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Lecture 7

BCH447 Lecture 7.docx

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Larry Moran

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BCH447 lecture 7 The evolution of sex... We are not talking about sex. We are talking about the reasons behind why sex developed. With yeast we get haploid yeast... And they can replicate and make more haploids.... Furthermore, the yeast also have a sequence that defines a sex... Some are a type and others are alpha type sex. It isimagined that in the beggining it was two different genes.... But the two different mating types can come together, fuse and form a diploid cell. And those diploid cells can also divide mitotically and keep surviving. The diploid cell will undergoe meiosis and form 4 spores... Usually form during difficult conditions. The question is, what is the advantage of going diploid and then going haploid again. What we are talking about is outcrossing. Redistributing alleles. We are also talking about recombination. Are diploid cells better off then haploid cells. Keep in mind that prokaryotes are still here... So the question is... Why do eukaryotes mainly do sexual haploid diploid stuff.... Perhaps being diploid helps mask recessive genes... But this is not an explanation of sex... Its an explanation for diploidy. Dont get confused between the evolution of sex and the maintanance of sec. Once evolved... Things may have happened to make sex irreversible... We wanna know the origin and evolution of sex. Note begging the question means avoiding the question. Mixing of genes allows for a population to survive because it contains more variation in the proteins formed.... The assumption is that sex was an accident... That a diploid evolved by accident and was maintained because of genes allowing populations to survive.... Mixing of genes may also allow for the evasion of deleterious mutation buildup... This is mullers ratchet... Muller said that the accumulation of slightly deleterious mutations over time in a population could cause the extinction of that population... Perhaps sexual reproduction allows the reshuffling of genes... So that mullers ratchet is no longer a problem. Sometimes the slightly deleterious population may survive over wildtype... As this happens over and over again... We will get a species with too much burden. Mixis... Sex is advantageous because sex provides more variability. The assumption is that either sex was an accident. Orit was faster and better at mixing up alleles. With mixis argument it may be faster to mix two alleles that have recessive regions that will complement each other. But, there is a lot of chance involved... And as such it may not be such a good argument. Is mixis and recombination enough to induce a selective advantage for sex?.. No. The probelm with these explanations is that we are selecting for the future. Sex is important because we can bring together alleles that are on different homologous chromosomes... Here we are not talking about the advantage of sex, but the advantage of recombination. Recombination may help increase diversity in a population. In order for recombination to occur is so that the alleles must be on the same chromosome... And close enough on the chromosomes that they segregate together... They are linked. If they are on different chromosomes then we are not talking about recombination, bu
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