Nucleosome

Definition

A nucleosome is the basic structural unit of DNA packaging in eukaryotic cells, consisting of a segment of DNA wound around a core of histone proteins. This structure plays a critical role in the organization and compaction of DNA, allowing it to fit within the nucleus while also regulating gene expression through its accessibility. Nucleosomes are essential for maintaining the overall topology of DNA, influencing processes such as replication and transcription.

5 Must Know Facts For Your Next Test

1. Nucleosomes consist of about 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 presence of nucleosomes allows for higher-order structures such as chromatin fibers, which further condense DNA into chromosomes during cell division.
3. The spacing of nucleosomes along the DNA strand can influence gene accessibility; regions with fewer nucleosomes are often associated with active gene expression.
4. Nucleosomes can be modified through various post-translational modifications of histones, such as acetylation and methylation, which play significant roles in gene regulation.
5. Nucleosome positioning is not random; specific sequences can affect where nucleosomes are placed on the DNA, impacting both transcription factor binding and chromatin accessibility.

Review Questions

• How do nucleosomes contribute to the compaction and organization of eukaryotic DNA?
  • Nucleosomes serve as the fundamental units of chromatin structure, with DNA wrapped around histone proteins to create a compact form. This organization allows eukaryotic cells to fit large amounts of genetic material within the nucleus. By forming higher-order structures like chromatin fibers, nucleosomes facilitate the overall compaction necessary for DNA storage while still allowing for dynamic regulation during processes like replication and transcription.
• In what ways do histone modifications impact nucleosome function and gene expression?
  • Histone modifications, such as acetylation and methylation, can alter nucleosome function by changing how tightly or loosely the DNA is wrapped around histones. These modifications can either promote or inhibit access to specific genes by transcription factors, influencing gene expression levels. For example, acetylation generally relaxes nucleosome structure, making DNA more accessible for transcription, while certain methylation patterns can lead to a more compact structure that represses gene activity.
• Evaluate the significance of nucleosome positioning on gene regulation and its implications for cellular functions.
  • Nucleosome positioning plays a critical role in determining which regions of DNA are accessible for transcription factors and other regulatory proteins. Proper positioning can facilitate active gene expression by ensuring that important regulatory sequences are exposed, while improper positioning may lead to silenced genes or altered cellular functions. Understanding how nucleosomes are arranged on the genome helps researchers comprehend mechanisms behind cellular differentiation and responses to environmental signals, highlighting their importance in overall cellular function and health.

Related terms

chromatin: Chromatin is a complex of DNA and proteins that forms chromosomes within the nucleus, existing in two forms: euchromatin, which is less condensed and active in transcription, and heterochromatin, which is tightly packed and generally inactive.

histones: Histones are highly alkaline proteins around which DNA is wrapped to form nucleosomes, playing a key role in the structural organization of chromatin and influencing gene regulation.

supercoiling:

Supercoiling refers to the over- or under-winding of DNA strands, which helps to further compact the DNA in chromosomes and can affect the accessibility of the DNA for transcription and replication.