<s>[INST] What classes of histones are found as a pair in the nucleosome? [/INST] The two classes of histones found as a pair in the nucleosome are H2A and H2B.
H2A and H2B form a dimer, which is the basic unit of the nucleosome. This dimer is then wrapped around by DNA to form the nucleosome. </s><s>[INST] Name some of the post-translational modifications of histones and how could that be used to regulate transcription. [/INST] Post-translational modifications of histones play a crucial role in regulating gene expression. Some common modifications include:

Acetylation: Adding an acetyl group to lysine residues on histones can relax the chromatin structure and increase accessibility to the DNA, promoting transcription.

Methylation: Methylation of lysine residues on histones can either activate or repress gene expression, depending on the specific lysine residue and the number of methyl groups.

Phosphorylation: Phosphorylation of histone tail residues can influence the chromatin structure and alter the interaction between histones and DNA, leading to changes in gene expression.

Ubiquitination: Attaching a ubiquitin molecule to histones can signal for their removal from the chromatin, leading to changes in gene expression.

These modifications are dynamic and can be reversed by specific enzymes, allowing cells to rapidly respond to changes in their environment and regulate gene expression accordingly. </s>