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.Carey, Nessa 2017. Junk DNA: a journey through the dark matter of the genome. Columbia University Press. ISBN 9780231539418

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,Elgar G. & Vavouri T. 2008. Tuning in to the signals: non-coding sequence conservation in vertebrate genomes. Trends in Genetics. 24 (7): 344–52.  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 suggested in September 2012 that over 80% of DNA in the human genome "serves some purpose, biochemically speaking".  This conclusion was strongly criticized by some other scientists.

 Historical note 
The term "junk DNA" was first used by Charles Ehret and Gérard de Haller in a paper.Ehret C.F. & De Haller G. 1963. Origin, development, and maturation of organelles and organelle systems of the cell surface in Paramecium. Journal of Ultrastructure Research 23: SUPPL6:1–42. Widely influential was Susuno Ohno's 1972 paper.Ohno S. 1972. So much junk in our genome. In Brookhaven Symposia in Biology. New York: Gordon & Breach. ed: Smith H.H. 23: p366–370. The term has been replaced by the more neutral term "non-coding DNA".

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Category:DNA