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MCAT Epigenetics, Chromatin and DNA Methylation Practice Questions

By StudyStore · Published 2026-11-04 · 8 original questions · about 12 min

Eight free MCAT-style practice questions on euchromatin and heterochromatin, histone acetylation, DNA methylation, imprinting and X inactivation, with explanations.

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Quick concept primer

Epigenetic regulation changes gene expression without changing the DNA sequence. Eukaryotic DNA wraps around histone octamers to form nucleosomes. Tightly packed heterochromatin is transcriptionally silent, while loosely packed euchromatin is accessible and active. Histones carry positive charge from lysine and arginine, so they bind the negatively charged DNA phosphate backbone.

Histone acetylases (HATs) add acetyl groups to lysine, removing its positive charge, loosening the histone-DNA contact and promoting transcription. Histone deacetylases (HDACs) reverse this and silence genes. DNA methylation occurs mainly at cytosines in CpG islands and generally represses transcription. Imprinting silences one parental allele, and X inactivation condenses one X chromosome in female mammals into a Barr body. These marks can be inherited through cell division.

8 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

Which statement best describes heterochromatin?

  1. Loosely packed and transcriptionally active
  2. Tightly packed and generally transcriptionally silent
  3. Found only in prokaryotes
  4. Packed without histones
Show answer and explanation

Answer: B. Tightly packed and generally transcriptionally silent

Heterochromatin is condensed, so transcription factors and RNA polymerase have limited access to the DNA and gene expression is low. Euchromatin is the loosely packed, active form. Both forms contain histones, and prokaryotes do not have nucleosomes in the same way.

Question 2

Acetylation of lysine residues on histone tails is expected to

  1. strengthen histone-DNA binding and repress transcription
  2. have no effect on charge
  3. weaken histone-DNA binding and promote transcription
  4. cause DNA to be methylated at adenine
Show answer and explanation

Answer: C. weaken histone-DNA binding and promote transcription

Acetylation neutralizes the positive charge on lysine, so the histone tail binds the negatively charged DNA less tightly. The chromatin relaxes and transcription machinery gains access. HDACs remove the acetyl groups and favor repression.

Question 3

A drug inhibits histone deacetylases (HDACs). What is the most likely effect on gene expression?

  1. Increased expression of affected genes
  2. Decreased expression of affected genes
  3. Increased DNA methylation at all genes
  4. Breakdown of nucleosomes into free DNA
Show answer and explanation

Answer: A. Increased expression of affected genes

With HDACs inhibited, acetyl groups stay on histones, chromatin stays relaxed and genes are more readily transcribed. Inhibitors of HDACs are used in some cancer treatments for this reason. Nucleosomes remain intact, and the drug does not directly cause methylation.

Question 4

DNA methylation in mammals most commonly occurs at which site, and what is its usual effect?

  1. Adenine in promoters, increasing transcription
  2. Cytosine in CpG islands, decreasing transcription
  3. Thymine at stop codons, ending translation
  4. Guanine in introns, increasing splicing
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Answer: B. Cytosine in CpG islands, decreasing transcription

DNA methyltransferases add a methyl group to cytosine, mostly where it is followed by guanine (CpG). Methylated promoters recruit proteins that condense chromatin and block transcription factors, so gene expression falls. Hypermethylation of tumor suppressor promoters can contribute to cancer.

Question 5

Which feature of epigenetic marks makes them different from mutations?

  1. They alter the nucleotide sequence of the gene
  2. They are never passed to daughter cells
  3. They can change gene expression without changing the DNA sequence
  4. They occur only in germ cells
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Answer: C. They can change gene expression without changing the DNA sequence

Epigenetic marks such as methylation and histone modification change how accessible a gene is while the base sequence stays the same. Many marks are copied during cell division, so daughter cells keep the pattern, and they can also change in response to environment.

Question 6

In female mammals, X inactivation results in which of the following?

  1. One X chromosome in each somatic cell condensed into a Barr body
  2. Both X chromosomes silenced in all cells
  3. Loss of one X chromosome from the genome
  4. Transcription of only the paternal X in all cells
Show answer and explanation

Answer: A. One X chromosome in each somatic cell condensed into a Barr body

One X is randomly inactivated early in development, forming a condensed, mostly silent Barr body. The choice is random in each cell, and all descendants keep it, so females are mosaics. The chromosome is not lost, and both X chromosomes are not silenced.

Question 7

Genomic imprinting differs from X inactivation in that imprinting

  1. silences an allele depending on which parent it came from
  2. silences an entire chromosome at random
  3. occurs only in prokaryotes
  4. is caused by a change in the DNA sequence
Show answer and explanation

Answer: A. silences an allele depending on which parent it came from

In imprinting, a gene is expressed or silenced according to parent of origin, often by DNA methylation set in the germline. This means that the two alleles are not functionally equal. X inactivation is random with respect to parent and affects a whole chromosome.

Question 8

A nucleosome core particle consists of DNA wrapped around

  1. a histone octamer containing two each of H2A, H2B, H3 and H4
  2. four copies of histone H1
  3. a single H3 molecule
  4. an RNA polymerase complex
Show answer and explanation

Answer: A. a histone octamer containing two each of H2A, H2B, H3 and H4

The core particle has about 147 base pairs of DNA wrapped around an octamer of two copies each of H2A, H2B, H3 and H4. Histone H1 is the linker histone that binds the DNA between nucleosomes and helps compact chromatin.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.

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