The structure in chromatin seen as 'beads-on' string' when viewed under electron microscope are called

Options

A. nucleotides
B. nuclesides
C. histone octamer
D. nucleosomes.

Explanation

**Step-by-Step Solution:**

1. **Understanding Chromatin Structure**: Chromatin is a complex of DNA and proteins found in the nucleus of eukaryotic cells. It plays a crucial role in packaging DNA into a more compact, dense shape.

2. **Identifying the 'Beads-on-a-String' Structure**: When viewed under an electron microscope, chromatin appears as a 'beads-on-a-string' structure. This description refers to the way DNA is organized around histone proteins.

3. **Exploring the Options**:
- **Nucleotides**: These are the building blocks of DNA, consisting of a sugar, a phosphate group, and a nitrogenous base. They do not form the 'beads-on-a-string' structure.
- **Nucleosides**: These consist of a sugar and a nitrogenous base but lack the phosphate group. They also do not resemble the 'beads-on-a-string' structure.
- **Histone Octamer**: This is a protein complex made up of eight histone proteins (H2A, H2B, H3, and H4). While it is involved in the formation of nucleosomes, it does not appear as 'beads-on-a-string'.
- **Nucleosome**: This is the fundamental unit of chromatin. It consists of a segment of DNA wrapped around a core of histone proteins. When viewed under an electron microscope, nucleosomes appear as 'beads' on the 'string' of DNA.

4. **Conclusion**: The structure in chromatin that appears as 'beads-on-a-string' when viewed under an electron microscope is called the **nucleosome**.

**Final Answer**: Nucleosome

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