Nucleosomes

Definition

Nucleosomes are the fundamental structural units of chromatin in eukaryotic cells, consisting of a segment of DNA wrapped around a core of histone proteins. This arrangement allows for the efficient packaging of DNA into a compact form, facilitating both gene regulation and DNA replication. Nucleosomes play a critical role in genome organization by influencing the accessibility of DNA to transcription machinery and maintaining the structural integrity of chromosomes during cell division.

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, forming a 'bead-on-a-string' structure that is crucial for DNA packaging.
2. The positioning and spacing of nucleosomes on DNA can influence gene expression by affecting the accessibility of transcription factors to specific genes.
3. Histone modifications, such as acetylation or methylation, can alter the structure and function of nucleosomes, impacting chromatin dynamics and gene regulation.
4. During cell division, nucleosomes help ensure that DNA is properly packaged and distributed to daughter cells, maintaining genetic integrity.
5. Nucleosome remodeling complexes are essential for altering the positioning of nucleosomes, allowing for dynamic changes in chromatin structure in response to cellular signals.

Review Questions

• How do nucleosomes contribute to the regulation of gene expression in eukaryotic cells?
  • Nucleosomes play a key role in regulating gene expression by controlling the accessibility of DNA to transcription factors. The arrangement and spacing of nucleosomes can either promote or hinder access to specific genes, depending on whether they are tightly packed or more loosely organized. Additionally, post-translational modifications on histones can further influence nucleosome stability and positioning, thus impacting overall gene expression.
• Discuss the significance of histone modifications in relation to nucleosome function and chromatin dynamics.
  • Histone modifications are critical for determining nucleosome function and chromatin dynamics. These chemical changes, such as acetylation and methylation, can influence how tightly or loosely nucleosomes are packed. For example, acetylation typically relaxes chromatin structure, allowing for increased accessibility for transcription machinery. Conversely, methylation can lead to a more compact structure that represses gene expression. This interplay between histone modifications and nucleosome positioning is essential for proper gene regulation.
• Evaluate the impact of nucleosome positioning on the efficiency of DNA replication and repair mechanisms in eukaryotic cells.
  • Nucleosome positioning significantly impacts the efficiency of DNA replication and repair mechanisms. Properly spaced nucleosomes allow replication machinery to access the DNA template efficiently during S phase. If nucleosomes are positioned too tightly or irregularly, it can hinder replication fork progression and increase the likelihood of replication errors. Similarly, during repair processes, accessible nucleosomes facilitate the recognition and binding of repair proteins to damaged sites. Thus, understanding nucleosome dynamics is crucial for grasping how cells maintain genomic integrity.

Related terms

chromatin:

The complex of DNA and protein found in eukaryotic cells, which condenses to form chromosomes during cell division.

histones: The family of small, positively charged proteins that associate with DNA to form nucleosomes, playing a key role in chromatin structure and gene regulation.

supercoiling: The coiling of DNA upon itself, which can occur in response to the winding and unwinding of the double helix, impacting the overall structure and accessibility of the genetic material.