Statement Analysis

A. Protein bodies. Incorrect—nucleosomes contain both histone proteins and DNA, not pure protein aggregates.

B. Protein–DNA complexes. Correct—the basic chromatin unit where DNA wraps around histone octamers.

C. Nucleic acids are covalently linked to proteins. Incorrect—DNA associates electrostatically with positively charged histones (H2A, H2B, H3, H4); no covalent bonds between them.

D. Protein octamers are wrapped with approximately 200 bp of DNA. Correct—precisely 147 bp core DNA (1.67 superhelical turns) around H3/H4 tetramer + 2×H2A/H2B dimers, plus ~50 bp linker DNA totals ~200 bp per nucleosome.

E. 10 nm fibers for DNA packing. Correct—”beads-on-a-string” structure (11 nm diameter nucleosomes + linker DNA) forms the 10 nm chromatin fiber.

Option Breakdown

• (1) A, B, D Only: Wrong—A false (not pure protein).

• (2) B, D, E Only: Correct—all three define nucleosome structure/packing role.

• (3) C, D, E Only: Wrong—C false (electrostatic, not covalent).

• (4) A, C, B Only: Wrong—A, C both false.

Nucleosomes serve as the fundamental units of chromatin, where protein octamers are wrapped with approximately 200 bp of DNA to achieve 7-fold compaction.

Core Structure

Each nucleosome contains:

• Histone octamer: 2×H2A, 2×H2B, 2×H3, 2×H4 (H1 linker histone optional).

• Core DNA: 147 bp making 1.67 left-handed superhelical turns.

• Linker DNA: 20-80 bp connects adjacent nucleosomes.

Chromatin Hierarchy

• 10 nm fiber: “Beads-on-string” (nucleosomes + linkers).

• 30 nm fiber: Zigzag/solenoid folding.

• Higher-order: Loops → metaphase chromosomes.

Key Exam Distinctions

NEET/CSIR tests non-covalent protein-DNA association vs. viral nucleoproteins. ~200 bp total (147+linker) vs. precise 147 bp core.

Packaging math: 2 m human DNA → ~200 bp/nucleosome = ~10^7 nucleosomes/cell.