3.9 Gene Regulation and Epigenetics

Gene expression is the use of genetic information to produce a functional RNA or protein and thereby contribute to phenotype. The genome is the complete DNA set, the transcriptome is the collection of RNA transcripts present under specified conditions, and the proteome is the set of proteins expressed. Most cells of one organism share a genome but have different transcriptomes and proteomes.

Regulation at transcription determines how many primary RNA molecules are made. RNA processing determines which mature isoforms exist. Nuclear export controls access to cytoplasmic ribosomes. mRNA stability determines how long translation can continue, and translation initiation affects how often ribosomes load. Protein modification and degradation finally determine active-protein abundance. A measured fall in protein therefore does not automatically identify transcription as the controlled step.

Nucleases degrade mRNA, and transcript lifetimes range from minutes to much longer periods. A short-lived mRNA lets a cell alter protein synthesis rapidly when a signal changes. Small non-coding RNAs can pair with target mRNAs and guide complexes that degrade the RNA or inhibit translation. RNA silencing therefore changes expression without altering the target gene's DNA sequence.

Epigenetics concerns persistent changes in gene activity associated with chromatin state rather than changes in nucleotide sequence. Chemical marks and their binding proteins form part of an epigenome. Because marks can be copied or re-established when cells divide, differentiated cell lineages can maintain distinct expression programs even though they carry the same genes.

DNA methyltransferases add methyl groups to cytosine, commonly at CpG sites. Heavy methylation of a promoter CpG island is often associated with reduced transcription because it can hinder factor binding or recruit proteins that build repressive chromatin. This is a strong general relationship, not a rule that every methyl group anywhere turns off the nearest gene.

Histone tails can be chemically modified. Acetylation of lysine residues generally reduces the positive charge involved in DNA–histone association and recruits proteins associated with open chromatin, increasing accessibility. Histone deacetylation is often associated with condensation and repression. Histone methylation can mark active or inactive chromatin depending on which residue is modified, so “methylation always silences” is unsafe when discussing histones.

A muscle cell and an intestinal epithelial cell contain largely the same DNA but require different proteins. Networks of transcription factors activate lineage-specific genes, while chromatin modifications help stabilize accessible and inaccessible regions. The cell identity is not caused by removing unused genes. It is maintained by a regulatory state that can persist through mitosis and still respond to signals.

Epigenetic change alters phenotype through expression, not genotype. It can be reversible and can occur faster than a DNA-sequence change spreading through a population by selection. That speed provides phenotypic flexibility, but it does not mean an environmentally induced epigenetic state is necessarily adaptive or inherited. Evidence must distinguish changes within one organism, mitotic inheritance across cells and transmission through gametes.