would be .7^2 = .49 or 49%

A student begins with a sequence of dsDNA. He then adds the enzymes necessary for DNA replication but substitutes artificial bases G', C', T', and A' for the normal bases. He allows the DNA to replicate once, then removes the excess artificial bases from the reaction environment and replaces them with another set of artificial bases, G'', C'', T'', and A''. Finally, the DNA is allowed to replicate two more times. At the end of the experiment, how can the DNA molecules present be described with regard to their composition? (In the notation below, the / symbol separates the two strands of the double-stranded DNA.) A. Two GCTA / GCTA DNA molecules, two G'C'T'A' / G'C'T'A' molecules, four G''C''T''A'' / G''C''T''A'' molecules B. Two GCTA / G'C'T'A' molecules, six G''C''T''A'' / G''C''T''A'' molecules C. Two GCTA / G''C''T''A'' molecules, two G'C'T'A' / G''C''T''A'' molecules, four G''C''T''A'' / G''C''T''A'' molecules D. Eight DNA molecules made up of random assortments of natural and artificial bases

C is correct. DNA replicates in a semiconservative manner, meaning that each replicating strand of double-stranded DNA (dsDNA) produces two daughter molecules, each of which contains one parental and one new strand. Here, we begin with one molecule of dsDNA that is GCTA / GCTA. After the first round of replication, we have two molecules: GCTA / G'C'T'A' and GCTA / G'C'T'A'. After the next round, we now have two GCTA / G''C''T''A'' and two G'C'T'A' / G''C''T''A'' molecules. In the final round of replication, we are left with two GCTA / G''C''T''A'', two G'C'T'A' / G''C''T''A'', and four G''C''T''A'' / G''C''T''A'' molecules. A: This might make more sense if DNA were replicated according to the conservative model, in which parent strands stick together. B: The two GCTA / G'C'T'A' strands would be present at the end of the first round of replication, but they would separate in subsequent rounds. D: This would be correct if DNA followed the dispersive model of replication, in which bases segregate randomly to daughter strands.

In the U.S. population, the frequency of the allele for colorblindness, Xc, is 8%. Which of the following is the frequency of colorblind women and colorblind men in the population, respectively? A. 8%, 0.64% B. 0.64%, 4% C. 0.64%, 8% D. 64%, 8%

C is correct. For a woman to be colorblind, she must get two copies of the Xc gene. Since the frequency of this gene is 0.08, the odds of being homozygous for the gene are 0.0064, or 0.64%. Males only need a single copy of the Xc gene to be colorblind (since their other gene is Y), making the odds of a male being colorblind 8%.

When a person's gender identity does not match his or her physical sex, the person would most likely experience which of the following? A. Paraphilia B. Sexual dysfunction C. Gender dysphoria D. Androgyny

C is correct. Gender dysphoria is the feeling of stress or discomfort in which one's sense of one's gender does not align with the gender assigned at birth (overwhelmingly the physiological sex of the person as determined by primary sex characteristics). In the older nomenclature, this was termed gender identity disorder, but (much like homosexuality before it), this condition has since been removed from the DSM as a "disorder" and come to simply be understood as one point on the wide spectrum of human sexual and gender characteristics. A: Paraphilia is the experience of intense sexual arousal to atypical objects, situations, or individuals. B: Sexual dysfunction is difficulty experienced by an individual or a couple during any stage of sexual activity, including physical pleasure, desire, preference, arousal or orgasm. D: Androgyny is the combination of masculine and feminine characteristics. Sexual ambiguity may be found in fashion, gender identity, sexual identity, or sexual lifestyle. It can also refer to biological intersex physicality.

ω-Conotoxin binds irreversibly to calcium channels in myocytes, locking the calcium channels open and preventing calcium from being isolated. Exposure to this toxin would most likely cause: A. a single short muscle spasm followed by flaccid paralysis. B. inability of the muscle to contract. C. tetanic muscle contraction. D. transient weakness that would be compensated for within minutes.

C is correct. High calcium concentrations allow muscle contraction to take place. Calcium binds to troponin, which causes tropomyosin to stop blocking the binding site on the thin filaments for myosin. If the calcium channels are locked open, the muscle will not be able to stop contracting. A state of constant contraction is tetany.

The results in Table 1 suggest: A. that only a small portion of Z mRNA is translated to protein. B. that pre-transcriptional control of gene expression is of primary importance in regulating cytokine Z. C. that post-transcriptional control of gene expression is of primary importance in regulating cytokine Z. D. that cytokine Z gene expression is poorly regulated.

C is correct. In examining Table 1, you should notice that the plasma concentrations of Z increases far more than the concentrations of Z pre-mRNA during exercise. This implies that the rate of transcription is not increasing as much as the rate of translation due to exercise; if pre-transcriptional control was most important to the exercise mechanism, the concentrations of pre-mRNA would increase more significantly. This indicates that the regulatory mechanism that causes plasma [Z] to increase does so by increasing the translation of Z, not its transcription. This is referred to as post-transcriptional control.

During an isoelectric focusing procedure, cytokine Z migrated closer to the cathode than the control protein. This is most likely because Cytokine Z has a higher concentration of which amino acid? A. D B. E C. K D. T

C is correct. Isoelectric focusing (IEF) is used to separate proteins by isoelectric point, which depends on the amino acid composition of the protein. In an IEF setup, the cathode is the negatively-charged electrode. Because Z migrated closer to the cathode than the control protein, it likely has a larger net positive charge than the other protein. Therefore, it would experience a greater electrostatic attraction towards the cathode. Lysine (K) is the only positively charged amino acid listed, so it must be correct. A, B: Aspartic acid (D) and glutamic acid (E) are negatively charged at physiological pH. Note that since both of these amino acids have extremely similar charge properties (and since both answers cannot possibly be correct at once), we can eliminate both even if we do not fully understand isoelectric focusing.

