Q.53 The number of polypeptide chains in a core nucleosome is ____.
The number of polypeptide chains in a core nucleosome is 8.
Question Solution
This CSIR NET-style question tests core knowledge of chromatin structure in molecular biology. The core nucleosome, or nucleosome core particle (NCP), excludes linker histone H1 and linker DNA, focusing solely on the histone octamer around which ~146 bp of DNA wraps. Each of the four core histones (H2A, H2B, H3, H4) exists as two identical copies, forming dimers (H2A-H2B, H3-H4) that assemble into the octamer—thus, 4 histones × 2 polypeptides each = 8 polypeptide chains total.
Option Analysis
CSIR NET questions often provide numerical choices like 4, 6, 8, or 10. Here’s why each fails or succeeds:
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4: Incorrect—represents only one set of core histones, ignoring the dimeric nature; the octamer requires two copies each.
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6: Incorrect—no structural basis; possibly confuses with incomplete assembly (e.g., tetramer + one dimer), but full core needs all eight.
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8: Correct—two molecules each of H2A, H2B, H3, H4 form the histone octamer, confirmed by X-ray crystallography (e.g., PDB structures).
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10: Incorrect—might include H1 (linker histone, 1 copy), but core nucleosome excludes it; full nucleosome with H1 has 9 chains.
Core Nucleosome Structure
The histone octamer features a central H3-H4 tetramer with two H2A-H2B dimers, creating a spool-like structure for DNA supercoiling. Histones are basic polypeptides with histone-fold domains (three α-helices), enabling tight DNA binding via electrostatic interactions. This 11 nm × 5.5 nm particle compacts DNA ~7-fold, essential for eukaryotic genome packaging.
Exam Relevance
For CSIR NET Life Sciences (Unit 2: Cellular Organization), memorize: core = 8 polypeptides (no H1); full nucleosome adds H1 (9 total). Practice visualizing the octamer to differentiate from 30 nm fiber (higher-order chromatin).
