Biology MCAT
Regulation of transcription
(see photo) transcriptional regulation is the means by which a cell regulates the conversion of DNA to RNA (transcription), thereby orchestrating gene activity. It is orchestrated by transcription factors and other proteins working in concert to finely tune the amount of RNA being produced through a variety of mechanisms. Prokaryotic organisms and eukaryotic organisms have very different strategies of accomplishing control over transcription, but some important features remain conserved between the two. Most importantly is the idea of combinatorial control, which is that any given gene is likely controlled by a specific combination of factors to control transcription. A single gene can be regulated in a range of ways, from altering the number of copies of RNA that are transcribed, to the temporal control of when the gene is transcribed. This control allows the cell or organism to respond to a variety of intra- and extracellular signals and thus mount a response. Some examples of this include producing the mRNA that encode enzymes to adapt to a change in a food source, producing the geneproducts involved in cell cycle specific activities, and producing the gene products responsible for cellular differentiation in multicellular eukaryotes, as studied in evolutionary developmental biology.
reflex arc components
A reflex is either a subconscious or involuntary response to an external stimulus. Whether it is blinking when an object comes close to the eyes or snatching your hand back when touching something hot - reflexes are designed to adjust to these stimuli to protect the body from harm. While sensory neurons are responsible for detecting a stimulus and motor neurons are responsible for stimulating a muscular or glandular response, interneurons serve as the connection point between these two pathways.
The amount of NE (norepinephrine) released by sympathetic nerve terminals will be most strongly influenced by a change in which of the following? A. Alpha receptor sensitivity B. Alpha receptor density C. Extracellular [Ca2+] D. COMT activity
Answer C. Extracellular [Ca2+] The question asks the examinee to identify the factor among the options listed that will most strongly influence the amount of NE released by the sympathetic nerve terminal. Of these options, only C, extracellular [Ca+], will influence the amount of NE released by the sympathetic nerve terminal. When an action potential reaches a nerve terminal it triggers the opening of Ca+ channels in the neuronal membrane. Because the extracellular concentration of Ca+ is greater than the intracellular Ca+concentration, Ca+ flows into the nerve terminal. This triggers a series of events that cause the vesicles containing NE to fuse with the plasma membrane and release NE into the synapse. Thus, C is the best answer.
Capillaries in the kidney and elsewhere in the body maintain fluid homeostasis by balancing hydrostatic and osmotic pressures. Which of the following is the initial effect of a blood clot forming on the venous side of a capillary bed? A. Net fluid flow in the direction of interstitial spaces will increase. B. Net fluid flow in the direction of interstitial spaces will decrease. C. Capillary osmotic pressure will increase. D. Capillary osmotic pressure will decrease.
Answer: A. Net fluid flow in the direction of interstitial spaces will increase. After introducing the idea of fluid homeostasis, which is obtained by balancing hydrostatic and osmotic pressures, the question asks the examinee to predict the consequences of a blood clot on the venous side of a capillary bed. To answer this, it is necessary to know that blood flows from arteries to capillaries and then to veins. If flow is blocked at the venous side, blood would accumulate in the capillaries. Thus, hydrostatic pressure would build up in the capillaries, causing a net increase in fluid flow into the interstitial spaces. Realize that the interstitial compartment is the space between the capillaries and the cells. Fluids support the matrix and cells within the interstitial space. The intracellular compartment is separated from the interstitial space by a cell membrane.