Negative punishment is a common example of extrinsic motivation. Which of the following best illustrates this type of motivation? A. making a child do extra work for bad behavior. B. ending a school suspension early for completing community service. C. a child losing time from an enjoyable activity for not eating his vegetables. D. a student receiving extra dessert for helping his sibling.

C is correct. Negative punishment involves the loss of something pleasurable in response to an undesirable behavior. A: This represents positive punishment (the administration of something unpleasant in response to an undesirable behavior). B, D: Both of these options can be immediately eliminated because they represent reinforcement, not punishment. Specifically, choice B constitutes negative reinforcement (the removal of something unpleasant in response to a desirable behavior), while choice D represents positive reinforcement (the administration of something pleasant in response to a desirable behavior).

A research subject is shown the following list of the following words: wing, engine, tail, and window. When later presented with and asked to recall whether the presented word was on the original list, the participant identifies "runway" as having been on the list. This is an example of: A. the context effect. B. a miss. C. a false alarm. D. déjà vu.

C is correct. The subject is primed with airplane-related words, then incorrectly identifies another airplane-related word as having been a part of the list. This is a false alarm because the participant incorrectly responds positively when the stimulus was in fact not present. The concept of a false alarm is part of signal detection theory, which focuses on the mechanisms by which individuals detect certain stimuli over others. The diagram below shows a false alarm in relation to other concepts from signal detection theory, in the context of the question stem. A: Do not fall for this answer simply because the word "context" looks correct here! In psychology, context effects refer to increased recall when the subject is in a similar environment as the one in which the original learning took place. B: A "miss" implies that the stimulus was indeed present on the list, but the participant did not recognize it. D: Déjà vu is an example of a context effect in which a person believes he has experienced an event before.

By approximately what percentage of its original length is a free DNA strand shortened by the coiling around a single histone? (Note: A histone has a diameter of 11 nm; assume inter-histone length is negligible.) A. 25% B. 50% C. 75% D. 100%

C is correct. This question asks us to determine how much each histone shortens the DNA strand. We can imagine this linking as winding a string around a tennis ball. From paragraph 3, we know that 200 nucleotides wrap around 1 histone. Initially, a 200-nucleotide sequence will be 200 (0.3 nm) = 60 nm in length. Once coiled around the protein, we can assume (without getting into unnecessary surface area calculations) that a single layer of DNA strand will be wrapped around the protein (strand thickness = 2 nm), meaning we now have the strand condensed to a ~ 4-nm layer (2 nm on each side) around the 11-nm-diameter protein. Thus, about 60 nm of DNA is replaced by 15 nm, meaning there is approximately a 45-nm decrease in length. 45/60 = 0.75 = a 75% decrease in length.

It was discovered that in those whose stress levels are significantly elevated by traumatic events, the incidence of bedwetting was increased. During which stage of sleep is bedwetting most likely to occur? A. 1 B. 2 C. 4 D. REM

C is correct. Typically, sleepers pass through five stages: 1, 2, 3, 4, and REM (rapid eye movement) sleep. Stage 4 is referred to as Delta sleep because of the delta waves that occur during this stage. Stage 4 is a deep sleep that typically lasts about 30 minutes. Sleepwalking and bed-wetting typically occur at the end of Stage 4. During this "deep sleep," there is no eye movement or muscle activity. This is when some children may also experience sleepwalking, or night terrors. A: Stage 1 represents light sleep, where one drifts in and out of sleep and can be awakened easily. In this stage, the eyes move slowly and muscle activity slows. B: In stage 2 sleep, eye movement stops and brain waves become slower, with only an occasional burst of rapid brain waves called sleep spindles. D: During REM sleep, breathing becomes more rapid, irregular and shallow; eyes jerk rapidly, and limb muscles are temporarily paralyzed. Brain waves during this stage increase to levels experienced when a person is awake. Also, heart rate increases, blood pressure rises, and the body loses some of the ability to regulate temperature.

In the first trial of the PCR procedure, only one primer was added to the mixture. What was the most likely outcome during this trial? A. Both of the DNA strands were linearly amplified. B. Only one strand of the DNA was exponentially amplified. C. Only one strand of the DNA was linearly amplified. D. PCR amplification was unable to proceed.

C is correct. When performing PCR, if only one primer is added to the mixture, then that primer will bind to one of the strands and initiate replication of that strand to produce the complementary strand. For one double-stranded template DNA molecule, after one cycle, we are left with two of the complementary strand while still having only one of the strands where the primer binds. In the next cycle, the primer again binds to the strand to which it is complementary. Therefore, only the complementary strand will be replicated again; this time we have three of the complementary strand. As you can see here, one of the strands is being replicated linearly (1 → 2 → 3 →...n), while the other strand is not replicated at all. After 30 cycles, for example, assuming that we began with one copy of each strand, we would have 30 copies of one strand and only one copy of the other. This differs from PCR that includes two primers (which is typical), where we would expect exponential amplification (230 copies of each strand). A: Only one of the primers is added to the mixture, so one of the strands will not have a primer to be replicated. B: The amplification is linear, not exponential (2n copies would be produced after n cycles), as explained above. D: PCR amplification will operate because one of the strands can still be replicated.

Classical Genetics

Classical genetics is often associated with the work of Gregor Mendel in the 19th century. Although modern research has added significantly to the field, some key concepts of classical genetics are tested on the MCAT. A fundamental distinction exists between phenotypes and genotypes. A phenotype is best thought of as the physical manifestation of a genetic trait. In Mendel's experiments, the phenotypes of interest were characteristics visible to the naked eye, such as the color or shape of peas, but in modern genetic research, phenotypes may not be immediately obvious and may take considerable effort to measure. An example might be the efficiency with which cells carry out a certain metabolic pathway. The genotype, in contrast, describes the combination of genes responsible for that phenotype. A certain phenotype can correspond to multiple genotypes, because alleles, or variants of genes, can be described as dominant or recessive. For dominant alleles, only one copy is necessary for its associated phenotype to be expressed, meaning that heterozygous individuals express the dominant phenotype. For recessive alleles, both alleles must be recessive for the phenotype in question to be expressed, meaning that heterozygous individuals do not express the phenotype. A key assumption of Mendelian genetics is the law of independent assortment, which states that the alleles for multiple genes are inherited independently. This is grounded in molecular genetics. It is logical that the alleles of genes on different chromosomes will be inherited independently, due to the random orientation of homologous pairs on the metaphase plate in metaphase I of meiosis. However, independent assortment also applies to genes on the same chromosome due to crossing over during prophase I of meiosis. Pedigrees, or records of the phenotypes observed within a given lineage, can be carefully used to infer genotypes from phenotypes. A common method of genotype analysis for the MCAT is the use of Punnett squares, in which the genotype for one parent is written top-to-bottom and that of the other parent is written left-to-right, and then combinations of those alleles generate the possible offspring genotypes.

