Biology Session 2

Give a definition for: 1) Mutualism 2) Commensalism 3) Parasitism

1) Mutualism: Relationship is beneficial for both 2) Commensalism: Beneficial for one and not affect the other 3) Parasitism: Beneficial for one and detrimental to the other

What will happen in LAC operon under these circumstances: 1) glucose, no lactose 2) lactose, no glucose 3) no glucose, no lactose 4) lactose and glucose

1) No lactase gene transcribed 2) Lactase gene transcribed at high levels 3) No lactase gene transcribed 4) Lactase gene transcribed at low levels

Give a definition for each of the following mutations: 1) Point 2) Missense 3) Neutral 4) Silent 5) Frameshift 6) Nonsense

1) Point: Changes a single base pair of nucleotides in a double strand of DNA. 2) Missense: a point mutation in which a single nucleotide is changed, and may or may not result in a codon that codes for a different amino acid 3) Neutral: No change in protein function 4) Silent: No change in amino acid 5) Frameshift: When an insertion/deletion occurs in multiples other than 3, and each sequence after the mutation will be grouped incorrectly. 6) Nonsense: When a base pair substitution or insertion/deletion mutation creates a stop codon.

Provide definition for: 1) malignant 2) benign 3) metastasis 4) protoncogenes 5) oncogenes 6) carcinogens

1) malignant: dangerous to health; characterized by progressive and uncontrolled growth (especially of a tumor) 2) benign: A benign tumor does not invade surrounding tissue or spread to other parts of the body. A benign tumor may grow but it stays put (in the same place). 3) metastasis: The process by which cancer spreads from the place at which it first arose as a primary tumor to distant locations in the body. 4) protoncogenes: Certain genes that stimulate normal growth in human cells. 5) oncogenes: genes that cause cancer 6) carcinogens: mutagens such as UV, chemicals or random mutations.

What are the three different shapes of bacteria?

1. Bacilli: Rod-shaped 2. Cocci: Spherical-shaped 3. Spirilla: Spiral-shaped

What is the difference between mitosis and binary fission?

1.Binary fission occurs among prokaryotes (cells that do not contain a nucleus). 2.Mitosis occurs among eukaryotes (cells that have a nucleus). 3.Binary fission does not include spindle formation (mitotic apparatus) and sister chromatids in its process making it a faster means of cellular division than mitosis. 4.Binary fission does not have the four distinct cellular phases (from G1 down to the final mitotic phase) that are seen in mitosis.

Provide definition for bidirectional, semi-conservative, and semi-discontinuous.

Bidirectional: Two replisomes proceed in opposite directions along the chromosome. Semi-conservative: When a new double strand is created, it contains one strand from the original DNA, and one newly synthesized strand. Semi-discontinuous: Formation of one strand is continuous and the other fragmented.

How do bacteria reproduce?

Binary fission: This process results in the reproduction of a living prokaryotic cell (or organelle) by division into two parts that each have the potential to grow to the size of the original cell (or organelle). This type of division takes place without the formation of spindles. The single DNA molecule first replicates, then attaches each copy to a different part of the cell membrane. When the cell begins to pull apart, the replicate and original chromosomes are separated. The consequence of this asexual method of reproduction is that all the cells are genetically equal, i.e have the same genetic material.

What is a vaccine, how does it work?

Can be either an injection of antibodies or an injection of a nonpathogenic virus with the same capsid or envelope. The nonpathogenic virus vaccine allows the host immune system to create its own antibodies. A biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins. The agent stimulates the body's immune system to recognize the agent as foreign, destroy it, and "remember" it, so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.

What is the basic structure of a bacteria?

Capsule Peptidoglycan cell wall Cellular membrane No complex membrane-bound organelles Single circular DNA

What division ("troph") are fungi?

Eukaryotic heterotrophic chemotrophs.

When determining probabilities for genes, If both events must occur simultaneously, what do you do to the probabilities of each event occurring individually?

Multiply them

What type of cells are bacteria?

Prokaryotes

Which DNA bases are purines and which are pyrimidines?

Purines: Adenine and Guanine Pyrimidine: Thymine and Cytosine

What are the Hardy-Weinberg Equations and how do you use them?

p²+2pq+q²=1 p+q=1 If 80% of the alleles are A, the probability that two As come together is 0.8²=0.64. The probability that two a's come together is 0.2²=0.04. And the remaining zygotes will be 2*0.8*0.2=0.32

Describe the process of translation.

-5' mRNA binds to small subunit of rRNA. -tRNA with 5'-CAU- binds Methionine and settles into P site, which causes the large subunit to bind and form the initiation complex (initiation) -Elongation: tRNA w/ its a.a. binds to A site at expense of two GTPs. C-terminus of Met binds to N-terminus of the new a.a. Through translocation the ribosome shifts 3 nucleotides and the Met tRNA moves to E-site. The A-site tRNA moves to P-site and a new tRNA binds A-site. -Termination: stop codon reaches A-site and water is added to end of polypeptide allowing it to be freed.

What are the three basic mechanisms of gene regulation?