Passage states, "A problem encountered in the design of antisense drugs was that oligonucleotides may only persist for a matter of minutes before they are degraded by cellular processes. Antisense drugs became feasible when phosphorothioate analogs of the oligonucleotides were developed; these analogs can exist for days or weeks within the cell." Question: If oligonucleotides such as mRNA were not degraded rapidly by intracellular agents, which of the following processes would be most affected? A. The production of tRNA in the nucleus B. The coordination of cell differentiation during development C. The diffusion of respiratory gases across the cell membrane D. The replication of DNA in the nucleus
Answer: B. The coordination of cell differentiation during development Explanation: From the question stem we can discern that an accumulation of mRNA will occur which would normally be degraded. Thus, the answer would have to directly result from excess mRNA in the intracellular agent. While A and D can be modified by the translation of mRNA that help make tRNA and DNA replication proteins, as the explanation states, excess mRNA will not alter the "production" of tRNA or "replication" of DNA which are not time sensitive. Perhaps if these answer directly stated an "increase in production of tRNA" or "increased rate of replication of DNA" it could be a better choice. C would only seem possible of the mRNA in question is specific to a protein that degrades the membranes ability to diffuse gases, which is not supported in the question stem or passage. We know that cell differentiation involves stem cells becoming unique proteins. Even if you did not know the exact mechanisms of cell differentiation, we can assume that this process will require the use of many different mRNA to translate for the proteins needed during development. The "timing of mRNA turnover" is the main point of this question. Once a specific protein is express through the transcription of mRNA, it must be degraded for new proteins to translated efficiently. Development, a time period sensitive to specific protein creation, relies on removal of mRNA to allow efficient translation of specifically needed protein.
Mitosis vs. Meiosis In mammals, which of the following events occurs during mitosis but does NOT occur during meiosis I? A. Synapsis B. The splitting of centromeres C. The pairing of homologous chromosomes D. The breaking down of the nuclear membrane
Answer: B. The splitting of centromeres Explanation: One of the key differences between mitosis and meiosis occurs during their respective anaphases. During anaphase of mitosis, sister chromatids are pulled apart at the centromeres, each becoming an independent chromosome in the two diploid daughter cells. During anaphase I of meiosis I, homologous pairs of chromosomes are separated into the two daughter cells. However, each chromosome still consists of two sister chromatids joined to each other at the centromere. It is not until anaphase II of meiosis II that the centromere is split and the sister chromatids separate.
Which of the following nucleotide sequences describes an antisense molecule that can hybridize with the mRNA sequence 5′-CGAUAC-3′? A. 5′-GCTATG-3′ B. 5′-GCUAUG-3′ C. 3′-GCUAUG-5′ D. 3′-GCAUAG-5′
Answer: C. 3′-GCUAUG-5′ The question asks the examinee to identify the sequence of an antisense molecule that could hybridize with the mRNA sequence 5′-CGAUAC-3′. When the RNA molecules hybridize, the antisense molecule would line up in an antiparallel fashion with the sense molecule, meaning its 3′ end would line up with the 5′ end of its complement. The nitrogenous bases would form the following pairs: A with U, G with C. The only correct sequence is C, which is 3′-GCUAUG-5′.
Why are high concentrations of sodium included in the dialysate (Table 1 see pic)? A. To induce water movement from the blood into the dialysate fluid B. To maintain a high osmotic pressure in the dialysate solution C. To maintain isotonicity of the dialysate solution with blood D. To compensate for the urea nitrogen and creatinine in the blood
Answer: C. To maintain isotonicity of the dialysate solution with blood The question asks the examinee to correctly identify the reason why high concentrations of sodium are included in the dialysate. As shown in Table 1, the dialysate contains 128-140 mEq/L of sodium, which is equivalent to the sodium concentration in normal plasma, as well as to the sodium concentration in the blood of a patient with renal failure. Consequently, A is incorrect because solvent will not move in response to the sodium concentration in the dialysate, if it is the same as the concentration in the patient's blood. B is incorrect because the sodium in the dialysate is not maintaining a high osmotic pressure if the patient's blood and the dialysate have the same sodium concentration. C is correct because examination of the concentrations of all ions in plasma and dialysate suggests that sodium levels must remain high in the dialysate so that the dialysate is isotonic to blood. D is incorrect because the sodium concentration of the dialysate would not compensate for the levels of urea nitrogen and creatinine nitrogen in the blood. Thus, C is the best answer.