Which of the following objections to the study would NOT likely be raised by an ethics committee? I. Potentially harmful medication should not be given to otherwise healthy subjects. II. Necessary medication should not be denied to patients for the sake of research. III. Patients suffering from schizophrenia were unable to provide informed consent for this experimental protocol. A. I only B. II only C. II and III only D. I, II, and III

D is correct. Administering potentially harmful medication to healthy volunteers is a common practice in research, and is only unacceptable if the patient does not give informed consent, or if the danger of the medicine clearly outweighs the potential benefits of the study (I). Likewise, necessary medication is often denied to sick patients during clinical trials, and is only unacceptable if the patient has not given informed consent, or if the danger of withholding the medication outweighs the potential benefits of the study (II). Roman numeral III may seem like a more tempting objection, but we cannot necessarily assume that all, or even most patients, suffering from schizophrenia are unable to give informed consent (III).

Researchers discover another cytokine (X) that is upregulated even more than Z as a result of exercise. Cytokine X binds to receptors in skeletal muscle to stimulate angiogenesis. After strenuous exercise, local [X] would be LEAST similar to post-exercise [Z] in: A. subjects in Group D. B. subjects in Group C. C. subjects in Group B. D. subjects in Group A.

D is correct. After you exercise, your body repairs or replaces damaged muscle fibers by fusing muscle fibers together to form new muscle protein strands (myofibrils). These repaired myofibrils increase in thickness and number to create muscle hypertrophy. These larger muscles require additional blood vessels. If cytokine X acts to promote angiogenesis in the new muscle tissue, this means we would expect large increases in the local concentration of X. Thus the group that is LEAST similar to this large increase in local [X] would be the group with the smallest increase in [Z]. That's group A.

Physician use of doxycycline is in decline due to the rise of antibiotic resistance. Which of the following adaptations would render doxycycline ineffective? I. Modification of the 30S ribosomal subunit to allow aminoacyl-tRNA binding in the presence of the drug II. Expression of a pump that removes doxycycline from the cell III. Thickening of the bacterial capsule A. I only B. I and II only C. II and III only D. I, II, and III

D is correct. Statement I is correct because doxycycline functions by competing for aminoacyl-tRNA for binding to the A site of the 30s ribosome subunit. If the ribosome can bind aminoacyl-tRNA even though the drug is bound, then doxycycline is not able to stop protein synthesis. Additionally, RN II is correct and is a common method of antibacterial resistance. Since doxycycline is a competitive inhibitor of the ribosome, keeping its concentration low will minimize its effect on translation. Finally, III is correct. Paragraph 2 tells us that Wolbachia is an endosymbiotic bacterium, and that doxycycline needs to be able to get through the cell membrane in order to effectively get in and kill its target via ribosomal binding. Gram + and gram - bacteria have a capsule (polysaccharide layer) outside their cell envelope, which protects them from insults. If the bacterium could adapt to thicken this protective layer, it is unlikely that the doxycycline would continue to be effective against them.

Hemiacetals and Acetals

Hemiacetals are compounds in which a terminal carbon atom is connected to (1) another carbon atom, (2) an -H atom, (3) an -OH group, and (4) an -OR group. Acetals are derivatives of hemiacetals in which the -OH group is replaced by an -OR' group. Hemiketals and ketals are analogous compounds, in which the carbon is non-terminal and is therefore has two carbon substituents (analogous to the difference between aldehydes and ketones). These functional groups are relevant for carbohydrates, as the cyclic forms of glucose and fructose are a cyclic hemiacetal and a cyclic hemiketal, respectively. Additionally, when a glycosidic bond is formed between two monosaccharides (isolated sugar molecules) to form a disaccharide, a hemiacetal or hemiketal is converted into an acetal or ketal. Acetals/ketals are also noteworthy in organic chemistry because they can be used as protecting groups. For instance, if we had a compound with a carboxylic acid functional group (-COOH) and a terminal carbonyl group (C=O), we might want to reduce the carboxylic acid group to an alcohol using LiAlH4 while not affecting the carbonyl group. The challenge here is that LiAlH4 is capable of reducing both groups. Treatment of the carbonyl carbon with two equivalents of an alcohol (often accomplished by using a single equivalent of a diol) results in the formation of an acetal group, which is not affected by LiAlH4 and can easily be removed under acidic conditions after the carboxylic acid functional group is reduced.

