Question

Describe the structure of a nucleosome, the basic unit of DNA packing in eukaryotic cells.

Short answer

A nucleosome is the unit of chromatin. It comprises eight histone proteins, two each of H2A, H2B, H3, and H4, around which the DNA is wrapped. One nucleosome is attached to other nucleosomes through a linker DNA.

Step-by-step solution

Differences in the genetic material of prokaryotic and eukaryotic cells

The structure of the genetic material, DNA, is different in prokaryotic and eukaryotic cells. In bacteria, the genetic material is a double-stranded, circular DNA molecule linked to tinyproteins.These are found in the nucleoid region of the bacteria.

However, the structure of the DNA molecule is very different in eukaryotic cells. Here, DNA is a long linear molecule linked to a large number of proteins. Moreover, the size of DNA is around 100 times more than prokaryotic DNA.

Chromosomes

DNA in eukaryotic cells is associated with various proteins to form chromatin.Each chromatin fiber contains one DNA molecule. Chromosomes are formed due to the condensation of chromatin before cell division.

As DNA is a linear molecule in a eukaryotic cell, it is tightly packaged to fit into the cell's nucleus.

Structure of nucleosome

The packaging of DNA begins with the formation of nucleosomes, where DNA is associated with highly basic proteins called histones. Five varieties of histones are found, namely: H1, H2A, H2B, H3, and H4.

The histone proteins interact with DNA to form a nucleosome. Two of H2A, H2B, H3, and H4 histones form an octamer histone cluster. DNA wraps twice around this octamer to form the nucleosome structure.

Each nucleosome formed is a disc-shaped structure about 10 nm in diameter. Thus, it is also called a 10 nm fiber. Each nucleosome is attached to an adjacent nucleosome through a linker, DNA, forming polynucleosomes.

The nucleosome attached to a DNA linker appears like beads on a string. Thus, the nucleosome forms the basic structural component of the eukaryotic chromosome.