Question

A typical nucleosome contains
(a) 100bp of DNA helix
(b) 200bp of DNA helix
(c) 300bp of DNA helix
(d) 400bp of DNA helix

Answer 1

They are present in the nucleus of the eukaryotic cell. They are the basic unit of DNA packaging into chromosomes. They are the repeating units in the chromatin thread, which give the beaded appearance.

Complete answer:
The human nucleus is only 6 micrometers in diameter and yet it contains 1.8 meters of DNA which are distributed among 46 chromosomes. So each chromosome contains a single DNA molecule of about 1.5 inches long. On this scale, an average chromosome would contain 40 km of DNA, and the nucleus which is only 6micrometers would have to contain 1800km of DNA which seems impossible.
To organize these contents in a systematic way the DNA can not just be crumbled in a ball of string and be fitted in the nucleus. Instead through molecular interaction with specific nuclear proteins, the DNA is precisely packaged. This forms a gene combination of DNA with protein and compact fibers called chromatin. This compaction of DNA is done by winding these compact fibers around a bunch of proteins called histones which are positively charged amino acids. Eight histones combine together to form the octamer around which DNA is wrapped around. The end product is a bead-like structure and is called the nucleosome. A typical nucleosome contains about 200 base pairs (bp) of the DNA helix while a human nucleosome contains 146 bp of the DNA helix.

Additional information:
-Histones are made up of positively charged amino acids that neutralize the negatively charged DNA.
-There are five types of histones.
-The four types of histones, called ${ H }_{ 2 }$A, ${ H }_{ 2 }$B, ${ H }_{ 3 }$, and ${ H }_{ 4 }$ each contribute two molecules each to form an octamer.
-The beadlike structure thus formed is called the nucleosome which can differ in length in different organisms.
-Some nucleosomes are coiled into solenoid structure by a fifth histone named ${ H }_{ 1 }$.
-${ H }_{ 1 }$ molecule binds at the region where the DNA enters and leaves each nucleosome.

So the correct answer is ‘200bp of DNA helix’.

Note:
Nucleosomes were earliest discovered as particles in the electron microscope by Don and Ada Olins in 1974. Each human cell possesses approximately 30 million nucleosomes. Some inactive DNA also gets wounded into the nucleosome that contributes to the selectivity of gene expression.