hemoglobin

Hemoglobin, a metalloprotein, plays an essential role in gas exchange in humans. Specifically, it functions in the transport of oxygen to the body tissues and of carbon dioxide to the lungs for exhalation. Hemoglobin is found in in red blood cells (erythrocytes), which are packed full of the protein; in fact, erythrocytes contain so much hemoglobin that they have no room for - and therefore lack - a nucleus or membrane-bound organelles. Erythrocytes are produced in the bone marrow. Under low-oxygen conditions, erythrocyte production increases in response to the secretion of a kidney hormone, erythropoietin (EPO). In humans, hemoglobin typically consists of four globular protein subunits, attached together to form the quaternary structure of the overall molecule. Each subunit contains one heme group, which is a structure consisting of a specific ring, termed a porphyrin ring. Each heme group contains one iron cation, which binds oxygen in the Fe2+ state. Since each hemoglobin molecule therefore includes four iron cations, each hemoglobin can carry up to four oxygen atoms. Binding of oxygen to any of the four binding sites causes an increase in the oxygen affinity of the remaining sites, a phenomenon known as cooperativity. Cooperative binding can be recognized by its signature sigmoidal, or S-shaped, shape; the steep part of the S denotes the sharply increased binding affinity that occurs as a result of the initial binding. The binding affinity between hemoglobin and oxygen is often displayed in graphical form. A hemoglobin-oxygen binding curve typically includes hemoglobin's percent saturation with oxygen on the y-axis and the partial pressure of oxygen on the x-axis. The higher the partial pressure of oxygen, the higher the oxygen saturation tends to be. Interestingly, under certain conditions, the binding curve can shift toward the right along the x-axis; this decreases the hemoglobin-oxygen binding affinity, allowing oxygen to be more easily dropped off in the tissues. These conditions include low (acidic) plasma pH and increased levels of carbon dioxide - both of which can indicate a shortage of oxygen. This rightward shift of the hemoglobin-oxygen binding curve is termed the Bohr effect. A compound known as 2,3-bisphosphoglyceric acid (2,3-BPG) can enhance these effects, further promoting the release of oxygen from hemoglobin.

Conjugation and Aromaticity

Hückel's rule: (4n + 2) = π-electrons (n must be an integer) Conjugation is a special case of resonance that occurs when three or more adjacent p-orbitals are aligned with each other, forming not just a π bond, but a π system. Electrons can delocalize throughout that π system. For the purposes of the MCAT, conjugation can be associated with structures containing alternating single and double bonds in carbon chains. An important characteristic of compounds with conjugated systems is that they absorb ultraviolet (UV) light, and can therefore be well visualized using UV spectroscopy. Aromatic compounds are conjugated cyclic molecules with a planar structure that also satisfy an additional criterion known as Hückel's rule: having (4n + 2) π-electrons, where n is an integer. The most important and well-known example of an aromatic compound is benzene. However, many other biologically relevant aromatic rings contain non-carbon molecules, for which reason they are known as heterocyclic rings. These include pyridine (present in the vitamin niacin), pyrimidine and purine (present in nucleic acids), imidazole (present in many important drugs), and pyrrole (which is a component of the porphyrins contained in heme). In contrast, antiaromatic molecules are highly unstable compounds with 4n π-electrons.

Isoelectric Point and Isoelectric Focusing

Isoelectric Focusing - cathode is negative Isoelectric focusing is an electrophoretic technique that separates proteins according to their isoelectric point (pI). The pI refers to the pH at which the net charge on a protein will be 0; this parameter varies across proteins, such that isoelectric focusing is a good way to separate proteins in a relatively specific manner. Isoelectric focusing requires the use of a gel that has a stable pH gradient. A protein that is in a region of the gel where the pH is below its pI will be positively charged and will migrate towards the negatively-charged plate (the cathode). The protein will cease to migrate when it reaches the region of the gel where the pH of the gel equals the pI of the protein. At this point, the protein will stop migrating because the net charge of the protein will be 0, so it will not experience any electric force. The proteins analyzed using isoelectric focusing will be separated based on the relative number of acidic and basic residues that they contain. The charge of a peptide at a given pH can be roughly predicted based on how many acidic and basic residues they contain. The side chains of the acidic amino acids, aspartic acid and glutamic acid, will be protonated and uncharged (-COOH) below a pH of roughly 3 and deprotonated and negatively-charged (-COO-) above that pH. Correspondingly, the side chains of the basic amino acids (histidine, lysine, and arginine) will be protonated and positively-charged at pH levels below the pKa values of their side chains (roughly 6.0, 10.5, and 12.5, respectively). The pI can also be estimated for amino acids in isolation. For diprotic amino acids (i.e., those amino acids for which the side chain is neither acidic nor basic), the pI can straightforwardly be captured as the average of the two pKa values. For triprotic amino acids (i.e., those that have an acidic or basic side chain), the pI can be obtained by averaging the two acidic pKa values for acidic amino acids or the two basic pKa values for basic amino acids.

Normality

M x H+ can be calculated by multiplying the molarity of the solution by the number of protons per molecule of acid

motivation

Motivation refers to the driving force or reasoning behind our actions and behaviors. Motivation can be broadly divided into two types: extrinsic motivations are created by external forces, while intrinsic motivations are created by internal forces. These forces include attitudes, or ways of thinking or feeling about people, places, and things that are reflected in our actions and our behaviors. Our motivations and behaviors can be shaped by several factors. Instincts are innate, unlearned, and usually fixed patterns of behavior that are, in general, present in all members of a species. One example found in humans is the instinct to suckle: babies do not need to be taught how to suckle milk from their mothers or bottles. Additionally, arousal describes how even when all instincts are fulfilled, people are still motivated to do things, sometimes out of boredom and sometimes out of curiosity. That is, some behaviors are motivated by a desire to achieve an optimal level of arousal (the physiological state of being reactive to stimuli). If a person is not stimulated enough and is below their optimal level of arousal, their desire for arousal may motivate an action, decision, or behavior. A drive is an urge that results from an urge to reach a goal or satisfy a need. Basic drives stem from states of physiological need like hunger or thirst. In these basic cases, the drive would be to eat or drink. Such drives are known as primary drives and are ways that the body is alerted to being out of equilibrium. In contrast, secondary drives are not based on biological needs. Instead, they stem from learning and experiences. Secondary drives can include feelings such as love and aggression. Finally, needs are rooted in higher-level desires. For example, a young person may aspire to be a doctor not just for the financial reward, but out of as desire to help others.

According to the experimental results, the elastic modulus of a sample of articular cartilage with mean CECT attenuation of 1500 HU is nearest: A. 0.1 MPa. B. 0.25 MPa. C. 0.3 MPa. D. 0.5 MPa.

