Question regarding DNA

Discussion in 'Health and medical' started by Michael Ragland, Feb 29, 2004.

  1. 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
     
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  2. Malcolm

    Malcolm Guest

    "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.
     
  3. Ron Okimoto

    Ron Okimoto Guest

    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.
     
  4. Guy Hoelzer

    Guy Hoelzer Guest

    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
     
  5. << 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.
     
  6. 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
     
  7. Jim Menegay

    Jim Menegay Guest

    [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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