Nucleosomes

Definition

Nucleosomes are the fundamental units of chromatin in eukaryotic cells, consisting of a segment of DNA wrapped around a core of histone proteins. They play a crucial role in the organization and compaction of DNA, allowing it to fit within the nucleus while also influencing gene expression and accessibility during transcriptional regulation.

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, which includes two copies each of histones H2A, H2B, H3, and H4.
2. The structure of nucleosomes helps to compact DNA into a more manageable form, allowing for efficient storage and access during processes like transcription.
3. Nucleosome positioning can affect gene expression by controlling the accessibility of transcription factors and RNA polymerase to the underlying DNA.
4. Acetylation and methylation of histones can modify nucleosome structure and influence transcriptional activity, demonstrating a direct link between nucleosomes and gene regulation.
5. The arrangement of nucleosomes can create higher-order structures known as chromatin fibers, which further condense to form chromosomes during cell division.

Review Questions

• How do nucleosomes contribute to the regulation of gene expression in eukaryotic cells?
  • Nucleosomes play a critical role in regulating gene expression by controlling the accessibility of DNA to transcription factors and RNA polymerase. When nucleosomes are positioned close to a gene's promoter region, they can hinder the binding of these proteins, thereby repressing transcription. Conversely, modifications to histones that influence nucleosome positioning can enhance accessibility, facilitating gene activation. This dynamic interplay between nucleosomes and regulatory proteins is essential for precise control of gene expression.
• Discuss the impact of histone modifications on the structure and function of nucleosomes in relation to transcriptional regulation.
  • Histone modifications, such as acetylation and methylation, significantly impact the structure and function of nucleosomes. Acetylation tends to loosen the interaction between histones and DNA, making it easier for transcription factors to access the underlying genetic material, thus promoting transcription. In contrast, methylation can either activate or repress transcription depending on which histones are modified. These changes in nucleosome structure directly influence how genes are expressed by altering chromatin accessibility and stability.
• Evaluate how nucleosome positioning affects chromatin structure and its implications for cellular processes beyond transcription.
  • Nucleosome positioning not only influences transcription but also plays a significant role in other cellular processes such as DNA replication, repair, and recombination. Proper positioning of nucleosomes is crucial for maintaining chromatin integrity and ensuring accurate replication during cell division. Mispositioned nucleosomes can lead to errors in these processes, resulting in genomic instability. Additionally, they can affect the binding sites for various proteins involved in repair mechanisms, impacting how cells respond to DNA damage. Therefore, understanding nucleosome dynamics is key to comprehending broader cellular functions.

Related terms

chromatin:

The complex of DNA and proteins, including histones, that make up chromosomes within the nucleus of eukaryotic cells.

histones: Proteins that help package and order DNA into structural units called nucleosomes, playing a key role in gene regulation.

transcription factors: Proteins that bind to specific DNA sequences to regulate the transcription of genetic information from DNA to messenger RNA.