practice MCAT questions and answers

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1/16/20 

THE CORRECT ANSWER IS B.

Genetic drift is the idea that over time, some alleles become more frequent than others due to random events (choice B is correct). Natural selection is a similar idea, but the alleles do not randomly become more or less frequent. Their frequency is determined by the phenotypic advantage or disadvantage displayed in the individual organism (choice C is wrong). The founder effect describes the idea that if a small population of a species becomes isolated from the rest of the species, their allelic frequency will predominate in future generations (choice A is wrong). Adaptations are structures or behaviors that enhance a specific function, causing organisms to become better at surviving and reproducing. They are produced by a combination of the continuous production of small, random changes in traits, followed by natural selection of the variants best-suited for their environment (choice D is wrong).

1/12/20 

THE CORRECT ANSWER IS C.

In the lysogenic cycle of bacteriophages, the genomic DNA of one bacterium can be transferred to another bacterium through a viral vector in a process called transduction (choice C is correct). Conjugation is the transfer of genomic material between bacteria via a physical bridge (choice A is wrong). Transfection and transformation are the process of incorporating DNA from the external environment in eukaryotes and prokaryotes, respectively (choices B and D are wrong).

1/9/20 

THE CORRECT ANSWER IS A.

The endosymbiotic theory describes a potential unicellular origin for both mitochondria and nuclei. If a researcher found that the inner lipid bilayer present in the nucleus possessed a fatty acid profile more similar to bacteria, it would lend support to the theory, as the unicellular organism (which likely possesses similarities to modern prokaryotes) might have been phagocytosed and incorporated into the cell in an earlier organism (choice B is wrong, and choice A is correct). The nuclear envelope contains two lipid bilayers, and given cholesterol is found in nearly all eukaryotic membranes, it would not support the theory proposed here (choices C and D are wrong).

1/5/20 

THE CORRECT ANSWER IS D.

The expression of genes is influenced by how the DNA is packaged in chromosomes and base modifications, such as methylation, can be involved in packaging. Regions that have low or no gene expression usually contain high levels of methylated cytosine bases; for example, cytosine methylation is important in X chromosome inactivation (choice A is true and can be eliminated). Bacteria methylate their genomes at the sequences recognized by restriction enzymes. This prevents binding of the bacterial genome to the active site of the enzyme, preventing digestion (choice B is also true). Methylation can lead to domination, leaving behind a thymine base where there used to be a cytosine; methylated cytosines are therefore particularly prone to mutations (choice C is true, as well). However, methylation of bases interferes with the ability of the DNA to interact with transcription factors; this decreases the rate of transcription in methylated regions (choice D is false and is the correct answer).

1/2/20 

THE CORRECT ANSWER IS D.

Euchromatin represents the least tightly packed sequence of DNA and thus the sequence that is most likely to be transcribed to produce needed cellular products. Given their state of "information readiness," they are least likely to contain confounding or unstable repetitious elements (choice D is correct).

12/22/19 

THE CORRECT ANSWER IS B.

In order to compute the voltage for the overall reaction, the two constitutive half reactions must be summed. The reactions in choices C and D result in negative cell potentials, and can therefore be eliminated. The overall size of the potential for the reaction Pb4+ → Pb2+ is much larger than any of the other reduction potentials. This points to the fact that the best overall equation is going to include this half-reaction in the forward (reducing) direction. Choice B includes the reduction of Pb4+, and in conjunction with the oxidation of Pb(s), has an overall cell potential of +1.82 V. This is far larger than possible for the Sn reaction in choice A.

12/19/19 

THE CORRECT ANSWER IS A.

The inability to encode new memories is called anterograde amnesia (choice A is correct). The inability to recall memories from the past is called retrograde amnesia (choice B is wrong). Proactive interference occurs when previously learned information interferes with the ability to recall information learned later; it does not prevent the encoding of memories but rather refers to the inability to recall them (choice C is wrong). Retroactive interference occurs when newly learned information interferes with the ability to recall previously learned information; it does not prevent the encoding of memories but rather prevents the ability to recall them (choice D is wrong).

12/14/19 

THE CORRECT ANSWER IS C.

A conditioned stimulus is a stimulus that produces a response following learning (choice C is correct). Continuous reinforcement is when a certain behavior is reinforced every time it occurs. Partial reinforcement is when a certain behavior is reinforced at certain times. For instance, there may be a reward attached to the behavior. Continuous and partial reinforcement are based on schedules in operant conditioning, not the stimuli themselves (choices B and D are wrong). A conditioned response is a response that has been learned: in this case, the greeting (choice A is wrong).

12/9/19 

THE CORRECT ANSWER IS B.

Longitudinal studies are best for exploring how variables develop over time. They involve conducting periodic measurements of the same individuals over many years to see how certain variables change. This is the ideal methodology for exploring the question presented in the stem.

12/4/19 

THE CORRECT ANSWER IS D.

Neutralization of a weak acid with a strong base will result in a basic solution, and our indicator should therefore change color at a pH greater than 7. Note that without the specific concentration of acetic acid and the base we begin with, it is not possible to determine the precise final pH upon complete neutralization (adding an equal molar quantity of strong base to the acetic acid).

11/30/19 

THE CORRECT ANSWER IS B.

While calcium does enter the cytoplasm of cardiac muscle cells from both the extracellular space and sarcoplasmic reticulum (SR), the predominate source is the SR (choices A and C can be eliminated). Note that all cardiac muscle cells must function the same way (choice D is wrong).

11/20/19

The correct answer is B.


Item I is true: the lymphatic system is responsible for returning water, dissolved solutes, and protein from the periphery to the blood (choice C can be eliminated). Item II is true: the lymphatic system also carries dietary fat, in the form of chylomicrons, including fat-soluble vitamins (choice A can be eliminated). Item III is false: under normal circumstances, red blood cells cannot exit the circulatory system and would therefore not be transported back to the blood via the lymphatic system (choice D can be eliminated, and choice B is correct).

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