-Rate of transcription: RNA has a short half-life, so gene products only continue to be expressed if DNA is continually transcribed. -Activators and Repressors: Certain substances upregulate DNA transcription (such as lactose in Lac Operon) and other substances down regulate transcription (glucose in the Lac Operon) -Permanent or Semi-Permanent Repression: methylation or other covalent modification that prevents transcription.

What is the difference between DNA and RNA?

-The hydroxyl group on the 2' carbon has been replaced with a hydrogen in DNA. -RNA is usually single-stranded, DNA is double-stranded -RNA contains Uracil bases, DNA contains Thymine bases -RNA exists in three forms (mRNA, tRNA and rRNA), DNA exists in only one form -RNA exits the nucleus into the cytoplasm, DNA always stays in the nucleus

Describe the functions of each of the three kinds of RNA.

-mRNA: transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein. -rRNA: Provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity. (Can be considered an enzyme) -tRNA: collects amino acids in the cytosol and transfers them to the ribosomes for incorporation into a protein.

Describe the following mutations that occur on the chromosomal level: 1) Duplications 2) Deletions (non-disjunctions) 3) Translocations 4) Inversions

1) Duplications: When a DNA fragment breaks free of one chromosome and incorporates into a homologous chromosome. Can occur with entire chromosomes or sets of chromosomes. 2) Deletions (non-disjunctions): When a portion of the chromosome breaks off, or when a portion of the chromosome is lost during homologous recombination and/or crossing over events. Can occur with entire chromosomes or sets of chromosomes. 3) Translocations: When a segment of DNA from one chromosome is inserted into another chromosome. 4) Inversions: The orientation of a section of DNA is reversed on a chromosome. (Transposons can excise themselves from a chromosome and reinsert themselves at another location. These can lead to translocations and inversions.)

Define: 1) Gene pool 2) Evolution 3) Polymorphisms 4) Niche 5) Survival of the fittest 6) Natural selection 7) Speciation 8) Adaptive radiation 9) Evolutionary bottleneck 10) Genetic drift 11) Founder effect 12) Carrying capacity

1) Gene pool: the complete set of unique alleles in a species or population. 2) Evolution: Change in the gene pool 3) Polymorphisms: when two or more clearly different phenotypes exist in the same population of a species 4) Niche: the relational position of a species or population in its ecosystem to each other, no two species can occupy the same niche indefinitely. 5) Survival of the fittest: one species will exploit the environment more efficiently, eventually leading to the extinction of the other within the same niche. The organism which can best survive to reproduce offspring. 6) Natural selection: process by which traits become more or less common in a population due to consistent effects upon the survival or reproduction of their bearers. 7) Speciation: the evolutionary process by which new biological species arise. 8) Adaptive radiation: Several separate species arise from a single ancestral species. 9) Evolutionary bottleneck: A species may face a crisis so severe as to cause a shift in the allelic frequencies of the survivors of the crisis. (The term bottleneck usually refers to entire populations being reduced to a few individuals, say due to hunting, or natural disasters.) 10) Genetic drift: One allele may be permanently lost due to the death of all members having that allele. Genetic drift is a change in the allele frequencies within a population due to something other than natural selection (ie genetic drift or the founder effect). 11) Founder effect: loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. 12) Carrying capacity: The maximum number of organisms that an environment can maintain.

What are the different classification levels?

1) Kingdom 2) Phylum 3) Class 4) Order 5) Family 6) Genus 7) Species

What are the Hardy-Weinberg Equilibrium assumptions?

1) Large population 2) No Mutation 3) No Immigration or Emigration 4) Random Mating 5) No Natural Selection

Provide a definition for: 1) Latent period 2) Virulent 3) Temperate 4) Dormant 5) Provirus 6) Reverse transcriptase 7) Retrovirus

1) Latent period: Period from infection to lysis 2) Virulent: A virus following a lytic cycle 3) Temperate: A virus in a lysogenic cycle 4) Dormant: While the viral DNA remains incorporated in the host DNA in a lysogenic cycle. 5) Provirus: a virus genome that is integrated into the DNA of a host cell. 6) Reverse transcriptase: Enzyme that creates DNA from RNA 7) Retrovirus: Carries reverse transcriptase

Provide definition for: 1) P1 generation 2) F1 generation 3) F2 generation 4) Test Cross 5) True-Breeding 6) Self-Pollinate 7) Mendelian Ratio 8) Phenotype 9) Genotype 10) Heterozygous 11) Homozygous 12) Gene 13) Allele 14) Locus

1) P1 generation: Parental generation 2) F1 generation: 1st filial 3) F2 generation: 2nd filial (grand children) 4) Test Cross: Determine if an individual exhibiting a dominant trait is homozygous or heterozygous for that trait. Breeding the individual with another individual that expresses a recessive version of the same trait. If all offspring display the dominant phenotype, the individual is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous. 5) True-Breeding: A kind of breeding in which the parents with a particular phenotype produce offspring only with the same phenotype, the parents are homozygous for a trait. 6) Self-Pollinate: Transfer of pollen from an anther to a stigma of the same flower. 7) Mendelian Ratio: 3:1 ratio 8) Phenotype: Expression of a trait 9) Genotype: Genetic make up 10) Heterozygous: Genotype with one dominant and one recessive trait. 11) Homozygous: Genotype with two dominant or two recessive traits. 12) Gene: Series of nucleotides that codes for the production of a single polypeptide. 13) Allele: One of two or more forms of the DNA sequence of a particular gene. Aa 14) Locus: Position of a gene on a respective chromosome

What are the three ways bacteria increase their genetic variability?

