Question regarding DNA



M

Michael Ragland

Guest
My biology text states the amount of adenine relative to guanine differs from one species to the
next but that the amount of adenine in DNA always equals that of thymine, and the amount of guanine
always equals that of cytosine. This is reflected in the formula A=T and G=C. What determines the
varying amounts of adenine relative to guanine amongst different species?

Michael Ragland
 
"Michael Ragland" <[email protected]> wrote in message
>
> What determines the varying amounts of adenine relative to guanine amongst different species?
>
This is something which isn't fully understood. The GC bond is more stable than the AT bond, and
the GC ratio isn't constant over the human genome. It may be that GC-rich areas are transcribed
less often.
 
Malcolm wrote:

> "Michael Ragland" <[email protected]> wrote in message
> >
> > What determines the varying amounts of adenine relative to guanine amongst different species?
> >
> This is something which isn't fully understood. The GC bond is more stable than the AT bond, and
> the GC ratio isn't constant over the human genome. It may be that GC-rich areas are transcribed
> less often.

Thermophiles have a very high GC content to their genomes. There is also codon bias in some
organisms. Mitochondrial genomes have extreme examples of codon bias. Just compare the codon
frequency in Drosophila compared to human mitochondrial DNA. This codon bias may be due to some bias
for a high AT genome in Drosophila. Chickens are sort of weird. They have a similar nucleotide
content to their genomes as mammals, but genes are GC rich. For all I know the higher GC content in
the coding regions is due to a bias for more stable codon tRNA interactions at the higher body
temperature that chickens have over most mammals.
 
in article [email protected], Michael Ragland at
[email protected] wrote on 2/29/04 10:34 AM:

> My biology text states the amount of adenine relative to guanine differs from one species to the
> next but that the amount of adenine in DNA always equals that of thymine, and the amount of
> guanine always equals that of cytosine. This is reflected in the formula A=T and G=C. What
> determines the varying amounts of adenine relative to guanine amongst different species?

The other 2 replies I have read are notably adaptationist in their perspectives. I think this source
of hypothesis formation should generally be tapped only after neutral hypotheses have been explored.
In this case the question itself seems badly phrased to me, because it seems to assume that the
answer must be an adaptive one. I would simply ask what would prevent GC content from varying among
species, just like any other aspect of the genome. If, for example, we take mutation to be random
with regard to GC content, then we would expect the GC content of independent gene pools to vary
under a neutral model of drift/mutation balance.

Guy
 
<< Thermophiles have a very high GC content to their genomes. There is also codon bias in some
organisms. Mitochondrial genomes have extreme examples of codon bias. Just compare the codon
frequency in Drosophila compared to human mitochondrial DNA. This codon bias may be due to some bias
for a high AT genome in Drosophila. Chickens are sort of weird. They have a similar nucleotide
content to their genomes as mammals, but genes are GC rich. For all I know the higher GC content in
the coding regions is due to a bias for more stable codon tRNA interactions at the higher body
temperature that chickens have over most mammals. >>

Also notice in a tRNA for ex. how heavy they GC content is in the stems.

If the GC pairs are used for more thermal stability, they support my hypothesis of life as energy
moderation.
 
On Sun, 29 Feb 2004 18:34:48 +0000 (UTC), [email protected] (Michael
Ragland) wrote:

>
>My biology text states the amount of adenine relative to guanine differs from one species to the
>next but that the amount of adenine in DNA always equals that of thymine, and the amount of guanine
>always equals that of cytosine. This is reflected in the formula A=T and G=C. What determines the
>varying amounts of adenine relative to guanine amongst different species?
>
There is a large range in bacterial species, from 25% GC genome content to 75% GC. In invertebrates
there is less range and in vertebrates it is narrower still (40%-45% GC). There are various
selectionist views on why this is so. The mutationist view is that it is basically caused by
differences in the DNA replication mutation rate. For example, if an AT-to-GC mutation error is 3
times more likely than a GC-to-AT mutation error, then, over long evolutionary time, the genome
will be 75%GC/25%AT absent any selection pressure. In a similiar vein, since the replication of
leading and lagging strands is somewhat different, there is reason to think the mutation rates
would also be different for each stand. This is an explanation for strand "skew", where the strands
form G=C (and A=T) pairings but one stand is more G and the other more C. William L Hunt
 
[email protected] (William L Hunt) wrote in message news:<[email protected]>...
> On Sun, 29 Feb 2004 18:34:48 +0000 (UTC), [email protected] (Michael Ragland) wrote:
>
> >
> >My biology text states the amount of adenine relative to guanine differs from one species to the
> >next but that the amount of adenine in DNA always equals that of thymine, and the amount of
> >guanine always equals that of cytosine. This is reflected in the formula A=T and G=C. What
> >determines the varying amounts of adenine relative to guanine amongst different species?
> >
> There is a large range in bacterial species, from 25% GC genome content to 75% GC. In
> invertebrates there is less range and in vertebrates it is narrower still (40%-45% GC). There are
> various selectionist views on why this is so. The mutationist view is that it is basically caused
> by differences in the DNA replication mutation rate. For example, if an AT-to-GC mutation error
> is 3 times more likely than a GC-to-AT mutation error, then, over long evolutionary time, the
> genome will be 75%GC/25%AT absent any selection pressure. In a similiar vein, since the
> replication of leading and lagging strands is somewhat different, there is reason to think the
> mutation rates would also be different for each stand. This is an explanation for strand "skew",
> where the strands form G=C (and A=T) pairings but one stand is more G and the other more C.
> William L Hunt

The phenomenon that Bill describes is known as "mutation pressure" and it has been known since the
1960s. Any good molecular genetics text will describe it - for example, Watson. Or, a search of the
WEB found a number of hits.
 
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