Noncoding DNA

In genetics, noncoding DNA describes components of an organism's DNA sequences that do not encode protein sequences. Some noncoding DNA is transcribed into functional noncoding RNA molecules (e.g. transfer RNA, ribosomal RNA, and regulatory RNAs), while others are not transcribed or give rise to RNA transcripts of unknown function. The amount of noncoding DNA varies greatly among species. For example, fully 98% of the human genome is noncoding DNA, while only about 2% of a typical bacterial genome is noncoding DNA.

Initially, a large proportion of noncoding DNA had no known biological function and was therefore sometimes referred to as "junk DNA", particularly in the lay press. Some sequences may have no biological function for the organism, such as endogenous retroviruses. However, many types of noncoding DNA sequences do have important biological functions, including the transcriptional and translational regulation of protein-coding sequences. Other noncoding sequences have likely, but as-yet undetermined, functions (this is inferred from high levels of homology and conservation seen in sequences that do not encode proteins but, nonetheless, appear to be under heavy selective pressure).

The Encyclopedia of DNA Elements (ENCODE) project reported in September 2012 that over 80% of DNA in the human genome "serves some purpose, biochemically speaking".