Nucleosome

Nucleosome is the basic unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound in sequence around eight histone protein cores. This structure is often compared to thread wrapped around a spool.

Structure

Nucleosomes are composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4. This structure is compact but also allows access to the DNA sequence when needed by the cell.

Function

The primary function of the nucleosome is to serve as a method for the cell to both compact DNA and to create a higher order of structure. This structure is necessary for the cell to fit all of its genetic material within the nucleus. Additionally, the structure of the nucleosome allows the cell to control gene expression, as the DNA sequence is not accessible when it is wrapped around the histones in the nucleosome.

Role in Gene Regulation

Nucleosomes play an important role in gene regulation. In general, genes that are active have less tightly packed nucleosomes, which allows transcription factors and other gene regulatory proteins to access the DNA. In contrast, inactive genes often have tightly packed nucleosomes that prevent access to the DNA.

See Also

• Chromatin
• Histone
• Epigenetics

References

Nucleosome Resources
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