Slightly less in brief:
The genome – the DNA that holds all the instructions for building an organism – doesn’t actually do anything. Proteins do most of the work in a cell. Proteins of all shapes and sizes, along with a few other types of molecule, are the cellular machinery.
The genome is just a blueprint for this machinery. Many short sections of the genome – the genes – describe how to build proteins. Certain proteins read genes and make temporary copies, which other bits of cellular machinery then use to build new proteins.
So which genes get read?
There are many ways to control gene expression. Some proteins suppress genes by covering them up. Other proteins wind up DNA like a spool, making it hard to read; others spread it out. A few proteins traverse large portions of the genome, attaching tiny molecules to each bit of DNA that block or encourage expression.
All this genome-modifying, epigenetic machinery is also expressed from genes. The expression or suppression of genes that create epigenetically-involved proteins can determine if a cell grows or dies, becomes a blood cell or a skin cell.
Some types of cancer grow when malfunctioning epigenetic machinery stops regulating growth genes. Other cancers arise when epigenetic malfunctions allow the genome to spread out, leading to genetic instability and errors.
Significantly, and unlike the genome, the epigenome – all the epigenetic controls on a cell’s genome – changes with time, influenced by bodily development, environmental factors, and medical drugs. This makes epigenetics a powerful tool for understanding and modifying life.
Image: Visualization of DNA wrapped around two histones (organizing protein spools). Credit: Walter and Eliza Hall Institute of Medical Research.