1. Conjugation: Transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. The prototypical conjugative plasmid is the F-plasmid, or F-factor. The F-plasmid is an episome (a plasmid that can integrate itself into the bacterial chromosome by homologous recombination). There can only be one copy of the F-plasmid in a given bacterium, either free or integrated, and bacteria that possess a copy are called F-positive or F-plus (denoted F+). Cells that lack F plasmids are called F-negative or F-minus (F-) and as such can function as recipient cells. If the F-plasmid that is transferred has previously been integrated into the donor's genome some of the donor's chromosomal DNA may also be transferred with the plasmid DNA.[3] The amount of chromosomal DNA that is transferred depends on how long the two conjugating bacteria remain in contact. 2. Transformation: Pick up DNA from environment. The genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material (exogenous DNA) from its surroundings and taken up through the cell membrane(s). 3. Transduction: Viruses accidentally incorporate host genetic material into their nucleic acids. DNA is transferred from one bacterium to another by a virus (bacteriophage - viruses that infect bacteria). It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. Transduction does not require cell-to-cell contact (which occurs in conjugation).

What happens in transcription?

1. Initiation occurs when an initiation factor finds a promoter on the DNA strand, and assembles a transcription initiation complex including RNA polymerase. (Promoter is a sequence of DNA nucleotides that designates a beginning point for transcription.) 2. RNA polymerase unzips the DNA and creates a transcription bubble. 3. During elongation only one DNA strand, the template strand or (-) antisense strand, is transcribed. The coding strand or (+) sense strand protects its partner against degredation. 4. The RNA polymerase moves in the 3' to 5' direction building RNA in the 5' to 3' direction. 5. No proofreading and much slower than replication. 6. Termination occurs when a termination sequence is reached. 7. Primary transcript is given a 5' cap to serve as an attachment site in protein synthesis and protection against degradation by exonucleases. It also has a poly A tail to protect from exonucleases. 8. The primary transcript is then cleaved into introns and exons. The introns are cleaved and remain in the nucleus, and the exons are spliced and exit nucleus to be translated. The final RNA codes for the polypeptides.

Describe the process of DNA replication.

1. Two replisomes proceed in opposite direction. 2. DNA helicase unwinds the double helix 3. DNA polymerase cannot initiate a strand from two nucleotides, but can only add nucleotides to an existing strand. 4. Primase creates an RNA primer to initiate the strand. 5. DNA polymerase reads the parental strand in the 3' to 5' direction, and creates the new complementary strand in the 5' to 3' direction. (DNA is written in 5' to 3' and this is considered downstream) 6. The lagging strand is looped around so that it can be read in the 3' to 5' direction, and is continuously interupted and restarted with a primer. The disconnected strands are called Okazaki fragments. 7. DNA ligase moves along the lagging strand and ties the Okazaki fragments together. 8. DNA polymerase automatically proofreads each new strand, and makes repairs when necessary.

Which of the following explanation best accounts for the fact the viral infection are often more difficult to reliably treat than are bacterial infections? A. Bacteria evolve more quickly than viruses, so a broad-spectrum antibiotic will usually kill a large spectrum of bacterial strains. B. Viruses evolve more quickly than bacteria, so a broad-spectrum antibiotic will usually fail to kill all viral strains. C. Viruses evolve more quickly than bacteria, so a broad-spectrum approach such as is used with bacteria usually fails. D. Bacteria and viruses evolve at approximately the same rate, but a virus cannot be destroyed with an antibiotic.

11) C ; Answers A and D are false because viruses do indeed evolve more quickly than bacteria. This is due to the lack of a proof-reading mechanism in their DNA replication and transcription processes. Answer B is false because an antibiotic will kill bacteria but usually will not kill any viruses. Answer C is thus the best answer. A single vaccine is often ineffective after a period of time due to the rapid evolution of the target virus.

All of the following are true of bacteria, Except: I. Gram positive bacteria form endospores and have two cell membranes. II. Gram negative bacteria stain pink and have thinner cell walls than do gram positive. III. Gram positive bacteria stain purple and have petidoglycan cell walls. IV. All bacteria contain organelles. A. I only B. II only C. III and IV only D. I and IV only

12) A ; Answer choice A is correct. Statement I is false because gram positive bacteria only have one cell membrane (it is gram negative that have two cell membranes). All of the remaining statements are true. You may be tempted to say Statement IV is false because bacteria are prokaryotes; however, recall that prokaryotes do not have "complex, membrane-bound" organelles, but do have simple ones such as ribosomes.

