Nucleosome

Definition

A nucleosome is the fundamental unit of chromatin, consisting of a segment of DNA wrapped around a core of histone proteins. This structure plays a crucial role in the organization and compaction of DNA within the cell nucleus, allowing for efficient storage and regulation of genetic information. Nucleosomes are essential for the overall structure of chromosomes and influence processes such as gene expression and DNA replication.

5 Must Know Facts For Your Next Test

1. Nucleosomes consist of approximately 147 base pairs of DNA wrapped around an octamer of histone proteins, which includes two copies each of histones H2A, H2B, H3, and H4.
2. The string of nucleosomes resembles 'beads on a string' when viewed under an electron microscope, with linker DNA connecting adjacent nucleosomes.
3. Nucleosome positioning is critical for regulating access to DNA; their arrangement can influence whether specific genes are turned on or off.
4. The histone tails extending from nucleosomes can be modified through processes like methylation and acetylation, impacting chromatin structure and gene activity.
5. Nucleosomes play a vital role in packaging DNA into chromosomes during cell division, ensuring accurate distribution of genetic material to daughter cells.

Review Questions

• How do nucleosomes contribute to the regulation of gene expression?
  • Nucleosomes influence gene expression by controlling access to DNA. The positioning of nucleosomes along the DNA strand can either expose or shield specific gene regions from transcription factors and other regulatory proteins. When nucleosomes are tightly packed, they can inhibit transcription, while modifications to histone tails can lead to a more open chromatin structure, promoting gene activation.
• Compare and contrast the roles of nucleosomes and chromatin in the organization of genetic material within the cell nucleus.
  • Nucleosomes are the basic structural units that form chromatin, with each nucleosome comprising a segment of DNA wrapped around histone proteins. Chromatin is the overall complex that includes both nucleosomes and linker DNA. While nucleosomes are essential for packaging DNA into a compact form, chromatin exists in varying degrees of condensation (euchromatin versus heterochromatin) depending on whether the genetic material needs to be accessed for transcription or replication.
• Evaluate the impact of histone modifications on nucleosome function and chromatin dynamics.
  • Histone modifications significantly affect nucleosome function and chromatin dynamics by altering how tightly or loosely DNA is associated with histones. For instance, acetylation typically leads to a more relaxed chromatin structure, facilitating access to DNA for transcription and replication. In contrast, methylation can lead to tighter packing of nucleosomes, silencing specific genes. Understanding these modifications helps reveal the mechanisms by which cells regulate gene expression in response to internal and external signals.

Related terms

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

histones: Small, positively charged proteins around which DNA is tightly coiled to form nucleosomes, playing a key role in the structure and function of chromatin.

gene expression: The process by which information from a gene is used to synthesize a functional gene product, typically proteins, involving various levels of regulation including chromatin structure.