Ken and others,
The "98% of the DNA
...(as),, 'non-coding'" may not be entirely correct. There
are more than 1,000 reports of evident to the contrary in diverse
organisms.
Perhaps it is better to state that the
"98%" (other sources say 95%) 'non-coding' is simply '98%' with unknown
function. Here are some of the contrary evidence - there are quite a
few studies that show that these so-called 'junk DNA' regions may be (1)
physically blocking transcription of adjacent genes, (2) regulating gene
_expression_ during development, (3) enhancers for transcription of proximal
genes, and (4) regulating translation of proteins.
There are also interspersed nuclear elements (L1 elements) which may be responsible for
the plasticity of the genome, which are likely jumping genes. There may be
50-60 of these in the human genome - certain sequences resemble similar
sequences in bacteria. For a nice discussion of the possibilities
that these 'non coding' DNA sections may actually be some kind of language, see
a nice article by F. Flam - "Hints of a language
in junk DNA", Science 266:1320, 1994.
If you also take a look at the genetic
code, there are six codons for arginine alone, which suggests that if one of the
bases in the middle of the codon was erroneous, it wouldn't matter during
transcription - arginine is still going to become part of the peptide
chain. So the redundancy of the code itself for some
amino acids may offset some of these mutations. More critical
would be amino acids that have only two codons, where it would make sense that
erroneous base duplications or substitutions here would have a higher
probability for the appearance of a mutation.
It also makes sense that the appearance of
a developmental mutation later rather than earlier in embryonic development
is likely to have a greater effect on the descendent cells versus mutations that
might occur in our post-natal cells.
I'm not certain that I'd want to
categorically admonish the 'deleterious' effects of mutations - even 'some'
of the time, rather than 'most' of the time. I don't think anyone
really has a good handle on mutation rates in general in the absence of
mutagenic substances, given the nature of the 'self-correction' that occurs
in nuclear enzymes that are guarding against such mutations, such as excision of
thymine-thymine dimers, and so on.
Surely there is abundant evidence that UV
light and nasty stuff like benzene 'up' these rates (and we'll never know for
sure how many of us were 'poisoned' by the aniline dye in ditto copies we
received in school - and now I'm really dating myself!!!).
It may very well be true that the
'non-coding' regions that show mutations have no observable effect.
But maybe reserving judgement on such is the most prudent way to
proceed.
Bob
The conventional wisdom is that most mutations are deleterious.
But if 98%
of the DNA is noncoding (and with certain
other assumptions that seem of
only peripheral
relevance), wouldn't it be more accurate to say that a
considerable majority of mutations are harmless?
Just a bit of food for thought.
Ken
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