Pay attention to which line of the 2 in the graph correlates and then make "box" to bottom and then use other line to fine crossing info with 2 lines in graph D is correct. Pay close attention to the caption of Figure 2! CECT attenuation is represented by the gray line. The point on that line that corresponds to 1500 Hounsfield units corresponds to a GAG content of about 6% (as shown below). Next, we need to remember what the question is asking for: elastic modulus in units of MPa. If we look at the black line, which is a plot of E (in MPa) versus GAG content, we can see that a 6% GAG content corresponds to an E value of 0.5 MPa.

If the study on the relationship between W. bancrofti and Wolbachia is correct, then combined with the results of the trials, significant numbers of microfilariae do not survive in circulation beyond what time period? A. 3 months B. 6 months C. 12 months D. 24 months

Pay attention!! B is correct. Paragraph two states that W. bancrofti is completely dependent on Wolbachia for replication. Treatment with doxycycline effectively sterilizes the nematodes. However, it does nothing about the microfilariae that are already in the blood prior to the treatment. They may still grow into adults, but they won't be able to reproduce and make more microfilariae. Since the vast majority of patients are free of microfilariae after six months, we can conclude that this is about the longest time that they can survive in circulation. A: About 60% of the patients treated with a combination therapy still have some microfilariae at three months. We can infer that the microfilariae live longer than three months. C, D: It might be tempting to try and infer this from the placebo + DEC line, but the passage states that DEC spares of many adults which are able to reproduce and release microfilariae. Therefore, we don't know if the microfilariae that are still alive at 12 or 24 months happened to be released yesterday or a year ago.

Sexual dimorphism

Sexual dimorphism refers to the degree to which males and females resemble each other. A species with low sexual dimorphism contains males and females that look mostly identical. High sexual dimorphism signals intense competition for mates, while animals from species with low sexual dimorphism typically form pair bonds and mate for life.

statistical significance and p value

Statistical significance is expressed using p-values, which express the likelihood of a certain result being due to chance given a certain null hypothesis, which usually refers to the absence of a relationship between the variables of interest. A p-value <0.05 indicates that there is a <5% chance of the observed relationship being due to chance, and this is the most commonly used threshold for deeming a result statistically significant. The power of a study refers to the ability of a study design to detect a real statistically significant effect, and it is primarily affected by the size of the study (larger samples have more power) and the size of the effect (larger effects are easier to identify).

muscle contraction

Striated muscle fibers contain long rod-like myofibrils that are composed of alternating units of thick (myosin) and thin (actin) fibers that overlap with each other. The basic mechanism of contraction is for the interwoven myosin and actin fibers to slip past each other, in what is sometimes known as the sliding filament model. The fundamental unit of contraction is the sarcomere, which is defined as consisting of a band of thick myosin fibers and half of each of the two adjacent bands of thin fibers. Sarcomeres are divided into the I-band, A-band, H-zone, Z-line, and M-line. The M-line defines the middle of the sarcomere, running through the middle of the thick filaments, while the Z-lines define the edges, running through the middle of the thin filaments. The I-band refers to the region where only thin actin filaments are present, and the A-band is everything else, that is, the entire region where thick filaments are present, including areas of overlap with the thin filaments. The H-zone refers to the region where only thick filaments are present, making it analogous to the I-band. During contraction, the M-lines and Z-lines come closer together, the A-band stays the same, and the I-band and H-zone become shorter. During contraction, the actin and myosin filaments slide past each other through what is known as a cross-bridge cycle, in which a cross-bridge is formed between myosin and actin, and a power stroke provides the force of contraction. Immediately after a power stroke, myosin and actin are bound together, and the cycle begins again. ATP binds to the myosin head, causing a conformational change that releases it from actin. Then, a protein called tropomyosin moves back into place to block strong interactions between actin and myosin. The ATP molecule is then hydrolyzed. This is a strongly exergonic reaction and is used to move the myosin head into the "cocked position," where it can interact weakly with actin. Tropomyosin is ultimately removed by Ca2+ through a somewhat complex mechanism. At this point, the myosin head can bind tightly to actin. The power stroke then occurs via a conformational change that happens when Pi is released, resulting in a force of about 2 pN. ADP is then released and actin and myosin are essentially stuck together until another ATP binds to myosin so that the process can start again. The signaling for contraction occurs when the neurotransmitter acetylcholine is released into the neuromuscular junction. Acetylcholine binds to receptors on the cell membrane, which is known as the sarcolemma in muscle cells, and the sarcolemma then depolarizes in response. This results in an action potential, and when the action potential reaches the sarcoplasmic reticulum, Ca2+ is released into the sarcoplasm (recall that this is just muscle-speak for the cytoplasm). Once in the sarcoplasm, Ca2+ can bind to troponin, which allows contraction to take place.

Evolution, symbiotic relationship

Symbiosis is also an important evolutionary mechanism. Of particular note for the MCAT is the endosymbiotic origin hypothesis, popularized by the pioneering 20th-century biologist Lynn Margulis. This hypothesis proposed that mitochondria derive from an original prokaryotic cell capable of aerobic metabolism that became engulfed in another cell, resulting in an endosymbiotic lineage. This hypothesis was developed to explain the remarkable fact that mitochondria contain independent DNA and self-replicating organelles.

anaerobic vs. aerobic respiration

The Krebs cycle (also known as the citric acid cycle) is also considered aerobic, even though it does not use oxygen directly, since this cycle requires the reduction of NAD+ to NADH. The opposite process (NADH to NAD+) happens in the ETC. When the ETC ceases to function in the absence of oxygen, then, NADH builds up and NAD+ becomes depleted, and the Krebs cycle eventually stops. Finally, glycolysis is anaerobic, since it does not require oxygen. In fact, cells use glycolysis to function in low-oxygen conditions, since it produces ATP - albeit less than the cell would create aerobically. Like the Krebs cycle, glycolysis requires NAD+, so a cytosolic process is required to regenerate it from NADH under anaerobic conditions. This process is fermentation. In yeast cells, ethanol fermentation converts pyruvate into ethanol and carbon dioxide. In human cells and some bacteria, lactic acid fermentation uses the lactate dehydrogenase enzyme to convert pyruvate into lactate. In both cases, NADH is converted to NAD+, allowing glycolysis to continue.