"A new strategy of drug design uses nucleic acid macromolecules to prevent the expression of a gene. The target of the drug can be a gene in a bacterial cell, a cancer cell, or a virus-occupied eukaryotic cell; the macromolecular drug can be either produced within a biological system or added to it. Normally produced messenger RNA (mRNA) molecules are known as the sense RNA. Antisense nucleic acids, which are complementary to a portion of the sense mRNA, can be synthesized. The antisense molecules will bind specifically to the sense mRNA and prevent the production of the natural gene product.... Genes that produce an antisense sequence can also be synthesized and added to the genome of organisms. In this manner antisense and sense RNA molecules could be produced simultaneously—in effect, preventing the production of the gene product permanently." Question: To be an effective therapy, an antisense gene that is incorporated into a genome that contains the target gene must be: A. on the same chromosome as the target gene but not necessarily be physically adjacent. B. on the same chromosome as the target gene and must be physically adjacent. C. regulated in a similar manner as the target gene. D. coded on the same strand of DNA as the target gene.
Answer: C. regulated in a similar manner as the target gene. Explanation: If you want the antisense RNA to have a therapeutic effect, it would need to be expressed whenever the target mRNA that it's meant to block is to be expressed. Thus the target gene and the antisense would have to be regulated in a similar manner. I.e same promoter Just because a gene is physically near another doesn't mean if one is expressed the other will be. Gene expression is based on a handful of regulatory processes.
Passage states, "The dialysis membrane is made of a thin semiporous material, which allows diffusion of solutes with molecular weights up to 1000-2000 daltons, depending on the size of the membrane pores. Protein molecules are too large to diffuse through the membrane." Question: Which of the following figures (A-D) shows expected solute filtration rates (mEq/mL-min) as a function of molecular weight for two dialysis membranes: Membrane 1 with large pores and Membrane 2 with small pores? (see picture)
Answer: Figure D Explanation: PAY ATTENTION TO WORDING OF QUESTION! Membrane 1 = LARGE pores > 1000-2000 Membrane 2 = small pores < 1000- 2000 Also, realize that large protein molecules will slow down diffusion rate. As molecular weight increases, the rate of filtration is expected to decrease, as shown in C and D. We can eliminate choices A and B. D is correct because it shows the trend of decreasing filtration rate with increasing molecular weight, and it shows that Membrane 1 (with larger pores) has a higher filtration rate than Membrane 2 for any molecular weight.
3 Na+(inside) + 2 K+(outside) + ATP4− + H2O → 3 Na+(outside) + 2 K+(inside) + ADP3− + Pi 2− + H+ Based on the reaction above, if all the energy produced from glycolysis were used to remove Na+ from a cell, how many molecules of Na+ would be removed per molecule of glucose? a. 3 b. 6 c. 9 d. 12
Answer: b. 6 Since glycolysis produces two ATP molecules per glucose molecule, and 3 Na+ molecules are extruded for each ATP molecule utilized by the sodium pump, a total of 6 molecules of Na+ are extruded per molecule of glucose.
Bacteria exchange genes by
Bacteria can exchange genes by three processes: 1. The process of conjugation involves production of a special conjugation pilus (sex pilus) by one bacterium and transfer through it of DNA to another bacterium. It requires special genes for the pilus and these are usually present on a plasmid, a separate extragenomic strand of DNA not incorporated into the bacterium's own DNA. This plasmid is referred to as the fertility or F factor. Conjugation is a feature of Gram-negative bacteria. It confers the advantages of sexual reproduction on the bacterium. The plasmid benefits by being able to move from one host bacterium to another through the conjugation pilus. Conjugation is the process described in the stem 2. In transformation bacteria take up DNA from their surroundings, the media in which they are immersed. This process does not require a plasmid. 3. Transduction is the process whereby genes are transferred by a virus. This process does not require a plasmid.