All of the following would prevent an autotrophic bacteria from surviving, Except: A. Adding an enzyme that breaks down peptidoglycan B. Preventing all access to sunlight C. Preventing all access to CO2 D. Exposure to a chemical that disrupts the cell wall

13) B ; Answer choice A would kill any bacteria by destroying their cell walls. Choice D, like choice A, would destroy any bacteria. Recall that autotrophic means only that the organism can rely on CO2 as a carbon source, whereas heterotrophs must consume organic materials for their carbon. Choice C would indeed be detrimental to an autotroph because they would be losing their carbon source. Choice B is thus the best answer. This could be detrimental to a phototroph (gets its energy from the sun) but would not necessarily be detrimental to an autotroph—as long as it was also a chemotroph (gets its energy from consuming organic materials).

Two alleles, I^A and I^B, are co-dominant, while i, a third allele at the same locus, is recessive. H is a dominant allele from a different locus and is necessary for expression of any of the A, B, or AB blood types. The recessive allele at this locus is h and a homozygous recessive individual will have no antigens on their blood cells regardless of the genes present at the AB locus. Individuals with the ii genotype have type O blood and those with the hh genotype are said to have the "Bombay" blood type. Which of the following statements is most likely true of an individual with the Bombay blood type? I. They lack the ability to produce substance H II. The hh genotype codes for a protein product that inhibits the production of blood antigens III. They lack both the I^A and I^B alleles A. I only B. I and II C. II and III D. Ik

18) A ; Statement I is true; Bombay individuals are hh and cannot produce the gene product of the H gene. Statement II is false because recessive phenotypes rarely if ever result in a gain of function. Statement III is false because people with the Bombay blood type can have any of the various alleles at the A-B locus. They lack antigens because they lack the necessary protein product of gene H, not because they lack the alleles for the blood antigens themselves. This makes answer A correct.

Hemophilia, a disease that causes excessive bleeding due to reduced ability of the blood to clot, is X-linked recessive. If a carrier female marries an affected male, and they have two children, what is the probability that they will both be affected males? A. 1/16 B. 1/32 C. 1/8 D. 1/4

19) A ; The easiest way to approach this problem is to calculate the probability of any boy child being affected and then square that fraction due to the fact that they both must happen. There is a ½ chance that a child will get the Y allele from his or her father. If the child gets this Y, it will be a boy, and there will be a ½ chance that the X he gets from his mother will be the abnormal allele instead of the normal allele (recall that she is a carrier only, not affected). That gives us ½ times ½ = ¼. This must happen twice, so we multipley ¼ by ¼ and get 1/16, answer A. Most students are surprised to realize that the chance is exactly the same for having two affected female children. Take a moment to figure out why this is true.

Each human chromosome is composed of a very long, single strand of double-helix DNA. Following replication which of the following statements is true? A. The newly formed strand is an exact copy of the DNA template strand. B. The newly formed strand is shorter than the template strand due to removal of introns. C. The newly formed strand is modified with a poly-A tail and a 5' cap. D. The newly formed strand contains fewer DNA nucleotides than the template strand.

2) D ; A is false because the new strand is NOT an exact copy of the template strand, it is the complement of the template strand and is an exact copy of the coding strand. B is false because introns are removed from pre-mRNA, not replicated DNA. C is false because, again, these modifications are made to pre-mRNA not DNA. D is the best answer. The newly formed strand does contain fewer DNA nucleotides because of the RNA primer used to start replication.

A man and a woman with longer-than-normal toes get married and have a child. The child also has long toes, but his toes are significantly longer than either of his parents. Assuming that toe-length is determined by alleles at a single locus, which of the following provides the best genetic explanation? A. The child is a heterozygote and toe-length is co-dominant B. The child is a homozygote and toe-length is co-dominant C. The child inherited the gene for long toes, which was amplified by environmental influences during development D. The child is homozygous recessive and his parents are heterozygotes.

20) B ; The trick here is to recognize that three phenotypes of increasing or decreasing degree is almost always an example of incomplete or "co-dominant" inheritance. The parents have longer-than-normal toes, so we know there must be another phenotype in the population of "normal" toes. The child has a third, phenotype, "really long toes." This is best explained if the parents are heterozygotes, the child is homozygous dominant, and the alleles for toe length are co-dominant. Genotypically, tt would be normal toe length, Tt would be longer-than-normal toes, and TT would be really long toes. You should always remember that in real life most phenotypes are better represented by incomplete or co-dominance than by strict dominant-recessive Mendelian inheritance.

While most vertebrates have a four-chambered heart, some classes, such as the chondrichthyes, or cartilaginous fishes, have a two-chambered heart. If embryologists were to discover that at a certain point of development Chondricthyes embryos do have a four-chambered heart, which of the following statements would be MOST supported? A. modern sharks represent an ancestral state B. modern sharks represent a degenerative state C. ancestral sharks were probably vertebrates D. sharks are a likely example of convergent evolution

23) B ; Things that show up during embryological development, but not in the adult are usually thought to be remnants of ancestral states. A good example in humans is the gill slits and tail that appear in embryos but not in adult human beings. Modern sharks, according to the stem, have two-chambered hearts, so they do NOT represent the ancestral state of 4-chambered hearts, making A false. Answer B is correct because modern sharks have a different heart than that of their ancestors. "Degenerative" is used because a two-chambered heart is perhaps thought to be less advantageous than a four-chambered heart. Answer C is true, but unrelated to the question. Answer D is false because convergent evolution is when two species evolve similar structures that lack a common evolutionary source (bat wings and bird wings); something that is entirely unrelated to this question.

