How does histone methylation control gene expression?

Histone methylation, first discovered in 1964, is a relatively stable post-translational modification (PTM) type, which derived from the interaction between the protein side chain with S-adenosyl methionine (SAM). This epigenetic modification, catalyzed by histone methyltransferases (HMTs), can occur at various sites of histones, but mainly occurs at the N-terminal lysine and arginine residues of H3 and H4, and it can be controlled by multiple positive and negative regulatory proteins to activate or inhibit transcription. This PTM is a very important and widespread chromatin modification that regulates gene expression by affecting the precise integration of chromatin or signal transduction, thereby affecting a variety of physiological processes in cells. The methylation of histones is reversible, and a considerable number of HMTs and histone demethylases (HDMs) have been identified, which jointly participate in mediating the methylation and demethylation of histones.

Explanation:

https://www.creativebiomart.net/epigenetics/category/products/proteins-enzymes/histone-modification-proteins/histone-methylation-proteins/

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.