Re: Layman's question on Newsweek article

Lorentz wrote:
Most of our DNA is not doing much other than maybe confusing viruses and
occupying space. A lot of it is old viruses. Random changes don't seem to
make much difference in many of these segments so you can use the average
rate of change in them to roughly measure time. If the stuff is active it's
a whole other story.

Is noncoding DNA used in these studies.

Sometimes yes, sometimes no.

That is, do they look at
mutations in codons that don't code for an amino acid?

Actually, if they don't code for amino acids (or are not stop codons),
then they aren't codons at all. A codon is a three-base sequence that,
when translated into RNA, is read by a ribosome.

The assumption
has long been made that this is "junk" DNA with no uses.

Not true. Most of it is junk, but a small percentage is not, and this
has long been known.

If that was
true, they would make the perfect DNA clock.

Not true either. There are differences in mutation rate among sequences
that would make a clock imperfect even if all evolution were neutral.

Recent studies show they
may have some selective value as place holders.
Although such DNA probably has some uses, I would think their bias
with respect to DNA dating would be far less than that of coding DNA.

Maybe. Depends on the coding DNA. There are unfortunately differences in
evolutionary rates among taxa and among sequences within taxa, even in
neutrally-evolving DNA.
.

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