Some non-coding DNA is transcribed into functional non-coding 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 non-coding DNA varies greatly among species. For example, over 98% of the human genome is noncoding DNA, while only about 2% of a typical bacterial genome is non-coding DNA.
Initially, a large proportion of non-coding DNA had no known biological function. It was known as "junk DNA", particularly in the press. However, it has been known for years that 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 likely, but 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.
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