Final answer:
Methylation can affect gene expression(A), and is influenced by various factors including stress and diet. It involves the addition of methyl groups to cytosine bases within the DNA, leading to gene silencing or other regulatory effects. External stimuli can also lead to chemical modifications of DNA and histones, affecting gene activity.
Step-by-step explanation:
Methylation affects gene expression(A), can be affected by stress, and can be affected by diet; therefore, all of these answers are correct.
DNA methylation occurs at CpG islands within the promoter regions of genes, which can lead to the silencing of genes or other regulatory effects. Environmental conditions such as diet, UV rays, and stress can modify these methylation patterns, thus influencing gene expression. The addition of methyl groups to cytosine bases in DNA can lead to the condensation of chromatin, rendering the DNA less accessible for transcription, resulting in suppressed gene activity.
Furthermore, chemical modifications such as histone acetylation and deacetylation also play a critical role in changing gene expression. Drugs designed to inhibit DNA methylation or prevent the removal of acetyl groups on histone proteins are being explored as potential therapies for cancer, as changing these epigenetic marks can reactivate silenced genes, which may help kill tumor cells.