How do nucleosomes aid in DNA compaction?

Nucleosomes aid in DNA compaction by coiling and folding the DNA into a more condensed structure.

Nucleosomes are the fundamental unit of DNA packaging in eukaryotes. They are composed of a segment of DNA wound around eight histone proteins, forming a 'bead-on-a-string' structure. This structure is the first level of DNA compaction and is crucial in the process of condensing the long DNA molecules into the microscopic space of the cell nucleus.

The DNA double helix wraps around the histone octamer approximately 1.7 times, forming a nucleosome core particle. This wrapping of DNA around histones reduces the space that the DNA occupies, thus aiding in its compaction. The nucleosomes are then further compacted through the formation of a 30nm chromatin fibre, which is a higher order of DNA packaging. This is achieved by the interaction of the nucleosomes with the histone H1, which helps to 'lock' the DNA into place around the histone core and promotes the formation of a more condensed and stable structure.

Furthermore, the compaction of DNA into nucleosomes is not just a passive process of packaging. It also plays a crucial role in regulating gene expression. The degree of compaction can influence the accessibility of the DNA to the transcription machinery, thereby controlling which genes are expressed and when. For instance, regions of DNA that are tightly packed into nucleosomes are generally less accessible and hence less likely to be transcribed, while regions that are more loosely packed are more likely to be transcribed.

In summary, nucleosomes aid in DNA compaction by wrapping the DNA around histone proteins, reducing its spatial volume, and promoting the formation of higher order structures. This not only allows the large DNA molecules to fit into the cell nucleus, but also plays a key role in the regulation of gene expression.