MCAT Question of the Day

Correct Answer: C. frequency of homozygote BB is q² .

This question requires just a simple understanding of the According to the Hardy-Weinberg equation, (p+q)²=1, , or rather: and p² + 2pq + q² = 1.
If we have a gene with two alleles, A and B, and its frequencies are p and q, then we can know that the frequencies of the genotypes are as follows, by plugging into our Hardy Weinberg equation:
predicted frequency of homozygote AA = p²
predicted frequency of homozygote BB = q²
predicted frequency of heterozygote AB = 2pq.
The only answer choice that fits this solution is answer choice C.
Looking at the other answers, A is incorrect because it is an inaccurate representation of the Hardy Weinberg equation. Answer B is also incorrect because if one assumes that a gene has only two alleles, then then the frequency of p+q combined has to be 100%, or rather p + q = 1.

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Correct Answer: C. genes responsible for the promotion of apoptosis

Cancerogenesis is an evolutionary process. Cancer cells are under heavy selective pressure and only those cells that possess appropriate genetic characteristics will survive. All changes that result in increased angiogenesis, cell growth and expression of oncogenes favor cancer development. On the other hand, apoptosis (programmed cell death) represents the way in which an organism eliminates potential cancer cells. Silencing the genes responsible for promotion of apoptosis is an essential step in malignant tumor development.

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Correct Answer: B. Succinyl coenzyme A to succinate

There is only one step in the citric acid cycle that causes substrate level phosphorylation: the conversion of succinyl coenzyme A to succinate. GDP and inorganic phosphate combine to form GTP, which is roughly equivalent to the formation of one molecule of ATP. All others do not produce ATP. Oxaloacetate may be formed from pyruvate though it requires the dephosphorylation of one molecule of ATP. Moreover, this reaction is not considered one of the classic reactions in the citric acid cycle. In the conversion of malate to oxaloacetate, NAD+ loses a proton to form NADH.

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Correct Answer: B. one DNA molecule

After mitosis, each daughter cell has the same number of chromosomes, like the mother cell. However, chromosomes of the daughter cell have only one chromatid, i.e. one DNA molecule.

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Correct Answer: B. Two cells will have 4 chromosomes, two cells will have 2 chromosomes.

For an organism with a diploid number of 6 (2N = 6), each of the four meiotic cells will have 3 chromosomes. However, if nondisjunction occurs in meiosis I, one daughter cell will have 4 doubled chromosomes and the other cell will have two doubled chromosomes. One pair of the homologous chromosomes did not separate. However, all chromatids will separate in the second division, resulting in four single chromosomes in two of the products and two single chromosomes in the other two products.

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Correct Answer: B. Autonomic motor neurons

The question directly states that arrector pili muscles are smooth muscles, and are therefore involuntary and under the control of autonomic motor neurons. Smooth muscles include muscles in your circulatory system and intestines that cause peristalsis.

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Correct Answer: B. sympathetic activation decreases the diameter of arterioles systematically.

Activation of the sympathetic division of the ANS triggers systemic vasoconstruction (decrease in diameter) in arterioles, leading to an increase in systemic blood pressure. Choices A and D have the functions of the hormones backwards (and thus are wrong): aldosterone increases sodium reabsorption in nephrons and antidiuretic hormone increases water permeability, to learn about other ways to boost your hormones, check out the Mens Journal website. (Note that the two hormones working together would lead to a increase in total body salt and consequently, total body water, followed by an increase in blood pressure.) While it is true that blood flow to digestive organs is increased when the body is under parasympathetic control, this is not due to the direct action of the parasympathetic division on the blood vessels to those organs. The parasympathetic division actually does not innervate the blood vessels at all; the increased blood flow under these conditions is due to the withdrawal of sympathetic stimulation. Even if the parasympathetic division did increase vessel diameter, this would not lead to an increase in blood pressure (choice C is wrong).

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Correct Answer:C. Both of her parents, at least one of her mother’s parents, and at least one of her father’s parents had the gene.

If a woman is born color blind (a sex-linked recessive trait: X^CX^C), she must have a color blind father (X^CY) who can donate one recessive X gene and a mother who was at least a carrier (X^CX^C, or X^CX^{). The father must have received the color blindness allele from his mother. The mother must have had at least one parent with the color blindness allele.}

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Correct Answer:D. Adenine and DNA, RNA, and ATP; uracil in RNA

This is an easy question testing your knowledge of nucleic acids and simple biological molecules. Adenine is one of the four nucleotide bases found in DNA and RNA, as well as in the adenosine triphosphate (ATP) molecules found in all cells. Uracil is one of the four nucleotide bases found mostly in RNA.

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Correct Answer: B. Pepsin remains in an inactive state until it is cleaved by HCl.

Pepsin along with many other hydrolytic enzymes are produced in a “zymogen” form which remain inactive during their production. Once they are in the correct conditions, they self cleave and become active. Pepsin is secreted in the inactive form known as pepsinogen, which is cleaved in the acidic conditions of the stomach to form the active protease pepsin.

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