Which of the following statements are correct with reference to packaging of DNA helix ?

A. Histones are organized to form a unit of eight molecules called histone octamer.
B. Histones are negatively charged basic proteins.
C. Histones are rich in the basic amino acid residues - lysine and arginine.
D. The positively charged DNA is wrapped around the histone octamer to form nucleosome.
E. The packaging of chromatin at higher levels requires an additional set of proteins called non-histone chromosomal proteins.

Choose the correct answer from the options given below :

Question

Which of the following statements are correct with reference to packaging of DNA helix ?

A. Histones are organized to form a unit of eight molecules called histone octamer.
B. Histones are negatively charged basic proteins.
C. Histones are rich in the basic amino acid residues - lysine and arginine.
D. The positively charged DNA is wrapped around the histone octamer to form nucleosome.
E. The packaging of chromatin at higher levels requires an additional set of proteins called non-histone chromosomal proteins.

Choose the correct answer from the options given below :

Accepted Answer

A. 1. **Recall how histones are organized.**

Given that DNA packaging begins with a core protein structure, eight histone molecules come together to form a **histone octamer**.

Hence, statement **A is correct**.

2. **Check the charge on histones.**

Since histones must attract the negatively charged DNA, they themselves are **positively charged basic proteins**, not negatively charged.

Hence, statement **B is incorrect**.

3. **Check the amino acid composition of histones.**

Given that a positive charge on a protein comes from basic amino acids, histones are indeed rich in **lysine and arginine**.

Hence, statement **C is correct**.

4. **Check the charge on DNA in the nucleosome.**

Since DNA carries negative charge from its phosphate backbone, it is the **negatively charged DNA** (not positively charged) that wraps around the histone octamer to form the nucleosome.

Hence, statement **D is incorrect**.

5. **Check what helps higher-order packaging of chromatin.**

Given that further compaction of chromatin beyond the nucleosome level needs extra support, this is provided by **non-histone chromosomal (NHC) proteins**.

Hence, statement **E is correct**.

**Conclusion:** The correct statements are A, C and E, so the answer is **A**.