Universal Genetic Code and DI

From: bivalve (bivalve@mail.davidson.alumlink.com)
Date: Wed Oct 10 2001 - 13:00:35 EDT

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    I have located my reference on the "universal" genetic code.
    Graur and Li, 2000, Fundamentals of Molecular Evolution
    2nd ed. References below are from their discussion.

    In any one genome, there are typically only a few
    departures from the universal code. No one organism has
    all the exceptions. For example, vertebrate mitochondrial
    genomes have four exceptions. Departures from the
    universal code typically occur in reduced genomes (e.g.,
    mitochondria) with GC mutational biases. Actually, Graur
    and Li claim that they are always associated with such
    conditions, but I do not know myself whether there might be
    some exceptions, e.g. some Paramecium species have
    non-standard codes coupled with inordinate amounts of
    DNA. At any rate, these nonstandard genetic codes
    typically occur in those situations where evolutionary
    pressures might be expected to favor them. This is not
    evdence against evolution.

    Jukes, 1985 proposed the codon-capture hypothesis to
    explain departures from the universal genetic code.
    Castresana et al., 1998 provide an example in which two
    steps in the process can actually be seen in living

    AAA normally codes for lysine. In hemichordate
    mitochondria, it is unassigned. In echinoderm
    mitochondria, it is asparagine. The evolutionary process
    starts with the replacement of AAA in the genome with AAG,
    which also codes for lysine, due to mutational pressures
    favoring G. Next, the tRNA that carries lysine mutates its
    anticodon from UUU, which paired with AAA or AAG, to
    CUU, which only pairs with AAG. AAA becomes unused.
    This is the condition in hemichordates (which appear from
    18S to be the sister taxon to echinoderms). The next step
    is a mutation in the asparagine tRNA. By changing the
    adjacent U to a C, the GUU of the asparagine tRNA
    becomes capable of recognizing AAA as well as its usual
    AAU and AAC. At this point, AAA can reappear as a
    functional codon if some of the AAU or AAC codons for
    asparagine mutate to AAA. This is the condition in
    echinoderms. Throughout all this, the translation of a
    particular gene would remain unchanges as long as the
    mutations kept pace with each other.
    Codon usage is highly uneven, so the loss of rare codons
    and capture by other tRNAs can occur with minmal impact
    on the genome.

    There is some evidence for a very different genetic code
    before the origin of any modern lineage of organisms.
    Inconveniently for the Discovery Institute, this consists of
    evidence for the evolution of a complex system (the modern
    universal code) rather than casting doubt on the
    evolutionary connectedness of organisms. Several tRNAs
    appear to represent ancient duplications of other tRNAs.
    This implies that the common ancestor of all living
    organisms was getting along with a greatly reduced
    version of the present system until gene duplication
    provided the opportunity to specialize into the modern level
    of complexity.

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