bacterial conjugation
Conjugation is the process by which one bacterium transfers genetic material to another through direct contact. During conjugation, one bacterium serves as the donor of the genetic material, and the other serves as the recipient. The donor bacterium carries a DNA sequence called the fertility factor, or F-factor. The F-factor allows the donor to produce a thin, tubelike structure called a pilus, which the donor uses to contact the recipient. The pilus then draws the two bacteria together, at which time the donor bacterium transfers genetic material to the recipient bacterium. Typically, the genetic material is in the form of a plasmid, or a small, circular piece of DNA. The genetic material transferred during conjugation often provides the recipient bacterium with some sort of genetic advantage. For instance, in many cases, conjugation serves to transfer plasmids that carry antibiotic resistance genes.
Cell Cycle: Mitosis
During the mitotic (M) phase, the cell divides its copied DNA and cytoplasm to make two new cells. M phase involves two distinct division-related processes: mitosis and cytokinesis. In mitosis, the nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a specialized structure made out of microtubules. Mitosis takes place in four stages: 1. prophase (sometimes divided into early prophase and prometaphase) 2. metaphase 3. anaphase 4. telophase. In cytokinesis, the cytoplasm of the cell is split in two, making two new cells. Cytokinesis usually begins just as mitosis is ending, with a little overlap. Importantly, cytokinesis takes place differently in animal and plant cells.
Brain parts and functions
Hippocampus (and other medial temporal lobe structures such as entorhinal cortex, perirhinal cortex, parahippocampal cortex): memory Hypothalamus: neuroendocrine (all the shit with the hormones and pituitary) and autonomics (hunger, well-being, distress, skin temperature, gut distension) Midbrain: probably the most "MCAT function" is the dopaminergic midbrain system, substantia nigra pars compacta with Parkinson's and Huntington's disease; also quite a few other functions such as temperature control, hearing, vision, attention, sleep/wake that you probably don't care about Cerebellum: error-based motor learning, i.e. you do some action, your brain gets an "error signal" if you did it incorrectly, then "bad" actions are suppressed; balance, coordination Basal ganglia: collection of structures involved in reward-based motor learning Thalamus: basically a signal relay station, where different signals come in and are re-routed to different parts of the brain for different functions; MGN = audition, LGN = vision Forebrain: higher, abstract, complex thinking; has some inhibitory control over the amygdala Amygdala: emotion Corpus callosum: a band of axons which connect the two hemispheres of the brain Wernicke's area: in the superior temporal gyrus; involved in speech comprehension Broca's area: in the inferior frontal gyrus; involved in speech production Brainstem: includes medulla, pons, and the midbrain; involved in sleep, heart rate, eating (basically a bunch of regulatory functions); also holds the nuclei of many of the descending axon tracts essential for motor movements Limbic system: a somewhat outdated view of the thalamus, hypothalamus, amygdala, and cingulate cortex as the "emotional system" And the four lobes in basic terms... Frontal lobe: reasoning, planning, parts of speech, movement, emotions, and problem solving Parietal lobe: movement, orientation, recognition, perception of stimuli Occipital lobe: visual processing Temporal lobe: sound and some smell; also contains Wernicke's area Edit: something about hypothalamus
heterozygous mating results in ..
If we call the recessive gene r and its dominant allele R, then a cross between two heterozygote strains Rr will produce the genotypes RR:Rr:rr in a ration of 1:2:1. The recessive gene will be masked so that its expression will not be observable if a dominant gene is present in the genotype. Only in the double recessive (rr) case will the presence of the recessive gene be observed.
Cell Cycle: Interphase
In eukaryotic cells, or cells with a nucleus, the stages of the cell cycle are divided into two major phases: interphase and the mitotic (M) phase. The prefix inter- means between, reflecting that interphase takes place between one mitotic (M) phase and the next. During interphase, the cell grows and makes a copy of its DNA. The G_1, S, and G_2 phases together are known as interphase. G_1 phase. During G_1 phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps. S phase. In S phase, the cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase. G_2 phase. During the second gap phase, or G_2 phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G_2 phase ends when mitosis begins.
