Non-coding DNA: Control Regions and Evolutionary Traces

The human genome spends only a small part of its DNA on coding for proteins. The rest makes no protein, and it was once called "junk DNA" simply because science had not worked out what it does. That name came from a gap in knowledge rather than from a measurement. The ENCODE Project began mapping biochemical activity across the genome in 2003, and its major releases in 2012 showed that cells use many noncoding regions. "Junk DNA" is therefore too simple a label. Regulatory regions, RNA genes, repeats, ancient viruses, centromeres, and telomeres all sit in the part that makes no protein and still change how cells behave. A centromere is the point where the cell pulls a chromosome apart during division, and a telomere is the protective cap at each end of a chromosome. Neither job shows up in the protein code.

Promoters, enhancers, and silencers are the switches. A promoter opens a gene for the copying machinery, an enhancer pushes its activity up, and a silencer pulls it down. Two cells with the same DNA can still end up as a skin cell and a nerve cell because they open different control regions. A segment that makes no protein can still decide which gene turns on, where it turns on, and for how long.