Gene Expression & Epigenetics

Every cell in your body carries the same DNA, yet a neuron and a skin cell look nothing alike. The difference is which genes are switched on — this is gene expression, and it's controlled by factors including the products of other genes (transcription factors) and the environment.

Controlling expression is what drives cellular differentiation: as tissues grow and develop, different genes are turned on and off.

Epigenetics

Epigenetic changes alter gene expression without changing the DNA sequence. Two main mechanisms:

• DNA methylation — adding methyl (–CH₃) groups, usually silencing a gene by blocking transcription.
• Histone modification — chemically tagging the histone proteins DNA wraps around, tightening or loosening access to genes.

Methylation of a gene switches it off without altering the underlying DNA sequence.

Because epigenetic tags respond to the environment, they explain phenotypic differences between identical twins and between clones — and, when they misfire on genes that control cell division, they can contribute to cancer.