Inflation of the lungs in mammals is accomplished by:
Inflation of lungs in mammals is accomplished by negative pressure pumping action. Because the lung stays in contact with the thoracic wall as it enlarges due to contraction of the diaphragm and the external intercostal muscles, a pressure that is lower than atmospheric pressure (negative pressure) is generated within the alveolar sacs. Conversely, positive pressure ventilation is mechanical. Instead of negative pressure created by the contraction of the diaphragm and intercostal muscles pulling air into the lungs to fill the alveoli, you use either a breathing machine (ventilator) via ET tube or non-invasive bag-mask, CPAB, BiPAP, etc to force air into the lungs, expand the lungs and deliver oxygen to the alveoli.
anaerobic respiration
Respiration that does not require oxygen. When the heart is not circulating oxygenated blood to the tissues, the cells would have had to rely entirely on anaerobic respiratory mechanisms for energy. Recall that anaerobic metabolic pathways can produce energy without using oxygen by glycolysis.
"Sarah, a scientist from New Orleans, takes two-week vacations to different locations every year to experience new sports. One year she went to the Caribbean Sea to learn skin diving. Although she was in excellent physical condition from daily swimming in the ocean, she noticed that the first time she went diving, she experienced an elevated pulse and ventilation rate. By the third time she went diving, her heart and breathing rate were no longer elevated. By the end of the two weeks, her skin had become darker. Another year she went skiing on snow in the mountains of Colorado. Again, she noticed that the first time she went skiing, her heart and ventilation rate were faster than usual. Although it was not as elevated by the end of the first week, her heart and breathing rates were still higher than usual. She also noticed that her appetite and caloric intake were considerably greater during her skiing vacation compared with her diving vacation. However, she noticed that her body weight did not change significantly." Question 1 : The initial increase in heart and breathing rates during the skin diving trip was probably a result of: A. activation of the sympathetic autonomic nervous system by the new experience. B. activation of the parasympathetic autonomic nervous system by the new experience. D. hypoxia caused by the inability of her blood hemoglobin concentration to supply sufficient oxygen for the strenuous exercise of swimming at sea level. D. elevated core body temperature caused by swimming in warm tropical waters. Question 2: The prolonged increase in heart and breathing rates during the snow skiing trip was probably a result of: A. activation of the sympathetic autonomic nervous system by the new experience. B. activation of the parasympathetic autonomic nervous system by the new experience. C. hypoxia caused by insufficient blood hemoglobin concentration to supply oxygen for exercise at the low oxygen pressure found at high altitudes. D. depressed core body temperature (hypothermia) caused by exposure to cold temperatures at high altitudes.
Solution 1: The correct answer is A. According to the passage, Sarah was in excellent physical condition prior to her trip to the Caribbean Sea to go skin diving. After her first diving experience, she noticed an elevated pulse rate and ventilation rate. The most likely explanation for her body's response was the activation of her sympathetic autonomic nervous system—the "fight or flight" response caused by adrenaline. Solution 2: The correct answer is C. According to the passage, Sarah went skiing in the mountains of Colorado. At first, she noticed an elevated pulse rate and ventilation rate. As the week progressed, these rates dropped, but were still higher than usual. This prolonged increase in heart rate and breathing rate was most likely the cause of hypoxia (insufficient oxygen to the body cells) caused by insufficient blood hemoglobin to supply oxygen for exercise at the low oxygen pressure found at high altitudes.
