non-coding DNA

Non-coding DNA is all the DNA that does not spell out a protein. Once dismissively called junk DNA, it makes up the overwhelming majority of a genome like ours — and far from being useless, much of it is busy doing other kinds of work.

It is a mixed category. Some non-coding DNA is regulatory: promoters, enhancers, and other switches that decide when, where, and how strongly nearby genes are turned on. Some is transcribed into RNA that is never translated into protein but still functions, such as transfer RNA, ribosomal RNA, and microRNA. Other portions are structural, like the centromeres and telomeres that keep chromosomes intact, while still others are repetitive sequences and remnants of ancient viruses.

In humans, roughly ninety-eight to ninety-nine percent of the genome is non-coding. How much of it is functional is still actively studied: some clearly matters a great deal, some appears to be inert, and the boundary between the two is an open scientific question rather than a settled fact.

The honest takeaway is that non-coding does not mean unimportant. Mutations in regulatory non-coding DNA can affect health and traits, even though no protein sequence changes, which is one reason genome studies look well beyond the coding regions.