Answer:
1. Histone methylation occurs at different amino acids: c. Activates and inactivates
2. A transcriptional binds an enhancer sequence: d. No effect
3. Histone acetyltransferase attach acetyl groups to the N-terminus of histones: a. Activates
4. Histone deacetylases remove acetyl groups from the N-terminus of histones: b. Inactivates
Explanation:
Epigenetics refers to the mechanisms associated with changes in gene expression which do no involve alteration in DNA sequence. Epigenetic marks include DNA methylation and histone modifications (acetylation, methylation, phosphorylation, etc). These mechanisms act together in order to regulate gene expression at specific genomic sites (<em>loci</em>). Histone methylation is an epigenetic mark involved both in transcriptional repression and transcriptional activation. For example, trimethylation of the histone H3 at lysine 4 (H3K4me3) has been associated with transcriptional activation, while dimethylation of histone H3 at lysine 9 (H3K9me2) is often involved in transcriptional repression. Moreover, histone acetylation by histone acetyltransferases (HATs) is an epigenetic mark that is involved in transcriptional activation and chromatin decondensation, while histone deacetylation by deacetylases (HDACs ) is generally associated with gene repression. However, it is important to highlight that there are exceptions to this rule (for example, acetylation of the histone H4 lysine 20 is associated with gene repression). Finally, transcriptional activators are proteins that bind to enhancers in order to recruit coactivator proteins/complexes and thus activate transcription.
