Question

Treating chromatin with a non specific nuclease yields a segment of about 168 bp which is bound to 9 histone molecules (H2A, H2B, H3, H4 and H1). This whole structure is known as:

• A. Histone Octamer
• B. Nucleosome
• C. Chromatosome
• D. Histosome

Hint:

Remember the progression of chromatin structure:

\textbf{Histone Octamer} (8 core histones) + \textbf{\( \sim \)147 bp DNA} = \textbf{Nucleosome}
\textbf{Nucleosome} + \textbf{Histone H1} (\( \rightarrow \) protection of \( \sim \)168 bp DNA) = \textbf{Chromatosome}
The key difference between a nucleosome and a chromatosome is the presence of the H1 linker histone.

Answer 1

The Correct Option is C

Step 1: Concept Overview:
The question identifies a specific chromatin structural unit based on its DNA length and histone protein composition, including linker histone H1.
Step 2: Detailed Explanation:
Here's a breakdown of chromatin structure levels:

Histone Octamer: The protein core of the nucleosome, composed of two copies each of histones H2A, H2B, H3, and H4. It lacks DNA and histone H1.
Nucleosome (or Core Particle): The basic repeating unit of chromatin, consisting of the histone octamer and approximately 147 base pairs (bp) of DNA wrapped around it. It excludes linker histone H1. Micrococcal nuclease (MNase) digestion yields this structure.
Chromatosome: A more complex structure than the nucleosome. It includes the nucleosome core particle {plus} linker histone H1, which binds to the DNA entry/exit points of the octamer. H1 binding protects a longer DNA segment from nuclease digestion. The chromatosome contains the histone octamer, linker histone H1 (9 histone molecules total), and roughly 168 bp of DNA. This precisely answers the question.
Histosome: This term is not commonly used to describe chromatin structure.
Step 3: Final Answer:
The structure comprised of 168 bp of DNA, the histone octamer, and the linker histone H1 is termed a chromatosome.