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September 14, 2017

Making sense of Meiosis


Singh.R.J. 1993. Plant Cytogenetics. CRC Press. Chapter 3.Cell Division pp.31-37.
Molecular Cell Biology, Lodish et al., Chapter 8
Introduction to Genetic Analysis, Griffiths et al., Chapter 3

Learning Checklist:
  1. Meiosis is the formation of four haploid gametes from a diploid cell.
  2. Meiosis creates genetic diversity in a population through independent segregation of chromosomes and through genetic recombination.
  3. The formation of haploid gametes each bearing a full complement of chromosomes requires chromosome pairing in meiotic prophase I. Random assortment of chromosomes would result in gametes that were missing some chromosomes and had duplicates of others.
  4. Chromosome pairing is also required for genetic recombination.
  5. Meiosis consists of two cycles: a disjunctional division that separates the homologous chromosomes, and a mitotic division, that separates the two sister chromatids for each chromosome. Know the stages of meiosis and be able to identify  cells in each stage. Also know the key events that occur during the five stages of meiotic prophase I.
  6. Understand how C-value changes during meiosis.
  7. Understand the data that demonstrate the correlation between DNA content per nucleus and length of the meiotic cycle.

I. Why Meiosis?

Meiosis is the physical basis of Mendel's laws of independent segregation and independent assortment. As we will see, it is  also an elaborate, metabolically costly, and sometimes faulty mode of reproduction. Considering the costs involved, what are the benefits?

A. Sexual reproduction creates genetic diversity

  1. Independent segregation of chromosomes

  2. Because each gamete receives chromosomes at random from one parent or the other,  there is a 50/50 chance of getting a given copy of any chromosome. Put another way, there are 2 possible outcomes for getting either the maternal or paternal chromosome. For a genome of n chromosomes, there are 2 n possible gametes from a single parent. For example, each human parent, with 23 chromosome pairs, can produce 2 23 =  8.4 x 10 6 possible gametes. Since each chromosome carries a unique combination of alleles for thousands of loci, progeny are always unique.
  3. Genetic recombination

  4. The unique combination of alleles contained on a given chromosome would be fixed if recombination didn't occur. Given a large enough population, recombination will occur between homologous chromosomes  at all loci, so that an almost limitless number of combinations of alleles, at different loci, can be tested by evolution.

B. In organisms with a haploid stage, meiosis may provide a "genetic cleansing" mechanism to eliminate deleterious alleles.

Flowering plants and fungi go through haploid stages as part of their life cycles. In plants, this is the gametophyte generation. Metabolically-active haploid cells must carry out all fundamental cellular functions. Since only one copy of each gene is present, there is strong selective pressure against deleterious alleles in the haploid state.

C. The problem: production of haploid gametes requires some mechanism to ensure that each gamete gets a complete set of chromosomes.

In mitosis, the two homologues for each chromosome can replicate and segregate independently and you'll always come out with a balanced set of chromosomes in each daughter cell. The strategy is simple: just attach a spindle fiber to each centromere from pole to pole, and as long as one chromatid migrates to each pole, each daughter cell will be complete.

But in meiosis, that wouldn't work. In the figure, a diploid with two sets of chromosomes, I and II, has just undergone a round of DNA replication, such that there are now two copies of each homologue. A single reduction division, as in mitosis, would still result in two balanced diploid cells. The problem arises in the second division. The figure at right shows one possible outcome for a meiosis in which spindle figers randomly attached to the kinetochore for one copy of each chromosome, during the second meiotic division. There is no orderly way to undergo a second cell division and be sure that each cell gets one and only one copy each of chromosomes I and II. This is why chromosome pairing (synapsis) is neccessary in meiotic prophase I . By keeping all homologues for a chromosome together,  a balanced segregation can occur. At the same time,  synapsis provides an opportunity for genetic recombination to occur.

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