Question

Nucleosome a repeating structural unit within eukaryotic chromatin is made up of

• A. 140 bp of DNA making 1.65 negative super helical turn wrapped around a hexamer of histones
• B. 146 bp of DNA making 1.65 negative super helical turn wrapped around a dimer of histones
• C. 140 bp of DNA making 1.65 negative super helical turn wrapped around an octamer of histones
• D. 146 bp of DNA making 1.65 negative super helical turn wrapped around an octamer of histones

Hint:

• Nucleosome Core Particle
  • DNA: ~146-147 bp wrapped ~1.65 times.
  • Histone Octamer: 2x(H2A + H2B + H3 + H4).
• Histone H1 (linker histone) binds to linker DNA and helps compact nucleosomes further.
• This "beads-on-a-string" structure is the first level of chromatin organization.

Answer 1

The Correct Option is D

The nucleosome is the fundamental repeating unit of chromatin in eukaryotic cells. It plays a crucial role in packaging DNA into a compact structure within the nucleus. A nucleosome core particle consists of:

• DNA: Approximately 146-147 base pairs (bp) of DNA.
• Histone Octamer: This DNA is wrapped around a core complex of eight histone proteins. The histone octamer is composed of two copies each of four core histone proteins: H2A, H2B, H3, and H4.
• The DNA wraps around the histone octamer approximately 1.65 to 1.7 times in a left-handed (negative) superhelical turn.

Linker DNA connects adjacent nucleosome core particles. Histone H1 (linker histone) is often associated with the linker DNA and helps in further compaction of chromatin. Let's analyze the options: (a) "140 bp ... hexamer of histones": Incorrect DNA length (closer to 146/147 bp) and histone core is an octamer. (b) "146 bp ... dimer of histones": Histone core is an octamer. (c) "140 bp ... octamer of histones": DNA length is slightly off. (d) "146 bp of DNA making 1.65 negative super helical turn wrapped around an octamer of histones": This is the most accurate description among the options. The DNA length is approximately 146-147 bp, it wraps about 1.65 times, and the core is an octamer of histones. Therefore, option (d) is the correct description. \[ \boxed{\parbox{0.9\textwidth}{\centering 146 bp of DNA making 1.65 negative super helical turn wrapped around an octamer of histones}} \]