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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]
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]
↑Carey, Nessa 2017. Junk DNA: a journey through the dark matter of the genome. Columbia University Press. ISBN 9780231539418
↑Elgar G. & Vavouri T. 2008. Tuning in to the signals: non-coding sequence conservation in vertebrate genomes. Trends in Genetics. 24 (7): 344–52. [1][permanent dead link]