Question

The eukaryotic histones which are nearly identical in amino acid sequence
(A). \(\text{H}_1\)
(B). \(\text{H}_2\text{A}\)
(C). \(\text{H}_3\)
(D). \(\text{H}_4\)
Choose the correct answer from the options given below:

• A. (A) and (B) only.
• B. (B) and (C) only.
• C. (C) and (D) only.
• D. (B), (C) and (D) only.

Hint:

Core histones \(\text{H}_3\) and \(\text{H}_4\) are among the most slowly evolving proteins known, because almost any mutation in their sequence disrupts chromatin structure and is lethal to the cell.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Histones are highly basic proteins around which DNA wraps to form nucleosomes in eukaryotic cells. The core histones show varying levels of evolutionary conservation across species.

Step 2: Detailed Explanation:
1. The nucleosome core consists of an octamer containing two copies each of histones \(\text{H}_2\text{A}\), \(\text{H}_2\text{B}\), \(\text{H}_3\), and \(\text{H}_4\).
2. Among these, \(\text{H}_3\) and \(\text{H}_4\) are extremely conserved throughout eukaryotic evolution, with nearly identical amino acid sequences from plants and yeast to mammals.
3. For example, the histone \(\text{H}_4\) of peas and cows differs by only two amino acid residues out of 102, despite their evolutionary divergence billions of years ago. This reflects their critical structural role in chromatin compaction and gene regulation.
4. Linker histone \(\text{H}_1\) is highly variable, and histones \(\text{H}_2\text{A}\) and \(\text{H}_2\text{B}\) are moderately conserved.
5. Thus, (C) and (D) are the most highly conserved and nearly identical histones.

Step 3: Final Answer:
The histones with nearly identical sequences are \(\text{H}_3\) and \(\text{H}_4\) (statements C and D only).