Non-coding DNA

Non-coding DNA sequences are parts of an organism's DNA which do not code for protein sequences. It is often called junk DNA.^[2]

Bladderwort (Utricularia) has 3% noncoding DNA,^[1] which is low for flowering plants.

Some non-coding DNA is transcribed into functional non-coding RNA molecules (e.g. transfer RNA, ribosomal RNA, and regulatory RNAs). Other DNA sequences are not transcribed, or give rise to RNA transcripts of unknown function. The amount of non-coding DNA varies greatly between species. For example, over 98% of the human genome is noncoding DNA,^[3] while only about 2% of a typical bacterial genome is non-coding DNA.

At first, much non-coding DNA had no known biological function. It was called junk DNA, particularly in the press. But many non-coding sequences are functional. These include genes for functional RNA molecules and DNA sequences such as "start replication" signals, centromeres, and telomeres.

Other noncoding sequences have not-yet-discovered functions. This is inferred from the high levels of sequence similarity seen in different species of DNA.

The Encyclopedia of DNA Elements (ENCODE) project^[4] suggested in September 2012 that over 80% of DNA in the human genome "serves some purpose, biochemically speaking".^[5] This conclusion was strongly criticized by some other scientists.^[6][7]

Historical note

The term "junk DNA" was first used by Charles Ehret and Gérard de Haller in a paper.^[8] Widely influential was Susuno Ohno's 1972 paper.^[9] The term has been replaced by the more neutral term "non-coding DNA".