TCA cycle

The citric acid cycle, also known as the Krebs cycle, is a major step in aerobic metabolism. It is the next step of glucose metabolism after glycolysis, but it is also the crossroads of various metabolic pathways in the body (that is, non-carbohydrate precursors can be fed into it, and its intermediates can be siphoned off to use as building blocks for other classes of molecules). The citric acid cycle does generate some ATP directly (through GTP), but its main value is that it generates several electron-carrying molecules that are fed into the electron transport chain to generate much larger amounts of ATP. In eukaryotes, the citric acid cycle takes place in the mitochondrial matrix, while in aerobic prokaryotes it is carried out in the cytosol. Before entering the citric cycle, pyruvate (a three-carbon molecule) must be converted into acetyl-CoA, a molecule that consists of a short two-carbon chain (the "acetyl" group) connected to CoA. This takes place in the mitochondria, in a special area called the pyruvate dehydrogenase complex (PDC), and results in 1 NADH and 1 CO2. The basic logic of the citric acid cycle is that the two-carbon molecule acetyl-CoA joins with the four-carbon molecule oxaloacetate to form a six-carbon molecule known as citrate. Citrate then undergoes a series of redox and decarboxylation reactions to generate the products of the citric acid cycle. The final product is the four-carbon compound oxaloacetate, which joins with acetyl-CoA to start the process again. Each turn of the citric acid cycle generates 1 GTP (which you can think of as functionally equivalent to ATP), 3 NADH, 1 FADH2, and 2 CO2. NADH and FADH2 are electron transporters that ultimately produce energy through the electron transport chain. Even though oxygen is not a reactant or a product, the citric acid cycle requires oxygen indirectly, because it is dependent on the aerobic electron transport chain to regenerate NAD+ and FAD for the process to continue.

Neurons and Neuronal Accessory Cells

The nervous system is composed of two types of cells: neurons and glial cells. Neurons are responsible for inter-cell communication and nerve impulse conduction, while glial cells are accessory cells that help maintain and repair neurons. Neurons are like most cells in that they have a nucleus, cell body, cell membrane, and so on. However, they have two highly specialized extensions that distinguish them from other cells and help them carry out their function of transmitting information. Dendrites, which have tree-like branching structures, gather information and relay it to each neuron's cell body. Axons are long, thin structures that carry the action potential away from the neuron's cell body toward other neurons or target tissues (muscle, gland, organ), with which the neuron connects via structures called synapses. Mirror neurons are nerve cells which fire both when an individual acts and when the individual observes the same action performed by another. The neuron "mirrors" the behavior of the other, as though the observer were the person perfomring the actions. Glial cells provide the neurons with nourishment, physical support, and protection. Glial cells also dispose of the waste generated when neurons die, and accelerate neural conduction by acting as an insulating sheath around certain axons, similar to a rubber coating around an electrical wire. There are three primary glial cells types in the mature human central nervous system. Oligodendrocytes (CNS only) and Schwann cells (PNS only) lay down the lipid-rich myelin that wraps around some, but not all, axons. Myelin significantly improves the velocity of action potential conduction. Microglial cells are small cells with a similar function to macrophages; however, microglia are limited to the nervous system. Microglial cells are scroungers that remove cellular debris from sites of injury or normal cell turnover. Following neural injury, a rapid increase in the number of microglia in the region is seen, either by proliferation of microglia already in the area, or by the transport of microglia via macrophages that migrate to the injured area from the circulation. Astrocytes are star-shaped cells found only in the CNS. The primary function of astrocytes is to maintain the proper chemical environment for action potential conduction and neuron signaling.

Sleep and Consciousness

There are various levels of alertness in which the brain can exist. A key distinction among them is whether the individual is conscious or not. Consciousness means being able to perceive and experience one's surroundings. Each day, your body loses consciousness intentionally, in a process called sleep. Scientists can monitor the electrical activity of the brain during sleep. Sleep is divided into four stages, each with its own associated electrical, psychological, and physical manifestations. The brain waves of a fully-awake person oscillate between a high-frequency, low-amplitude pattern (beta waves) and higher-amplitude, high-frequency, faster patterns (alpha waves), depending on the state of alertness. Alpha waves are more consistent (synchronous) than beta waves. When a person falls asleep, they enter the first of the four stages of sleep. Stage 1 sleep primarily shows theta waves (low amplitude, irregular frequency). In this stage, rolling movement of the eyes occurs with moderate skeletal muscle activity. In Stage 2 sleep, theta waves continue, but are now interspersed with K-complexes (single high-amplitude, low-frequency waves) and sleep spindles (bursts of multiple high-frequency, moderate-amplitude waves). During this stage, there is no eye movement, but skeletal muscle activity remains at a similar level as Stage 1. During Stage 2, heart rate, temperature, and respiration rate decrease. Stage 3 sleep marks the transition into slow-wave sleep, which includes both Stage 3 and Stage 4. In this stage, delta waves (high amplitude, low frequency) predominate. As Stage 3 progresses, higher-frequency waves disappear and Stage 4 sleep begins. During Stage 4, digestion and heart rate slow and growth hormones are released. In the final stage, rapid-eye-movement (REM) sleep occurs. During REM sleep, very little skeletal movement takes place. In fact, during REM sleep, brainwaves, heart rate, and respiration rate are very similar to what is observed when a person is awake. REM is also the stage when a person typically dreams. A sleep cycle is a complete progression through all of the stages of sleep, from Stage 1 to REM and over again. At the beginning of the night, a person spends most time in Stage 4, but as the night progresses, REM predominates, and a person may even have very brief moments of being awake. The length of the sleep cycle increases from childhood to adulthood, from about 50 minutes to 90 minutes, respectively.

