What are the components of a nucleosome?

A nucleosome is composed of a segment of DNA wound around eight histone proteins and a linker histone.

A nucleosome, the fundamental unit of chromatin, is made up of a segment of DNA wrapped around a protein core. This protein core consists of eight histone proteins, two each of four types: H2A, H2B, H3, and H4. These histones form an octamer, a structure that resembles a spool, around which the DNA strand is wound. Approximately 147 base pairs of DNA are wrapped around this histone octamer.

The DNA segment that is wound around the histone octamer is not continuous but is interrupted by a short stretch of linker DNA. This linker DNA, which can vary in length from about 20 to 60 base pairs, connects one nucleosome to the next in the chromatin fibre. The linker DNA is associated with a fifth type of histone, known as the linker histone or H1. The H1 histone binds to the DNA as it enters and exits the nucleosome, helping to stabilise the structure.

The nucleosome structure allows the long DNA molecules to be compacted and fit within the cell nucleus. It also plays a crucial role in gene regulation. The wrapping of DNA around histones can prevent the DNA from being accessed by the cellular machinery that reads and copies the genetic information. However, the nucleosome structure is not static. The histones can undergo chemical modifications that alter their interaction with DNA and other proteins, thereby regulating the accessibility of the DNA.

In summary, a nucleosome is a complex structure composed of DNA and histone proteins. The DNA is wound around a core of eight histones, with a segment of linker DNA connecting one nucleosome to the next. The linker histone H1 helps to stabilise the nucleosome structure. The arrangement and modification of nucleosomes play a key role in the compaction of DNA and the regulation of gene expression.