<P> Histone methylation is a process by which methyl groups are transferred to amino acids of histone proteins that make up nucleosomes, which the DNA double helix wraps around to form chromosomes . Methylation of histones can either increase or decrease transcription of genes, depending on which amino acids in the histones are methylated, and how many methyl groups are attached . Methylation events that weaken chemical attractions between histone tails and DNA increase transcription, because they enable the DNA to uncoil from nucleosomes so that transcription factor proteins and RNA polymerase can access the DNA . This process is critical for the regulation of gene expression that allows different cells to express different genes . </P> <P> Histone methylation, as a mechanism for modifying chromatin structure is associated with stimulation of neural pathways known to be important for formation of long - term memories and learning . Animal models have shown methylation and other epigenetic regulation mechanisms to be associated with conditions of aging, neurodegenerative diseases, and intellectual disability (Rubinstein--Taybi Syndrome, X-linked mental retardation). This modification alters the properties of the nucleosome and affects its interactions with other proteins, particularly in regards to gene transcription processes . </P>

What regulates the packing of dna into chromosomes