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MCAT Sample Questions : Biological Sciences

Passage VII

In humans, Hormone X is a protein that induces normal cells to admit more Ca2+.To examine the mode of action of Hormone X, a biologist screened thousands of families for inherited Hormone X deficiencies.The screen turned up two (unrelated) families in which many males but no females had a defect in their ability to respond to Hormone X.

The biologist performed a series of experiments on cells isolated from females and the affected males in the two families to determine:

  1. the pathway between hormone binding and Ca2+ entry in a normal cell, and
  2. 2) the point in the pathway where the defect occurs in the males.

The following results were obtained:

  1. In both families, none of the isolated cells from affected males had an elevated influx of Ca2+ after treatment with Hormone X.
  2. Hormone X bound to Protein R of all cells examined.
  3. Protein R was smaller than normal in cells from affected males of Family 1, but its size was absolutely normal in all other cells examined.
  4. In intact cells, Protein R phosphorylated Protein P in the cells from females only.
  5. When activated Protein R was isolated from females, it could phosphorylate Protein P isolated from affected males of Family 1, but not Family 2.
  6. Phosphorylated Protein P was necessary and sufficient to increase the rate of Ca2+ entry in cells from females

Following are some sample questions on this passage:

  1. Which of the following protein(s) is most likely to utilize ATP for its action?

    1. Hormone X only
    2. Protein R only
    3. Protein P only
    4. Both Hormone X and Protein P

    Answer: B

    Explanation: The passage tells us in statement 4 that protein R is capable of phosphorylating Protein P.Phosphorylation is the addition of high-energy phosphate groups by a kinase to another protein.A molecule such as ATP (adenosine triphosphate) donates the phosphate group.The only protein that utilizes ATP is protein R (answer B).Protein R must split ATP, releasing ADP (adenosine diphosphate) and attaching a phosphate molecule to Protein P.Thus, answer choice B is the best answer.

  2. Among the following choices, the most likely function of phosphorylated Protein P is as a:

    1. Na+/K+ pump.
    2. phosphatase.
    3. membrane channel.
    4. hormone receptor.

    Answer: C

    Explanation: Because phosphorylated Protein P was necessary and sufficient to increase the rate of Ca2+ into the cell (statement 6 in the passage), Protein P is probably a membrane channel, a pore in the membrane that allows Ca2+ to pass freely.Thus, answer choice C is the best answer.

  3. In the normal cellular response to Hormone X, the influx of Ca2+ should stop immediately when:

    1. Hormone X no longer binds to Protein R.
    2. the complex of Hormone X and Protein R undergoes endocytosis.
    3. synthesis of Protein P is terminated.
    4. Protein P is dephosphorylated.

    Answer: D

    Explanation: The phosphorylation of Protein P must alter it in some way that allows it to increase Ca+2 influx.The most immediate way to stop the influx of Ca2+ therefore should be to dephosphorylate Protein P (answer D).Ca2+ would continue for a time in all the other answer choices A-C.

  4. Based on the passage, the mutation in the DNA in affected males from Family 2 most likely results in:

    1. the replacement of a single amino acid in Protein R.
    2. the replacement of a single amino acid in Protein P.
    3. the introduction of a premature stop codon in Protein R.
    4. the introduction of a premature stop codon in Hormone X.

    Answer: B

    Explanation: According to statement 5, Protein P from affected males from Family 2 could not be phosphorylated.This means that the genetic defect in Family 2 must be in the gene that codes for Protein P.Therefore, the correct answer choice is B, that the alteration of one amino acid in Protein P is the likely result of the mutation in the DNA of affected males.

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