Chromatin Structure And Function

Nucleosomes are the fundamental units of chromatin, comprising:
- Histone core: Eight histone proteins (H2A, H2B, H3, H4) around which DNA is wrapped.
- Core DNA: The 147 base pairs of DNA wrapped around the histone core.

The linking number (Lk) represents the total twists and writhes of a DNA molecule. Adding +2 to the linking number of relaxed DNA changes the topology and introduces positive supercoiling. The new linking number is Lk = 202, which leads to torsional stress and a more tightly coiled DNA.

Topoisomerases are enzymes that alter the topology of DNA by changing its linking number. They are critical for processes like relieving supercoiling tension during replication, transcription, and chromosome segregation by cutting and resealing DNA strands.

DNA gyrase, a type II topoisomerase, introduces negative supercoils into circular DNA. This reduces torsional strain and facilitates processes like replication and transcription by making the DNA more accessible to enzymes and proteins.

Histone H1, often referred to as the linker histone, binds to the DNA that enters and exits the nucleosome core particle, stabilizing the chromatin's higher-order structure. Unlike core histones (H2A, H2B, H3, H4), H1 is not part of the nucleosome octamer but instead associates with the linker DNA between nucleosomes, facilitating chromatin compaction.

- Histone erasers such as Topoisomerase remove methyl groups from histones, modifying gene expression.
- Histone readers is facilitated by bromodomain proteins that interact with acetylatedlysines.
- Histone writers recognize histone modifications (e.g., acetylation or methylation) to mediate chromatin remodeling.
- DNA unwinding like deacetylases add or remove acetyl groups to regulate DNA accessibility.