Chromatin assembly is a rapid, stepwise process that occurs immediately following DNA replication, building structure around newly synthesized DNA using a mixture of existing and newly produced histone proteins.
Here's a breakdown of the process:
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Timing is Key: This process is tightly coupled with DNA replication. As the replication fork moves along the DNA, chromatin assembly occurs quickly behind it.
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Histone Supply: The newly replicated DNA needs histones to form nucleosomes, the basic repeating unit of chromatin. The cell uses a mix of:
- Old Histones: These are recycled from the parental chromatin that was disrupted during replication.
- Newly Synthesized Histones: The cell produces new histones to supplement the existing pool and ensure sufficient histone supply for the daughter DNA molecules.
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Stepwise Deposition: The histones are not deposited randomly. The process follows a specific order:
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H3-H4 Tetramer Deposition: A tetramer composed of two H3 and two H4 histone proteins is deposited onto the DNA first. These tetramers are crucial for nucleosome formation. They serve as a foundation upon which the rest of the nucleosome is built.
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H2A-H2B Dimer Addition: Next, two H2A-H2B dimers (each consisting of one H2A and one H2B histone protein) are added to complete the nucleosome core. This results in a complete nucleosome, consisting of two copies each of H2A, H2B, H3, and H4, around which the DNA is wrapped.
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Chromatin Assembly Factors (CAFs): These proteins play a critical role in facilitating chromatin assembly. They help to:
- Target histones to the newly replicated DNA.
- Properly assemble the nucleosomes.
- Prevent inappropriate histone aggregation.
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Post-translational Modifications (PTMs): Histones can be modified by the addition of chemical groups (e.g., acetylation, methylation, phosphorylation). These modifications can influence chromatin structure and gene expression. PTMs are added during and after nucleosome assembly, contributing to the establishment of epigenetic marks.
In summary, chromatin assembly is a carefully orchestrated process involving histone deposition (both old and new), specific histone ordering (H3-H4 tetramer first, then H2A-H2B dimers), and the assistance of chromatin assembly factors. This ensures proper packaging of newly synthesized DNA into chromatin.