Suppose a group of scientists is making an argument for the re-classification of a certain unique class of vertebrates into a new, previously non-existent sub-phylum. Which of the following findings would best support the need to make this re-classification. A. The class in question shares several characteristics with other vertebrates. B. The class in question shares several characteristics with members of another non-vertebrate subphylum. C. Unique characteristics found in the class in question appear to represent an ancestral state. D. Unique characteristics found in the class in question appear to represent a degenerative state.

25) D ; This question, and one previous one, were included simply because the MCAT had one fairly odd passage a few years ago regarding ancestral and degenerative states. It is my opinion that these types of questions would be quite hard unless you've had experience with that exact type of question before, so we've included a couple for you to practice. Shared ancestral characteristics are indicative of evolutionary similarity because it shows a likelihood that the two species share a common evolutionary background. Degenerative states are, by definition, different from ancestral states, indicating less similarity and less likelihood of common evolutionary origin. In fact, degenerative states could even be thought of as the process of speciation—the way in which new species diverge from their common ancestral heritage to form new species. Answers A and B both describe similarity with existing taxonomical categories, in which case there may not be any need for creation of a new sub-phylum. Answer D is true over answer C due to the explanation given above.

In a certain marine mammal, dorsal fin length is incompletely dominant, resulting in long, short and mid-length fins. In the same mammal, gray eye color is dominant to blue eye color. If these two genes are NOT linked and a mid-length fin, blue-eyed individual is crossed with a short fin, homozygous gray-eyed individual, which of the following combinations of phenotypes is possible? A. Mid-length fin/blue-eyed, short fin/gray-eyed, mid-length fin/gray-eyed and short fin/blue-eyed B. Short fin/gray-eyed, mid-length fin/gray-eyed, and long fin/gray-eyed C. Short fin/gray-eyed, short fin/blue-eyed, mid-length fin/gray-eyed, mid-length fin/blue-eyed, long fin/gray-eyed and long fin/blue-eyed D. Mid-length fin/gray-eyed and short fin/gray-eyed

26) D ; To answer these types of questions accurately, PLEASE draw Punnet Squares. The genotypes for the two individuals described would be Ff and ff for fin-length and gg and GG for eye color. Cross the two genes individually. The possibilities for fin-length are: Ff and ff. Thus, all offspring must be either mid-length finned or short-finned. This makes answers B and C impossible because they include long-finned individuals. The possibilities for eye color are: Gg only, meaning ALL individuals must have gray eyes. This is because we are told the gray-eyed individual is homozygous and gray eyes are dominant. Answer A is thus false because it includes blue-eyed individuals. Thus, D is the only correct answer.

A man and a women get married and have tow children. A karyotype for each child is compared with each parent and it is found that each child has a unique, random combination of parental and maternal chromosomes. This provides direct support for which of Mendel's original observation regarding pea plants? A. The Law of Independent Assortment B. Test Cross for heterozygosity C. The Law of Segregation D. The Law of Karyotype Variability

30) A ; Mendel's Law of Independent Assortment states that maternal and paternal homologues assort randomly at the metaphase plate during meiosis, resulting in each child getting a random combination of chromosomes from each parent. Answer D is nonsense and doesn't exist. Answer C is a true principle outlined by Mendel, but it explains how alleles assort independent of one another (i.e., are not linked; which we have actually proven does occur, but linkage is usually counteracted by frequent crossing over). Answer B is what Mendel did to determine if something was a heterozygote or a homozygote, but is unrelated to this question.

A small religious group migrated from upstate New York in the early 1800s and formed a colony in southern Pennsylvania. This colony has persisted thru multiple generations and a very high percentage of its members alive today suffer from rare inheritable diseases. This case study is an excellent example of the deleterious effects of: A. genetic drift B. genetic bottleneck C. natural selection D. K-selection

33) A ; Genetic drift is a change in the allele frequencies within a population due to something other than natural selection. Genetic bottlenecks and the founder effect are both examples of events that result in rapid genetic drift. The term bottleneck, however, usually refers to entire populations being reduced to a few individuals, say due to hunting, or natural disasters. In this case, the entire population did not decrease. Rather, a small set of individuals of random genetic make up separated themselves from the rest of the population. This is most precisely called the founder effect, but that is not offered as an answer choice. Because this is not a genetic bottleneck, but is an example of genetic drift, A is the best answer.

Translation occurs in all of the following cellular locations, EXCEPT: A. cytosol B. mitochondria C. endoplasmic reticulum D. lysosome

4) D ; It should be obvious that translation occurs at A and C at free-floating ribosomes and the rough ER, respectively. This leaves B and D. Remember that we're looking for the one that DOES NOT occur. Use the system I taught you for EXCEPT questions and read each choice individually with the stem and you won't mess up. Recall that mitochondria are somewhat self-sufficient and replicate their own DNA and you may guess that they do some translation as well. In reality, it depends on the organism. Translation inside the mitochondria is common in insects and other organisms, but less so in animals. This would normally be knowledge well beyond the MCAT, but you're not being tested on that knowledge, only on your ability to reason between two answers and eliminate the LEAST likely. Lysosomes budd off the golgi, have no independent abilities to replicate or reproduce and they are thus the LEAST likely place for translation to occur.