Ribosomes and Translation

Translation is the process in which an mRNA sequence is translated into a protein, with each codon corresponding to an amino acid. Transfer RNA, or tRNA, is a relatively small RNA molecule characterized by a hairpin structure that is responsible for "translating" between codons and amino acids. The other structure needed for translation is the ribosome, which is primarily made up of ribosomal RNA (rRNA). Ribosomes contain multiple rRNA strands with associated proteins, and have two major components: the large subunit (50S in prokaryotes and 60S in eukaryotes) and the small subunit (30S in prokaryotes, 40S in eukaryotes), with overall sizes of 70S for the prokaryotic ribosome and 80S for the eukaryotic ribosome. The large subunit catalyzes the formation of the polypeptide chain, while the small unit reads the RNA. Translation has three main steps: initiation, elongation, and termination. Initiation occurs when the mRNA sequence binds to the small ribosomal subunit, either at a region in the 5' untranslated region known as the Shine-Dalgarno sequence (in prokaryotes) or to the 5' cap in eukaryotes. The first tRNA is known as the initiator tRNA, and it binds to the start codon (AUG). The initial amino acid is methionine in eukaryotes, but N-formylmethionine in prokaryotes. Once this happens, initiation factors facilitate the binding of the small ribosomal subunit to the large ribosomal subunit, forming the initiation complex. Elongation is the next step. During elongation, the ribosome reads the mRNA in the 5' to 3' direction and synthesizes a polypeptide from its N terminus to its C terminus, which is one of the reasons why amino acid sequences are traditionally written in the N-to-C order. Proteins known as elongation factors help move this process along. Three main binding sites are involved in elongation. The A site contains the next aminoacyl-tRNA complex, and at the P site a peptide bond is formed between the growing polypeptide chain and the incoming amino acid. The tRNA, which is now no longer "charged" with an attached amino acid, briefly pauses at the E site and detaches from the mRNA. After all of the charged tRNA sequences have been translated, translation is terminated.

reflex arc

Voluntary muscles may contract involuntarily due to a reflex arc. The classic example is the patellar tendon reflex, in which sudden stretching of the patellar tendon leads to an involuntary contraction of the quadriceps. Such contraction occurs before the signal has even reached the brain. A typical reflex arc is shown below. This arc contains a sensory neuron, which carries sensory information from peripheral receptors toward the spinal cord, and a motor neuron, which carries a signal from the spinal cord to an effector muscle. This particular reflex arc also contains an interneuron, which is a neuron within the spinal cord that synapses on both the sensory and motor neurons, connecting them.

paratope and epitope

antigenic determinant that fits like a lock and key to the paratope on the Fab Region of the B cell which matches with the antigen Extensive random recombination of the antigen-recognizing area of the antibody (also known as the paratope)

Mitochondria

binary fission unique in that they are self-replicating organelles. They contain their own DNA (mitochondrial DNA, or mtDNA), which is circular in structure and inherited maternally, and undergo binary fission. This remarkable fact has been explained through the endosymbiotic origin hypothesis, according to which mitochondria derive from an original prokaryotic cell capable of aerobic metabolism that became engulfed in another cell, resulting in an endosymbiotic lineage.

Antibiotic Resistance Mechanisms

block entry, inactivation of resistance enzymes, alteration of target molecule, efflux of antibiotic

A pregnant couple is worried that their child will develop AI. Their respective genealogies are shown below. What is the probability that the first child of this couple will develop non-ENAM-related AI? Image A. 1/16 B. 1/9 C. 1/2 D. 8/9

boxes - males circles - females B is correct. In Figure 1 and, more importantly, in the figure in the question stem, we can see that there is an instance of two unaffected parents having an affected child. We can also see that the trait is found equally in males and females. This means that the trait is inherited in an autosomal recessive fashion. Image We now know that each of the couples' parents are heterozygous carriers of the AI gene. This means each parent in generation 1 has a 50% chance of passing on their affected gene. Neither member of the couple is affected, but they may be carriers. If we designate "A" as the wild-type allele and "a" as the mutated allele, then (as shown in the Punnett square below) the probability that each member of the couple is a carrier is 2/3. The probability is NOT 1/2, because the only possible genotypes of the couple are AA and Aa. Image In order for the couple to have an affected child, both would need to be carriers. Thus the odds that they both are carriers is 2/3 x 2/3 and they would need to pass their mutated allele (not the healthy allele) to their child. If they were both carriers, the probability they would have an affected child is ¼. Therefore, the probability of the child with the '?' being affected by AI is (2/3)(2/3)(1/4) = 4/36 = 1/9.

missense mutations

conservative or non-conservative. --conservative missense mutation, the amino acid in question is replaced with another amino acid that is not identical, but that has similar properties. In the image, for example, lysine is replaced with arginine; since both are basic amino acids, the final protein may function relatively normally. --nonconservative missense mutation involves the replacement of one amino acid with another that has dissimilar properties, like Lys (basic) and Thr (polar uncharged). Nonconservative mutations typically have a greater impact than conservative mutations on the final protein.

The molecule below is: I. aromatic. II. antiaromatic. III. pyrrole. A. I only B. II only C. I and III only D. II and III only

do NOT confuse pyrrole (N in ring) with furan (O in ring A is correct. The molecule depicted is furan, one of the classic aromatic heterocyclic compounds. Furan is a useful molecule to recognize for the MCAT.

myelin

high resistance around membrane of axon much lower resistance interior of axon As an insulator, myelin provides very high resistance around the membrane of the axon. In contrast, the interior of the axon - the inside of the cell - has much lower resistance, so the signal can propagate down the axon without "leaking" out of the neuron.

Eluting strength depends

how strongly a compound adsorbs onto the adsorbent. Since typical adsorbents are highly polar, eluting strength increases with increasing solvent polarity. Methanol is more polar than pentane and therefore has a greater eluting strength. also note: TLC depends on temperature as well!! The retention factor in a TLC procedure depends on the solvent system, temperature, and the adsorbent.

