Junk DNA

Junk DNA is all the DNA in a genome with no known function that is not part of a gene.

Some sections of DNA that are not part of a gene do have a known function. For instance DNA contains many origins of replication[?] per chromosome, which define the starting point of the DNA copying process. Also some parts of the DNA outside the genes are now known to be promoters of transcription or other regulatory sequences that figure in the control of gene activity. In some cases, the positioning, rather than precise sequence, may be important. There are known examples in the yeast Saccharomyces[?].

As more functions are found the amount of DNA we now call junk DNA will likely decrease. It is also believed that junk DNA contains a pool of now defunct genes and gene fragments, known as pseudogenes, that were cast aside during evolution.

As much as 95% of the human genome is now considered junk DNA. Other species may have much more DNA than humans. For example the onion has 12 times as much. It is unlikely that the onion needs more genes than we do. Therefore it seems that the ratio of functional and junk DNA differs widely per species.

It should be noted that junk DNA is a broader category than selfish DNA. The former is an expression of our ignorance, whereas the latter is a positive hypothesis which may or may not be descriptive of particular sub-sequences of a genome.