In genetics, "junk DNA" or noncoding DNA describes components of an organism's DNA sequences that do not encode for protein sequences.
In many eukaryotes, a large percentage of an organism's total genome size is noncoding DNA, although the amount of noncoding DNA, and the proportion of coding versus noncoding DNA varies greatly between species.
Much of this DNA has no known biological function. However, many types of noncoding DNA sequences do have known biological functions, including the transcriptional and translational regulation of protein-coding sequences.
Other noncoding sequences have likely but as-yet undetermined function, an inference from high levels of homology and conservation seen in sequences that do not encode proteins but appear to be under heavy selective pressure.
Junk DNA Term
Junk DNA, a term that was introduced in 1972 by Susumu Ohno,
is a provisional label for the portions of a genome sequence of a for
which no discernible function has been identified.
According to a 1980 review in ''Nature'' by Leslie Orgel and
Francis Crick, junk DNA has "little specificity and conveys little or
no selective advantage to the organism".
The term is currently, however, a somewhat outdated concept,
being used mainly in popular science and in a colloquial way in
scientific publications, and may have slowed research into the
biological functions of noncoding DNA.
Several lines of evidence indicate that many "junk DNA"
sequences have likely but unidentified functional activity, and other
sequences may have had functions in the past.
Still, a large amount of sequence in these genomes falls
under no existing classification other than "junk". For example, one
experiment removed 1% of the mouse genome with no detectable effect on
This result suggests that the removed DNA was largely
nonfunctional. In addition, these sequences are enriched for the
heterochromatic histone modification H3K9me3.
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