Histones: Major DNA-binding proteins involved in chromatin formation.
DNA: Genetic material, packaged in a highly organized structure.
Chromatin: Complex of DNA and proteins in the eukaryotic nucleus, facilitating compaction and gene regulation.
Types of Chromatin
Euchromatin:
Less condensed form of chromatin, generally transcriptionally active.
Features include:
CpG hypomethylation (lower methylation levels).
Early replication during the cell cycle.
Gene-rich areas.
Stains less intensely with DNA dyes.
Heterochromatin:
Highly condensed form of chromatin, usually transcriptionally inactive.
Types include:
Constitutive: Present in all cells, consists of repetitive DNA (satellite DNA, telomeric, centromeric).
Facultative: Can be converted to euchromatin; regulated during differentiation.
Chromatin Structure
Histones Composition:
60% protein, 30% DNA, 10% RNA with a bulk being histones.
Histones are alkaline and facilitate packaging of negatively charged DNA.
Nucleosomes:
Fundamental unit of chromatin, consists of DNA wrapped around a core of histone proteins (H2A, H2B, H3, H4).
Each nucleosome covers about 200 bp of DNA.
Linker DNA connects nucleosomes, bound by histone H1.
Levels of Chromatin Organization
10 nm Fiber: Represented as 'beads on a string', composed of nucleosomes.
30 nm Fiber: Organized structure that further compacts DNA during interphase and metaphase.
DNA Accessibility: Nucleosomes restrict access and determine the sites for transcription, affecting gene expression due to their phasing and positioning in relation to regulatory sequences.
Nucleosome Positioning
Intrinsic Factors: Favor certain positions based on DNA sequence (e.g., A-T rich regions are more flexible).
Extrinsic Factors: Include binding proteins and regulatory elements that exclude nucleosomes from specific regions, influencing transcriptional activity.
Consequences: Proper positioning affects access to DNA and gene regulation, determining whether sequences are exposed or require remodeling for transcription.
Chromatin Assembly and Dynamics
Replication-Coupled Assembly:
Involves the transfer of parental histones and de novo assembly facilitated by chromatin assembly factors (CAF1 and ASF1).
This process ensures rapid and proper repackaging of replicated DNA into chromatin.
DNase I Hypersensitivity
Definition: Sites more susceptible to DNase I cleavage are indicative of accessible chromatin (active transcription regions).
Significance: DHS sites are prominent near regulatory elements like enhancers and promoters of active genes.
Insulators and Gene Regulation
Insulators: DNA elements that prevent activation by enhancers or repression by heterochromatin; critical for maintaining specific gene expression patterns.
Activities: Can exhibit barrier or enhancer-blocking capabilities, ensuring discrete regulation between adjacent genes.
Special Chromatin Structures and Regulation
Heat Shock Genes: Exhibit special structures (e.g., scs regions) that become accessible during stress responses, indicating organization and regulation of chromatin by insulators and protein factors.
Loop Domains: Interactions among insulators facilitate the isolation of gene clusters for regulated expression, crucial during cellular stress events.