Principles Module 38


  • 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


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

cell after S phase

cell after S phase, before meiosis

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:

cells after meiosis I

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:

cells after meiosis II

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.