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The general structure of chromatin has been found to be remarkably similar in the cells of all eukaryotes. The most abundant proteins associated with eukaryotic DNA (somewhat more than half its mass) are histones, a family of basic proteins rich in the positively charged amino acids that interact with the negatively charged phosphate groups in DNA.

Which of the following amino acids acts as a component of histones and a precursor for Nitric oxide (NO)?

A. Arginine

B. Histidine

C. Lysine

D. Asparagine

E. Leucine

The correct answer is- A- Arginine.

Histones are nucleoproteins, rich in basic amino acids such as Arginine and Lysine. The net positive charge (due to basic amino acids), helps in binding of histones to negatively charged groups of DNA. Histones help in DNA packaging. The nucleosomes, the first level of DNA organization, are composed of DNA wound around a collection of histone molecules. Nucleosomes contain four types of histones: H2A, H2B, H3, and H4. H1 histones are the ones least tightly bound to chromatin (figure-1)

Role of histones

Figure-1- Role of histones in nucleosome formation

Arginine, a semi essential amino acid, is also a precursor for Nitric oxide that acts as a vasodilator and a smooth muscle relaxant. Nitric oxide is synthesized by nitric oxide synthase (NOS). The reaction catalyzed can be expressed as follows (figure-2)

Synthesis of Nitric oxide

Figure-2-Synthesis of nitric oxide, the reaction is catalyzed by Nitric oxide

As regards other options

B. Histidine- Histidine carries a net positive charge at physiological pH but it is not a predominant component of histones or a precursor for Nitric oxide.

C. Lysine- Lysine is a predominant component of histones, but it is not a precursor for nitric oxide.

D. Asparagine is an amide group containing amino acid; it is neither a component of histones nor a precursor for nitric oxide.

E. Leucine is also not a suitable option due to similar reasons.


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