Kaplan Test Prep MCAT

Try out the following MCAT sample questions to test your MCAT biology, physics and organic chemistry skills. If you've already answered the questions on a Pop Quiz flier, you can find the correct answers with full explanations below.

Biology Sample Question

Passage (Questions 1–2)
In an attempt to develop a vaccine for pneumonia, Fred Griffith performed a series of experiments in 1928 using mice and two strains of the pneumococcus bacteria: a virulent encapsulated strain and a nonvirulent unencapsulated strain. The encapsulated strain was called the "smooth strain" because the colonies looked smooth on a culture plate due to their polysaccharide capsules, whereas the unencapsulated strain was denoted as the "rough strain" due to the irregularity of its surface.

Four different groups of mice were injected with different combinations of the bacterial strains. The experimental results are shown in Table 1.

Table 1: Results of Griffith's experiment

Mice Injected Bacteria Result
Live smooth Died
Live rough Survived
Heat-killed smooth
Heat-killed smooth + live rough

Autopsies performed on Group 4 mice revealed blood samples filled with live smooth-strain bacteria. Some component had been transferred from the heat-killed smooth-strain bacteria to the live rough-strain bacteria, transforming them into the virulent smooth-strain bacteria. This process later became known as transformation. Griffith believed the protein from the dead smooth-strain bacteria was the active transforming agent.

  1. A colony of smooth strain bacteria is grown on a culture containing an experimental drug that cleaves nucleic acid base sequences wherever adenine is paired with uracil. Which of the following processes will be directly affected?
    1. Transcription
    2. Translation
    3. Transformation

    1. I only
    2. I and II only
    3. I and III only
    4. I, II and III

Answer: (B)

One of the ways that RNA differs from DNA is in the use of the nucleotide uracil. In RNA, uracil is substituted for thymine. In other words, RNA has uracil and DNA doesn't. During DNA replication, adenine pairs with thymine. During transcription (mRNA synthesis from a DNA template) and translation (translation of mRNA into a peptide chain), adenine pairs with uracil. Thus, if bacteria were exposed to a drug that cleaves adenine from uracil, transcription and translation would be disrupted, whereas transformation, which is the transfer of DNA from one bacteria to another, would not be directly affected by this drug. Therefore, statements I and II would be directly affected and statement III would not. Choice (B) is correct. Kaplan MCAT training will provide you with the content knowledge you need to quickly and confidently answer questions about molecular biology/DNA on test day.

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