Question

An assembly of nucleosomes constitutes

• A. Histochromosomes
• B. Chromatin
• C. Chromatosomes
• D. Histosomes

Hint:

Remember the levels of DNA packaging:
Double-stranded DNA $\rightarrow$ Nucleosomes ("beads-on-a-string") $\rightarrow$ 30 nm fiber $\rightarrow$ Loops $\rightarrow$ Chromatin $\rightarrow$ highly condensed Chromosomes (visible during mitosis).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
To fit the large eukaryotic genome inside the microscopic nucleus, the DNA must be highly compacted.
This structural packaging is achieved by wrapping the DNA around specialized proteins to form a complex known as chromatin.

Step 2: Detailed Explanation:
The fundamental repeating unit of eukaryotic chromatin is the nucleosome.
A single nucleosome consists of:
- An octameric core of histone proteins (two molecules each of H2A, H2B, H3, and H4).
- Approximately 147 base pairs of double-stranded DNA wrapped $1.65$ times around this histone core.
These individual nucleosomes are connected by stretches of linker DNA, creating a structure that resembles "beads-on-a-string."
An assembly of these repeating nucleosomes, along with linker histones (H1) and non-histone chromosomal proteins, folds and condenses further to form the fiber-like structure known as chromatin.
Let us review the other terms:
- Chromatosome (C): This refers specifically to a single nucleosome core plus the linker histone H1 and its bound linker DNA, not the larger macro-assembly.
- Histosomes (D) and Histochromosomes (A) are not standard terms used to describe DNA packaging.

Step 3: Final Answer:
Therefore, an assembly of nucleosomes constitutes chromatin, which corresponds to option (B).