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Principles Module 38

Objectives

• Explain the similarities and differences between meiosis and mitosis
• Describe how meiosis produces greater genetic variation

Key Points

Features of meiosis

• synapsis: homologous chr pair
  • form synaptonemal complex, binds hom. chrom. together
    • composed of variant of cohesin
• crossing over: exchange of bits of DNA btw hom. chrom.
  • results in recombination, mixing of genes
• 2 rounds of division
  • first separates hom. chrom.
    • reduces chrom number (diploid to haploid)
  • 2nd like mitosis, but no replication before it

Process

Starting with a cell that is beginning meiosis and has completed S phase:

After meiosis I, the chromosome number will be reduced from diploid to haploid, with each daughter cell receiving one of the two homologous chromosomes for each of the 2 different chromosomes in this example:

After meiosis II, the daughter cells each have a single copy of each chromosome, still having only one version of each chromosome from the previous meiotic division:

Comparison with Mitosis

• mitosis compares most directly with meiosis II in that both result in the separation of sister chromatids
• mitosis always maintains the same number of versions of chromosomes (like editions of a book)
• meiosis I reduces chromosome number by half, meiosis II separates sister chromatids
• there is no intervening interphase between meiosis I and II

In-class Activities

Questions for Practice

• Draw or describe the process of meiosis, labeling each resultant cell with its ploidy level (diploid or haploid).
• Explain two ways in which meiosis results in greater genetic variation.