Small DNA strands are sometimes used as a delivery method for drugs or enxymes targeted fro the nucleus. For such a DNA probe to be effective, it should be: A. identical to its target sequence B. complementary to its target sequence C. identical to its target sequence, but with Uracil substituted for Thymine D. complementary to the pre-mRNA strand that binds its target sequence.

44) B; This is a frequent MCAT question. For two DNA strands to anneal to one another they must be complementary, not identical. This makes B the only possible answer. Answer C confuses this with transcription of mRNA. Answer D is logical, but would actually be the exact same sequence as answer A.

The DNA strand shown below is from the coding strand of a section of human DNA. Which of the following gives the matching pre-mRNA sequence? 5'ATTCG3' A. 5'UAAGC3' B. 3'UAAGC5' C. 3'GCUUA5' D. 3'AUUCG5'

5) C ; To answer this correctly, you must differentiate "coding strand" from "template strand." The template strand is the one copied and the coding strand is the other strand, the complement to the template strand, which is NOT copied. As a result (visualize this in your mind) the new DNA strand OR the new pre-mRNA strand will be an exact copy of the coding strand (EXCEPT, in the case of pre-mRNA, T will be replaced with U). The next skill you need is to keep careful track of the 3' and 5' ends. Normally, we need the strands to run in opposite directions, so if the template strand was listed 5' to 3' the new strand would consist of the matching base pairs running 3' to 5'. However, note that here you are NOT given the template strand, but the coding strand. Thus, the coding strand and the new pre-mRNA strand will both run the same direction and will be identical except for replacing T with U. C is the only answer satisfying all of these requirements.

Hormone Q binds to receptor P on the cell membrane of liver cells. Receptor P causes the release of a small protein which activates adenylate cyclase, which in turn converts ATP to cAMP. Cyclic AMP initiates two additional steps, eventually leading to the phosphorylation of a trans-membrane protein, Protein Z, which actively transports glucose into the cell. Which of the following would most quickly decrease the flow of glucose into the cell? A. Immediate removal of all Hormone Q from the bloodstream B. Inhibiting the formation of cAMP C. Dephosphorylating Protein Z D. Increasing the glucose concentration inside the cell

7) C ; You should recognize this as a cascade or "second messenger system." Immediate removal of Hormone Q would stop the flow at the top of this cascade, however, all of the subsequent molecules would still continue their effect until used up. Inhibiting cAMP would stop the process further down the line, and thus more quickly. Increasing glucose concentration inside the cell would increase the gradient against which active transport must work, slowing the process slightly, but much less so than the other choices. The fastest way would be to dephosphorylate Protein Z, stopping the final step in the chain; in this case, it will not matter what previous molecules are still present.

A student is studying a newly discovered virus. She performs an assay on a sample of several thousand virions and finds three major biological components, two of which were phospholipids and ribonucleic acids. Which of the following is likely the identity of the third component? A. enzymes B. carbohydrates C. hormones D. ribosomes

9) A ; Recall that viruses can contain either RNA or DNA as their genetic material, however, they will not contain both. If they contain RNA, they will ALWAYS contain a specialized enzyme, reverse transcriptase. As is often the case with the MCAT, you are expected to pick up on this clue. Hormones would not be produced by a virus, nor found in a virion. A ribosome, would not be found in a virus either, they do not contain organelles. Even if you thought they might, ribosomes are made of RNA, one of the components already mentioned. Carbohydrates may be found on the cell surface of a virus, but would be a minor contributor compared to an enzyme.

The Lac Operon is an often studied bacterial gene that regulates the production of Lactase, an enzyme that degrades Lactose. Which of the following is true regarding the Lac Operon? A. If lactose is present, the operon will be transcribed and translated. B. If glucose is present, the operon will be transcribed and translated. C. If lactose is present but glucose is not, the operon will be transcribed, but not translated. D. If lactose is present but glucose is not, the operon will be transcribed and translated.

??? 15) D ; This should be a gimme point. The only real thing you need to remember about the Lac Operon is that lactose is a promoter, but glucose is an inhibitor. If lactose is present, its derivative, Allolactose binds to an inhibitor near the beginning of the operon, causing the inhibitor to release. This would allow the transcription of the gene EXCEPT for the fact that glucose acts as an inhibitor. If glucose is present at all, a second inhibitor will prevent transcription. If glucose is absent, then this inhibitor is not in place and all that is needed for transcription is the presence of lactose. C may be tempting, but there is no reason to believe that if the gene is transcribed, it won't be also translated.

What is a sex-linked trait? Why is it different for males.

A gene that is found on the sex chromosome (23rd). The Y chromosome for males does not generally carry the allele for the sex linked trait. So the allele that is carried on the X chromosome for the male is expressed whether it is dominant or recessive. (Hemophilia is more common in males because as a recessive trait if a man is a carrier he will have it, whereas a woman having both recessive alleles for hemophilia is more uncommon.)

