Nucleosomes give beaded appearance to chromosomes. They help in packing of DNA in the chromosomes. A nucleosome has

Options

A. about 5 turns of DNA
B. 8 histone molecules of 4 types (2 mols each of `H_(2)a`, `H_(2)b`,H_(3) and H_(4)`)
C. 400 nitrogen base pairs
D. all of the above

Answer

To answer the question about what a nucleosome has, we can break down the information step by step.

### Step-by-Step Solution:

1. **Understanding Nucleosomes**:
- Nucleosomes are the fundamental units of chromatin in eukaryotic cells. They consist of DNA wrapped around histone proteins.

2. **Components of Nucleosomes**:
- A nucleosome is made up of a core of histone proteins and DNA. The histone proteins are positively charged due to the presence of basic amino acids like lysine and arginine.

3. **Histone Octamer**:
- The core of the nucleosome is known as the histone octamer, which consists of eight histone molecules. Specifically, it includes two copies each of four types of histones: H2A, H2B, H3, and H4.

4. **DNA Packaging**:
- The DNA, which is negatively charged due to its phosphate backbone, wraps around this histone octamer. Typically, about 200 base pairs of DNA are associated with each nucleosome.

5. **Linker Histone**:
- There is also a linker histone, known as H1, which helps in the further organization of the nucleosomes into higher-order structures.

6. **Conclusion**:
- Therefore, a nucleosome consists of 8 histone molecules (2 of each type: H2A, H2B, H3, and H4) and approximately 200 base pairs of DNA.

### Final Answer:
A nucleosome has **8 histone molecules of 4 types (H2A, H2B, H3, and H4), with 2 molecules of each type**.

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