Nucleosomes

Definition

Nucleosomes are the fundamental units of DNA packaging in eukaryotic cells, consisting of a segment of DNA wound around a core of histone proteins. This organization allows for the compact storage of genetic material while also playing a crucial role in regulating gene expression and DNA accessibility. Nucleosomes form the first level of chromatin structure, setting the stage for higher-order folding and long-range interactions within the three-dimensional genome.

5 Must Know Facts For Your Next Test

1. Each nucleosome consists of about 146 base pairs of DNA wrapped around a histone octamer made up of four different histone proteins: H2A, H2B, H3, and H4.
2. Nucleosomes are spaced along the DNA molecule, creating 'linker' regions between them that can be further compacted into higher-order structures.
3. Nucleosome positioning is essential for gene regulation; specific locations on DNA can be occupied or evicted by nucleosomes to expose or hide regulatory elements.
4. Post-translational modifications of histones, such as acetylation and methylation, can influence nucleosome stability and function, thereby affecting gene expression.
5. Nucleosomes contribute to the three-dimensional organization of the genome by facilitating long-range interactions between distant regions of DNA, impacting overall gene regulation.

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 access to DNA. They can either block or expose specific regions of DNA depending on their positioning and modifications. When nucleosomes are positioned over promoter regions, they can inhibit transcription factors from binding, whereas when they are evicted or altered through histone modifications, they allow transcription factors access to initiate gene expression.
• Discuss the structural arrangement of nucleosomes and how this arrangement affects genome organization.
  • Nucleosomes are arranged in a repeating pattern along the DNA strand, forming a 'bead-on-a-string' structure that is further folded into higher-order chromatin structures. This arrangement allows for efficient packing of DNA within the nucleus while enabling flexibility for regulatory interactions. The spatial organization facilitated by nucleosomes contributes to the three-dimensional architecture of the genome, influencing how different genes can interact with one another across long distances.
• Evaluate how modifications to nucleosome components influence cellular processes at both the genetic and epigenetic levels.
  • Modifications to nucleosome components, such as histone acetylation or methylation, significantly influence cellular processes at both genetic and epigenetic levels. These modifications can alter the affinity of nucleosomes for DNA, affecting their stability and positioning. Such changes impact gene accessibility and expression patterns, thus influencing cellular identity and function. Furthermore, these modifications can be inherited through cell divisions, leading to epigenetic changes that affect gene expression without altering the underlying DNA sequence.

Related terms

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

histones: Basic proteins that help package DNA into nucleosomes by forming an octameric core around which DNA is wound, playing a key role in gene regulation and chromatin structure.

DNA accessibility: The degree to which DNA is available for transcription and replication, which can be influenced by nucleosome positioning and modifications that either promote or hinder access to the underlying genetic information.