The above diagram (see pic) represents the neural pathway that causes an individual to retract a stubbed toe. If one were to modify this diagram to represent the pathway involved in feeling pain in the stubbed toe, where could additional neurons be placed? A. At II and III B. At II and IV C. At III and IV D. At I and IV
Solution: The correct answer is A. At II and III Additional neurons would be placed at synapses II and III, which are located in the central nervous system. It is from these sites that ascending tracts to the brain, where pain is perceived, originate. The efferent nerve terminal, represented by IV, is located in the muscle and is a motor, not a sensory, site. Impulses arrive at IV from the central nervous system. Although the afferent leg of pain information (diagrammed as I) is part of the pain-perception pathway, additional pathways would be added in the spinal cord. Without such ascending pathways, a reflex could occur, but the sensation of the pain would be absent. Occasionally these ascending tracts are cut as a means of alleviating pain. Thus, answer choice A is the best answer.
action potential
Stages of an action potential: 1. Resting: cell at rest, sodium-potassium pump maintaining resting potential (-70 mV). Lots of sodium outside, lots of potassium inside. Ion channels closed so the established ion gradient won't leak. 2. Depolarization: sodium channels open, positive sodium rushes inside, membrane potential shoots up to +30 mV. Lots of sodium inside, lots of potassium inside. 3. Repolarization: potassium channels open, sodium channels close, positive potassium rushes outside [due to excess positive charge on the inside], membrane potential drops back down. Lots of sodium inside, lots of potassium outside (opposite of the resting state). 4. Hyperpolarization: potassium channels doesn't close fast enough, so the membrane potential actually drops below the resting potential for a bit. (> - 70 mV) 5. Refractory period: the sodium-potassium pump works to re-establish the original resting state (more potassium inside, sodium outside). Until this is done, the neuron can't generate another action potential. Absolute refractory period = from depolarization to the cell having re-established the original resting state. Relative refractory period = After hyperpolarization till resting state re-established.
sympathetic vs parasympathetic nervous system
Sympathetic = prepares body for activity = fight or flight response. -Increase heart rate, blood pressure -More blood flow to muscles, less to digestive system. -Pupil dilation. -Break down glycogen to release glucose into blood. Parasympathetic = prepares body to rest -Decrease heart rate, blood pressure. -Less blood to muscles, more to digestive system. -Pupil constriction. -Synthesizes glycogen for storage from glucose.
prokaryotic vs eukaryotic cells
The distinction is that eukaryotic cells have a "true" nucleus containing their DNA, whereas prokaryotic cells do not have a nucleus. Both eukaryotes and prokaryotes contain large RNA/protein structures called ribosomes, which produce protein. Prokaryotes lack mitochondria and chloroplasts.
3 germ layers
The ectoderm is what attracts you to a person (brains and looks) for skin, brain, nails, etc. Also, the adrenal medulla comes from the ectoderm. The mesoderm is the means-o-derm, it's the means of getting around so skeletal, muscular, excretory, circulation, etc. The adrenal cortex also comes from the mesoderm. - The mesoderm is basically all long/tubelike structures such as vessels (CV system), muscles and bones (MS system). The endoderm - Anus is your end (digestive and respiratory)
antibiotic resistant bacteria
antibiotic resistant bacteria: bacteria that undergo mutation to become resistant to an antibiotic and then survive to increase in number. Mutations are one way for bacteria to become resistant to antibiotics. Some spontaneous mutations (or genes that have been acquired from other bacteria through horizontal gene transfer) may make the bacterium resistant to an antibiotic (See: Resistance mechanisms for information about how bacteria resist antibiotic action). If we were to treat the bacterial population with that specific antibiotic, only the resistant bacteria will be able to multiply; the antibiotic selects for them. These bacteria can now increase in numbers and the end result is a population of mainly resistant bacteria.
Cancer is caused by:
genetic mutations occur in proliferating somatic cells
insulin vs glucagon
insulin lowers blood sugar; insulin puts sugar back "in" glucagon raises blood sugar
sense vs antisense strand
template = noncoding strand = anti-sense strand = (-) sense think coding like protein coding, this is the DNA/mRNA fragment that is NOT used during translation, within viruses, this strand does require transcription nontemplate = coding = sense = (+) sense identical to mRNA, if in reference to virus does not require transcription