Nucleosome

Definition

A nucleosome is the basic structural unit of chromatin, consisting of a segment of DNA wrapped around a core of histone proteins. This organization helps package the long DNA molecules into a compact form, making it essential for the regulation of gene expression and DNA accessibility. Nucleosomes play a crucial role in the overall structure and organization of chromatin, influencing various cellular processes such as replication, repair, and transcription.

5 Must Know Facts For Your Next Test

1. Nucleosomes are formed by wrapping approximately 147 base pairs of DNA around an octamer of histone proteins, which consists of two copies each of histones H2A, H2B, H3, and H4.
2. The arrangement of nucleosomes along the DNA strand resembles 'beads on a string,' where the nucleosomes are the beads and the linker DNA between them represents the string.
3. Nucleosome positioning is crucial for regulating gene expression, as the accessibility of DNA to transcription factors and other regulatory proteins depends on the positioning and modification of nucleosomes.
4. Chemical modifications to histones, such as acetylation and methylation, can influence the stability of nucleosomes and affect chromatin structure, thereby impacting gene expression.
5. The dynamic nature of nucleosomes allows for reversible changes in chromatin structure, which is important during processes like DNA replication, repair, and transcriptional activation.

Review Questions

• How do nucleosomes contribute to the regulation of gene expression?
  • Nucleosomes play a critical role in regulating gene expression by controlling the accessibility of DNA to transcription factors. The positioning and modifications of nucleosomes can either promote or inhibit access to specific genes. When nucleosomes are tightly packed, it can prevent transcription factors from binding to the DNA, while modifications such as acetylation can loosen nucleosome packing, facilitating gene activation.
• Discuss the structural features of nucleosomes and how they interact with DNA.
  • Nucleosomes consist of an octamer of histone proteins around which about 147 base pairs of DNA are tightly wrapped. This structure not only condenses DNA but also creates specific interactions between the histones and the negatively charged phosphate backbone of DNA. The linker DNA between adjacent nucleosomes contributes to higher-order chromatin structure by allowing further folding and compaction.
• Evaluate the implications of histone modifications on nucleosome stability and chromatin dynamics during cellular processes.
  • Histone modifications can significantly alter nucleosome stability and dynamics, impacting various cellular processes such as replication and transcription. For instance, acetylation typically reduces histone-DNA interactions, leading to a more open chromatin configuration conducive to gene activation. In contrast, methylation can either promote or repress gene expression depending on the specific context. This dynamic regulation through histone modifications allows cells to respond rapidly to environmental changes and maintain proper gene expression patterns.

Related terms

Chromatin: A 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: Proteins that help package and organize DNA into nucleosomes, playing a key role in regulating access to genetic information.

Euchromatin: A less condensed form of chromatin that is generally associated with active gene expression and accessible regions of DNA.