Junk DNA usually refers more to the non-coding part of the genome. A hint
that it is not essential comes from the widely ranging amount of it in
different organisms. Closely related species may have a lot or a little.
Junk DNA probably has structural functions in parts of it, as another post
mentioned, but much is probably also unnecessary.
Gene duplications and pseudogenes (non-functional genes) are a somewhat
different situation. After duplication of a gene, as long as one copy
remains functional the other may mutate into a new function. Similarly, a
pseudogene may undergo further mutation to become functional again.
Relatively similar proteins can do very different things, so this can be a
very effective way of generating new molecular functions. On the other
hand, extra copies can sometimes be harmful. Most plants and various
animals do not seem to mind multiple copies of genes, but most large
duplications in humans are generally harmful.
Several important genes, including those involved in some complex molecular
systems, result from gene duplication and mutation. Examples include the
genes for color vision in primates, tRNA genes, and hemoglobin genes.
Introns, spacers, and synonymous substitutions are additional examples of
DNA for which the sequence does not seem to matter, even though they have
some functional importance.
David C.
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