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: Understanding the Concept:
This question asks for the specific name of a chromatin structural unit defined by its DNA length and the complement of histone proteins, including the linker histone H1.
Step 2: Detailed Explanation:
Let's define the different levels of chromatin structure:

Histone Octamer: This is the protein core of the nucleosome. It consists of eight histone molecules: two copies each of the core histones H2A, H2B, H3, and H4. It does not include DNA or histone H1.
Nucleosome (or Core Particle): This is the fundamental repeating unit of chromatin. It consists of the histone octamer with approximately 147 base pairs (bp) of DNA wrapped around it. It does not include the linker histone H1. Mild digestion of chromatin with micrococcal nuclease (MNase) yields this structure.
Chromatosome: This is the next level of organization. It consists of the nucleosome core particle {plus} the linker histone H1, which binds to the DNA where it enters and exits the octamer. The addition of H1 protects a longer stretch of DNA from nuclease digestion. The chromatosome comprises the histone octamer, the linker histone H1 (total of 9 histone molecules), and about 168 bp of DNA. This description perfectly matches the question.
Histosome: This term is not standard in the context of chromatin structure.
Step 3: Final Answer:
The structure containing 168 bp of DNA, the histone octamer, and the linker histone H1 is called a chromatosome.