# DBT JRF 2022 — Question 43

Exam: DBT JRF 2022 · Section: B · Topic: Cell Biology › Histone-DNA Interactions · Difficulty: Medium

## Question

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R. Assertion A: Histones bind to DNA with high affinity. Reason R: Histones are small proteins rich in valine and isoleucine residues. In the light of the above statements, choose the correct answer from the options given below:

## Options

- **A.** Both A and R are true and R is the correct explanation of A.
- **B.** Both A and R are true but R is not the correct explanation of A.
- **C.** A is true but R is false.
- **D.** A is false but R is true.

## Answer

Option C

## Explanation

Amino acid composition of histones: Histones are the basic proteins that package DNA into chromatin. They must carry a net positive charge to interact electrostatically with the negatively charged phosphate backbone of DNA. Positively charged amino acids: • Lysine (K): especially rich in H2A, H2B, H4 (contributes +1 charge per residue) • Arginine (R): especially rich in H3, H4 (contributes +1 charge per residue, through guanidinium group) Histones are typically ~20–25% Lys+Arg by composition. Amino acids NOT enriched in histones: • Valine and isoleucine: hydrophobic branched-chain amino acids; not characteristic of histones. Note: Histone tails with many Lys residues are also sites of post-translational modifications (acetylation, methylation, ubiquitination) that regulate chromatin structure. Answer: Lysine and Arginine (the positively charged residues).

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