- #1
kotreny
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The purpose of meiosis is to create genetic variation unattainable through mitosis by, in effect, randomly replacing one homolog from each homologous chromosome pair with another one from a different individual. There is also an exchange of genes between homologs before this replacement occurs, generating even more variation. Variation, of course, is a favorable trait.
To achieve the results of meiosis, it is necessary to separate the two members of every homologous pair, so that each set of homologs may end up in a gamete ready to combine with another. This occurs in meiosis I. For crossover, it is necessary to place the homologs closely adjacent to each other, so that they may form chiasmata prior to anaphase I. This, too, occurs in meiosis I.
What I don't understand is where DNA replication comes into this. As a result of sister chromatid formation earlier in S phase of the cell cycle, meiosis is forced to proceed with a second phase, meiosis II, in which the sisters are pulled apart like in mitosis.
This brings me to my question: Does chromosomal duplication have any role whatsoever in producing the positive results--with respect to evolution--of meiosis? Or is it merely a vestige of mitosis, which I suppose meiosis evolved from? My guess is that S phase in cells destined to produce gametes is simply an evolutionary artifact of mitosis, and that S phase was not disposed of because then the cell would have to produce two signals (or one long-lasting signal) in order to both cancel S phase and differentiate meiosis from mitosis. This, I suppose, is harder than generating only one quick signal to initiate meiosis as we know it, even though this form of meiosis involves processes that might otherwise be superfluous.
Thanks in advance for any replies, even if only a simple affirmation of what I've written.
To achieve the results of meiosis, it is necessary to separate the two members of every homologous pair, so that each set of homologs may end up in a gamete ready to combine with another. This occurs in meiosis I. For crossover, it is necessary to place the homologs closely adjacent to each other, so that they may form chiasmata prior to anaphase I. This, too, occurs in meiosis I.
What I don't understand is where DNA replication comes into this. As a result of sister chromatid formation earlier in S phase of the cell cycle, meiosis is forced to proceed with a second phase, meiosis II, in which the sisters are pulled apart like in mitosis.
This brings me to my question: Does chromosomal duplication have any role whatsoever in producing the positive results--with respect to evolution--of meiosis? Or is it merely a vestige of mitosis, which I suppose meiosis evolved from? My guess is that S phase in cells destined to produce gametes is simply an evolutionary artifact of mitosis, and that S phase was not disposed of because then the cell would have to produce two signals (or one long-lasting signal) in order to both cancel S phase and differentiate meiosis from mitosis. This, I suppose, is harder than generating only one quick signal to initiate meiosis as we know it, even though this form of meiosis involves processes that might otherwise be superfluous.
Thanks in advance for any replies, even if only a simple affirmation of what I've written.