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2011-10_Muta - SMOUSE SPRING 2011 EVOLUTION(11:704-486...

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E VOLUTION (11:704-486) M UTATIONAL V ARIATION S MOUSE S PRING 2011 1 M UTATIONAL V ARIATION Motivation: Darwin talked about descent with modification . Next time, we’ll talk about descent . But first, we need to talk about modification . Over the short term, most organisms have a large enough reservoir of genetic variation within the population to produce a lot of variety, small to medium-size variations on a single theme. Over the long haul of evolutionary time, though, they probably do not have enough of the right kind of variation to allow a dinosaur to become a bird. For that, we have to generate something quite different, maybe several different somethings, which is what mutation is all about, changes in the genetic material. Even the resident variation represents mutation from the past. There is a steady stream of mutation, pumping novelty in, while natural selection sorts out what works the best; it’s time to talk about that stream. Nucleotide Changes - Start with simple changes in the DNA code. Recall that for eukaryotes, a gene consists of exons (that code), introns (that do not), and flanking sequences involved in gene regulation. For the moment, concentrate on the exons, but there is quite a bit of speculation to the effect that many of the important developmental changes we see in evolution may be a consequence of mutation in the introns or flanking sequences, and there is precedent. We can substitute purines for purines (A G), pyrimidines for pyrimidines (T C), called transitions . We can substitute purines for pyrimidines, or the reverse (A or G T or C), called transversions . Recall that the code involves triplets, such as: DNA RNA AA DNA RNA AA CTT GAA glutamate CAT GUA valine A transversional mutation from T A in the 2 nd position of the DNA codon yields a corresponding change of A U in the mRNA and the substitution of v aline for g lutamate in the protein chain. The 6 th AA in the -globin chain is normally a glutamate , but the sickle ( S ) allele has a valine instead. That 6 glu val change makes all the difference in the world. The triplet code is redundant, of course, and transitions are generally not serious. In several cases, a transition in the 1 st position, and most transversions in the 3 rd position don’t even change the AA at the back end. Transversions in the 1 st and 2 nd positions usually do lead to a change (see Futuyma, or the next page of this handout, for the code). In addition, we have insertions or deletions ( indels ) that shift the reading frame. A completely different set of AAs result, downstream from a frameshift, rendering protein non- functional. If a triplet of nucleotides is either removed from or substituted into the current chain, reading frame is restored, but with an extra AA or a gap of an AA in the protein.
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