When determining probabilities for genes, If either events occurring fulfills the requirement, what do you do to the probabilities of each event occurring individually?

Add them

What does the Law of Segregation mean?

Alleles segregate independently of one another when forming gametes. When any individual produces gametes, the copies of a gene separate so that each gamete receives only one copy. A gamete will receive one allele or the other. The Law of Segregation states that every individual possesses a pair of alleles for any particular trait and that each parent passes a randomly selected copy (allele) of only one of these to its offspring.

What is telomerase? Why would a cancer with upregulated telomerase activity be especially harmful?

An enzyme that continually adds additional telomere with each cell division. Cancer cells divide continuously and eventually lose their telomeres which causes the cell to stop replicating the DNA. With upregulated telomerase activity, the DNA would be able to be replicate indefinitely.

What shows evidence of linkage?

Any variance from expected ratios or random assortment. In a dihybrid cross you should get a 9:3:3:1 ratio and in a mono-hybrid cross you should get 3:1. Anything other than these ratios is evidence of some type of linkage.

Why are nitrogen-fixing bacteria found in the soil so important? What type of "troph" are they?

Chemoautotroph Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen (N2) in the atmosphere is converted into ammonia (NH3). This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and amino acids for proteins. Nitrogen fixation also refers to other biological conversions of nitrogen, such as its conversion to nitrogen dioxide. Most plants are unabel to use ammonia, and must wait for other bacteria to further process the nitrogen in a process called nitrification. This is a two step process that creates nitrates from ammonia. Chemoautotrophy is an inefficient mechanism for acquiring energy, so chemoautrotrophs require large amounts of substrate. This means that chemoautotrophs have a large environmental impact.

Distinguish Chemotroph vs. Phototroph and Autotroph vs. Heterotroph

Chemotroph: organisms that obtain energy by the oxidation of electron donors in their environments. Phototroph: organisms that carry out photosynthesis to acquire energy. They use the energy from sunlight to convert carbon dioxide and water into organic material to be utilized in cellular functions such as biosynthesis and respiration. Autotroph: organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) from simple inorganic molecules using energy from light (by photosynthesis) or inorganic chemical reactions (chemosynthesis). They are the producers in a food chain, such as plants on land or algae in water. They are able to make their own food and can fix carbon. Therefore, they do not use organic compounds as an energy source or a carbon source. Heterotroph: organism that cannot fix carbon and uses organic carbon for growth. Heterotrophs, by consuming reduced carbon compounds, are able to use all the energy that they obtain from food for growth and reproduction, unlike autotrophs, which must use some of their energy for carbon fixation. Heterotrophs are unable to make their own food, however, and whether using organic or inorganic energy sources, they can die from a lack of food. This applies not only to animals and fungi but also to bacteria. All animals and fungi are heterotrophic, as well as most protists and prokaryotes. All organisms can be classified as one of each of these types.

What does degenerative and unambiguous mean when talking about the genetic code?

Degenerative: more than one series of three nucleotides may code for any amino acid. Unambiguous: Any single series of three nucleotides will code for one and only one amino acid.

Differentiate between divergent and convergent evolution.

Divergent: 2 or more species evolving from the same group maintain a similar structure from the common ancestor Convergent: 2 species independently evolve similar structures. (wings of bats and birds)

Fungi have _____ made of ______.

Fungi have septa made of chitin. Septa: Cell wall Chitin: polysaccharide

What are hyphae? What stage are hyphae in?

Fungi in their growth state (haploid) consist of a tangled mass (mycelium) of multiple branched thread-like structures called hyphae. Haploid.

What does the Law of Independent Assortment mean?

Genes located on different chromosome assort independently. Alleles of different genes assort independently of one another during gamete formation. Separate genes for separate traits are passed independently of one another from parents to offspring. That is, the biological selection of a particular gene in the gene pair for one trait to be passed to the offspring has nothing to do with the selection of the gene for any other trait. More precisely the law states that alleles of different genes assort independently of one another during gamete formation. The gamete can end up with any combination of paternal or maternal chromosomes.

Why is it important to distinguish between somatic and germ cells with mutations?

Germ cell mutations will be carried on to every cell in the offspring. Somatic cell mutations will only be carried to few cells.

What stage do fungi stay in most of their life? (in reference to chromosomes)

Haploid. Their growth state is haploid. Hyphae=haploid

Why do fungi alternate between sexual and asexual reproduction?

If conditions are good for the parent, they will be good for asexually reproduced offspring that are exactly like the parent, but if conditions are bad for the parent, they many not be bad for sexually reproduced offspring that are different from the parent.

Where does translation occur?

In the cytosol at either free ribosomes (proteins for cytosol) or rough ER (proteins for plasma membrane or extracellular proteins).

What is an endospore? What type of bacteria are they found in?

It is a stripped-down, dormant form to which the bacterium can reduce itself. Endospore formation is usually triggered by a lack of nutrients, and usually occurs in gram-positive bacteria. In endospore formation, the bacterium divides within its cell wall. One side then engulfs the other. Endospores enable bacteria to lie dormant for extended periods, even centuries. The endospore consists of the bacterium's DNA and part of its cytoplasm, surrounded by a very tough outer coating.

