How do nucleosomes contribute to the supercoiling of DNA?

Sourav PanNovember 9, 2024

Answered

Nucleosomes play a crucial role in the organization and supercoiling of DNA, which is essential for various cellular processes, including transcription, replication, and DNA repair. Here’s how nucleosomes contribute to DNA supercoiling:

Structure of Nucleosomes

1. Composition:
   • A nucleosome consists of a segment of DNA (about 146 base pairs) wrapped around a core of eight histone proteins, forming a structure resembling “beads on a string.” Each nucleosome is composed of two copies each of histones H2A, H2B, H3, and H4, which form the histone octamer.
2. DNA Packaging:
   • Nucleosomes compact DNA into a more manageable structure that can fit within the cell nucleus. This initial level of compaction is approximately seven-fold linear reduction in length. The arrangement of nucleosomes allows for further folding into higher-order chromatin structures.

Contribution to Supercoiling

1. Supercoiling Mechanism:
   • Supercoiling refers to the over- or under-winding of the DNA double helix. It occurs when the DNA is twisted beyond its normal relaxed state. Nucleosomes influence supercoiling by creating torsional stress during processes such as transcription and replication.
2. Transcription-Induced Supercoiling:
   • During transcription, RNA polymerase moves along the DNA strand, unwinding it to synthesize RNA. This process generates positive supercoils ahead of the polymerase and negative supercoils behind it. The presence of nucleosomes can either stabilize or destabilize these supercoils:
   • Positively Supercoiled Regions: Nucleosomes on positively supercoiled DNA may be destabilized, facilitating transcription by allowing easier access to the underlying DNA.
   • Negatively Supercoiled Regions: Conversely, nucleosomes on negatively supercoiled DNA can relieve superhelical stress, promoting stability and accessibility for transcriptional machinery.
3. Nucleosome Dynamics:
   • The dynamic nature of nucleosomes allows them to be repositioned or remodeled in response to changes in supercoiling. Chromatin remodeling complexes can slide or remove nucleosomes to relieve torsional strain and facilitate access to DNA for transcription or replication.
4. Role in Higher-Order Structures:
   • Nucleosomes also contribute to the formation of higher-order chromatin structures that further compact DNA and influence its accessibility. These structures are essential for regulating gene expression and maintaining genomic integrity