Under low-oxygen conditions, erythrocyte production

increases in response to the secretion of a kidney hormone, erythropoietin (EPO).

calcitrol

increases serum Ca2+ through GI and affects phosphate levels as well Although it is not always considered a hormone, vitamin D plays a crucial role in the regulation of serum calcium and phosphate levels. Vitamin D exists in multiple forms, not all of which are relevant for calcium regulation. Cholecalciferol is the inactive form of vitamin D3, which is processed to form calcitriol, the biologically active form that affects calcium and phosphate levels. Calcitriol has a similar function to PTH in that it increases serum calcium levels, but it does so primarily through a different mechanism: it promotes the absorption of Ca2+ from the gastrointestinal tract.

availability heuristic

individuals pay attention to more extreme cases, which they then use to generalize events as occurring at greater rates than they actually are.

phenomenology

involves subjective experiences, rather than objective (not subject to bias) realities.

Signal detection theory

is a framework for explaining how the mind transforms sensory stimuli into perceptions. This theoretical framework points out that the perception of stimuli can be affected not only by the stimuli themselves, but also by nonsensory considerations like expectations, experiences, and motives. In this way, psychological and environmental context can alter our perception of stimuli. Signal detection theory accounts for response bias, which is the tendency of people to habitually respond to a certain stimulus in a certain way because of nonsensory factors.

Collagen

is a primary component in connective tissue, such as tendons, cartilage, and blood vessels.

Anchoring

is the act of relying too much on the first information encountered.

affect heuristic

is the process of making a judgment based on emotions that are evoked.

Chromatography

is used to separate, identify, and purify the components of a mixture. Four common MCAT-tested techniques are ion exchange, surface adsorption, partition, and size exclusion. Although there are many chromatography techniques, they share the principle that the molecules in a mixture are applied onto a stationary phase (usually a solid), while a fluid known as the mobile phase (generally a solvent chosen to match the target molecules, e.g. polar or nonpolar) containing the molecules of interest travels through the stationary phase. Molecules of interest in the mobile phase will interact with the stationary phase with different levels of intensity. Molecules that interact more strongly with the stationary phase will take longer to pass through it, whereas molecules that interact more weakly with the stationary phase will pass through it more quickly. Common factors that shape these interactions include molecular characteristics related to adsorption, polarity- or charge-based affinity for the stationary/mobile phase, and differences in molecular weight. A chiral stationary phase can also be used to separate stereoisomers based on the principle that the various enantiomers of a compound may interact differently with such a stationary phase. The list below contains some common types of chromatography that may appear on the MCAT. All are based on the principles outlined above. As such, when studying a chromatography technique, it is less helpful to focus on the details of the setup than it is to determine which molecules will interact more closely with the stationary phase, leading them to be retained longer. Column chromatography Ion-exchange chromatography Size-exchange chromatography Paper chromatography Thin-layer chromatography (TLC) Gas chromatography (GC) High-pressure liquid chromatography (HPLC) Gel chromatography Affinity chromatography

Right shift of the oxygen dissociation curve means

less affinity of Hb for O2 These conditions include low (acidic) plasma pH and increased levels of carbon dioxide - both of which can indicate a shortage of oxygen. This rightward shift of the hemoglobin-oxygen binding curve is termed the Bohr effect.

hyperventilation and hypoventilation

pay attention!! hyper - respiratory alkaloidosis hypo - respiratory acidosis

elimination reaction

produces an alkene The NaOH would produce a hydroxyl group, which is a powerful base and a powerful nucleophile. With the chlorine on the secondary carbon, the OH- could act as either one, carrying out an elimination or a substitution reaction. We see that there are no alkenes listed among the answer choices, so the question must be asking about a substitution reaction.

Osmolarity

refers to the total amount of solute in solution.

Statistical power

strengthened by increasing sample size Statistical power refers to the potential or likelihood of a study to detect a "real" difference in the outcome of interest. As such, it depends on the sample size and the magnitude of the effect of interest.

rater bias

tendency for raters to rate in the middle of a scale.

Alleles

think BB or bb or Bb genes for example 2 alleles needed for recessive, only one needed for dominant https://www.youtube.com/watch?v=oc9fJCAIRJs

A large steel water storage tank with a diameter of 20 m is filled with water and is open to the atmosphere (1 atm = 101 kPa) at the top of the tank. If a small hole rusts through the side of the tank, 5.0 m below the surface of the water and 20.0 m above the ground, assuming wind resistance and friction between the water and steel are not significant factors, how far from the base of the tank will the water hit the ground? A. 5.0 m B. 10.0 m C. 20.0 m D. 30.0 m

v = sqrt(2gdeltay) then get time: y = 1/2gt^2 then x: d = rt C is correct. We first need to determine the velocity of the water that comes out of the hole, using Bernoulli's equation. P1 + ρgy1 + 1/2 ρv1^2 = P2 + ρgy2 + 1/2 ρv2^2 The atmospheric pressure exerted on the surface of the water at the top of the tank and at the hole are essentially the same. Additionally, since the opening at the top of the tank is so large compared to the hole on the side, the velocity of water at the top of the tank will be essentially zero. We can also set y1 as zero, simplifying the equation to: 0 = ρgy2 + 1/2 ρv2^2 -gy2 = 1/2 v2^2 Inserting the value for gravitational acceleration (g = -10 m/s^2), and the level of the hole below the surface (y2 = 5.0 m) into the equation, we get: (10 m/s^2)(5.0 m) = 1/2 v^2 50 = 0.5 v^2 10^2 = v^2 10 m/s = v This velocity will be in the horizontal direction, and we will assume that the water acts like a projectile. Now, we need to determine the time that it will take the water to fall to the ground. We can assume that the initial vertical velocity of the water is zero and the displacement of the water is -20 m, because the hole is 20.0 m above the ground. The kinematic equation is: d = vit + 1/2 gt^2 -20 = 0 + 1/2 (-10) t^2 -20 = -5 t^2 4 = t^2 2 s = t Finally, we can determine the range, or displacement in the horizontal direction. Since the acceleration in the horizontal direction is zero, the equation becomes d = vit d = (10 m/s)(2 s) d = 20 m