Re: Re: junk DNA or buffer DNA?

CurtAdams (
Wed, 18 Mar 1998 04:21:58 EST

In a message dated 3/17/98 9:57:19 PM, you wrote:

>I would think lower wavelength EM radiation would be screened by the
>massive clumps of DNA that exist in a eukaryotic cell, especially
>non-coding sequences which(interestingly enough) tend to clump up to a very
>high degree in a bundle of nucleotides and histones. Maybe the low
>wavelength I'm talking about it not very common (cosmic/gamma rays, maybe?)

Even an x-ray barrels through 4 gigabases of DNA like it's not there 99.999%
of the time. The only wavelengths DNA picks up well are some of the UV,
and even there DNA absorption is negligeable compared to protein absorption.

>Try most carcinogens. Most carcinogens work by binding to areas of DNA

>during cell division. The replication process has to skip this part of the

>DNA, causing a massive deletion. Obviously, if such a mutagen is bound, it

>will only mutate that particularly area, and no others. Thus, it is

>"cleared" in this sense.

Only relevant for a carcinogen that spends an inordinately large proportion
of its time bound to DNA. For a carcinogen that preferentially binds to DNA
as opposed to other cell constituents, yes, junk DNA would shield.

>hmmm... I think I may have something there, what do you think?

I should think you could find a carcinogen with a strong preference for
binding to DNA. That would indicate the junk has value in at least some
situations. To show that it's evolutionarily significant, you'd need
to show that a reasonably large proportion of evolutionarily significant
carcinogens had this property of preferentially binding DNA. That would
be hard, because, AFAIK, there's no good grip on which carcinogens are
evolutionarily significant.

>> The idea that "junk" DNA provides a distraction for all those transposons

>> hopping around has been suggested, although I don't know where.

>Also a good explanation. A good way to test this hypothesis would be to

>look for transposon binding/recombination sequences in junk DNA.

Not necessary. Junk DNA doesn't have to attract things better than useful
DNA, just equally well. In the case of transposons that's no problem.
However, junk DNA often does attract transposons as transposons often
need DNA repeats or inverted repeats to do their thing. Repeats are
relatively uncommon in coding DNA. Watch out for your causation, though;
a big reason for all those repeats in junk DNA is that transposons (
which have lots of repeats) blew their exits and got stuck.