Nucleosome

Definition

A nucleosome is the fundamental unit of DNA packaging in eukaryotic cells, consisting of a segment of DNA wrapped around a core of histone proteins. This structure helps to compact and organize DNA within the nucleus, playing a crucial role in gene regulation and accessibility. By forming higher-order structures, nucleosomes enable the efficient storage and management of genetic information.

5 Must Know Facts For Your Next Test

1. Nucleosomes consist of about 146 base pairs of DNA wrapped around an octamer of histone proteins, specifically H2A, H2B, H3, and H4.
2. The organization of nucleosomes creates a 'beads-on-a-string' structure, which further coils and folds to form higher-order chromatin structures.
3. Each nucleosome is connected by a short stretch of linker DNA, which can be associated with the linker histone H1, contributing to the overall stability of the chromatin.
4. Nucleosomes play a critical role in gene regulation by controlling access to DNA; modifications to histones can influence whether genes are expressed or silenced.
5. The dynamic nature of nucleosomes allows for the rearrangement and repositioning on DNA, facilitating processes like replication and repair.

Review Questions

• How do nucleosomes contribute to the compaction and organization of DNA in eukaryotic cells?
  • Nucleosomes contribute to DNA compaction by wrapping around histone proteins, creating a structural unit that organizes the lengthy DNA molecule into a more manageable form. This wrapping reduces the overall length of DNA and facilitates its organization within the nucleus. Furthermore, by forming higher-order structures through interactions between nucleosomes, they enable efficient storage and regulation of genetic information.
• Discuss the role of histone modifications in regulating gene expression via nucleosome dynamics.
  • Histone modifications, such as acetylation, methylation, and phosphorylation, play a crucial role in regulating gene expression by influencing the dynamics of nucleosomes. These modifications can alter the interaction between histones and DNA, either loosening or tightening the nucleosome structure. Such changes impact accessibility for transcription factors and RNA polymerase, thus determining whether specific genes are turned on or off in response to various cellular signals.
• Evaluate the implications of nucleosome positioning on genomic stability and gene regulation during cellular processes such as replication or repair.
  • Nucleosome positioning is vital for maintaining genomic stability during cellular processes like replication and repair. Properly positioned nucleosomes ensure that essential genes remain accessible for transcription while protecting other regions from inappropriate expression. During replication, nucleosomes must be reassembled correctly behind the replication fork to maintain chromatin structure. Disruption in nucleosome positioning can lead to errors in gene regulation and increase susceptibility to mutations or genomic instability.

Related terms

Chromatin: The complex of DNA and proteins that forms chromosomes within the nucleus, existing in two forms: euchromatin (less condensed) and heterochromatin (more condensed).

Histones: Basic proteins that play a key role in the organization of DNA into nucleosomes, facilitating the compaction of DNA in eukaryotic cells.

Euchromatin: A form of chromatin that is less densely packed, allowing for active transcription and greater accessibility to the genetic material.