What are the different classification levels for Humans?

Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Primates Family: Hominidae Genus: Homo Species: sapiens

A man is found to be HIV positive. Is this an example of a virus in a lytic or lysogenic cycle?

Lysogenic

Explain the difference between the Lytic and the Lysogenic cycle.

Lytic: The virus commandeers the cell's reproductive machinery and begins reproducing new viruses. The cell may fill with new viruses until it lyses or bursts, or it may release the new viruses one at a time in a reverse endocytotic process. The period from infection to lysis is call the latent period. A virus following a lytic cycle is called a virulent virus. Lysogenic: The viral DNA is incorporated into the host genome, or, if the virus is an RNA virus and it possesses reverse transcriptase, DNA is reverse-transcribed from RNA and then incorporated into the host cell genome. When the host cell replicates its DNA, the viral DNA is replicated as well. A virus in a lysogenic cycle is called a temperate virus. A host cell infected with a temperate virus may show no symptoms of infection. While the viral DNA remains incorporated in the host DNA, the virus is said to be dormant or latent, and is call a provirus. The dormant virus may become active when the host cell is under some type of stress.

What is R-Selection and K-selection

R-Selection: producing large numbers of offspring that mature rapidly with little or no parental care (Rabbits) K-Selection: small brood size with slow maturing offspring and strong parental care (Kids)

What are telomeres?

Repeated six nucleotide units from 100 to 1000 units long that protect the chromosomes from being eroded through repeated rounds of replication. (Each time DNA is replicated the chromosome is shortened and this prevents vital info from being eroded.)

All fungi are _______? (Mechanism for ingestion)

Saprophytic: a process of chemoheterotrophic extra-cellular digestion involved in the processing of dead or decayed organic matter that occurs in saprotrophs or heterotrophs, and is most often associated with fungi. Feeding, absorbing or growing upon decaying organic matter.

How do fungi reproduce?

Sexually and asexually. Sexually: when life is hard (stress, little food, bad environment, etc) Asexually: when life is good

Why do vaccines lose their effectiveness?

Spike proteins encoded from the viral nucleic acids protrude from the envelope. These proteins bind to receptors on a new host cell causing the virus to be infectious. However, it is also the spike proteins that human antibodies recognize when fighting the infection. Since RNA polymerase does not contain a proofreading mechanism, changes in the spike proteins are common in RNA viruses. When the spike proteins change, the antibodies fail to recognize them, and the virus may avoid detection until new antibodies are formed.

What is the start codon and the three stop codons? Do you include the start and stop codons when counting number of amino acids that will be transcribed.

Start Codon: AUG Stop Codon: UAA, UAG, UGA Count the start codon (AUG), but do not count the stop codons.

What happens during post-translational modification and where does it occur?

Sugars, lipids, or phosphate may be added to amino acids. Occurs in golgi and ER.

What is the difference between the DNA strand that is NOT transcribed and the pre-RNA formed.

The DNA has Thymine residues and the RNA has Uracil residues.

Briefly how does lac operon work?

The lac operon codes for enzymes that allow E. coli to import and metabolize lactose when glucose is not present in sufficient quantities. 1. In the absence of lactose, the lac repressor protein binds to the operator and keeps RNA polymerase from transcribing the lac genes. (Operon is turned off by the regulatory protein) 2. When lactose is present, the lac genes are expressed because lactose binds to the lac repressor and keeps it from binding to the lac operator. RNA polymerase can then bind to the lac promoter and transcribe the lac genes. Lac genes will continue to be expressed as long as lactose is present in the cell. 3. In the absence of glucose, high levels of cAMP are present. The cAMP binds and activates CAP which binds to the CAP site and greatly enhances binding of RNA polymerase to the lac promoter. Thus increasing transcription. 4. In the presence of glucose, the cAMP-CAP binding to the CAP site does not occur thus resulting in non-efficient binding of RNA polymerase to the lac promoter and low levels of lac mRNA transcribed.

How do fungi obtain their food?

They obtain it through absorption rather than by ingestion: they secrete their digestive enzymes outside their bodies and then absorb the product of digestion.

What is the difference between gram positive and gram negative bacteria?

This type of classification reveals the type of cell wall that the bacteria possess. Gram Positive: Stain purple due to the very thick petidoglycan cell wall which prevents the gram stain from leaking out. Form endospores Single cell membrane Gram Negative: Stain pink due to their thin peptidoglycan cell wall which allows most of the gram stain to be washed off. Do not form endospores Contain two cell membranes: one inside the cell wall and one outside the cell wall

How do yeasts reproduce almost exclusively?

Through budding: a smaller cell pinches off from the single parent cell.

What is a virus? Is it alive? What are the major components of all viruses?

Virus: A virus is a biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus, at an extraordinary rate. Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell. Is it alive: Some debate, however viruses are not currently classified as living organisms. They do not belong to any of the taxonomical kingdoms of organisms. Major components: Capsid: a protein coat Nucleocapsid: inner shell around the DNA or RNA DNA of RNA (Never contain both) Envelope: lipid-rich bubble found in some/most virus