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Chapter 16 · Part 5 of 10
  1. Chapter overview
  2. 1Chromosome sets and homologous pairs
  3. 2The cell cycle and its checkpoints
  4. 3Sister chromatids and the spindle
  5. 4The phases of mitosis
  6. 5Meiosis I separates homologs
  7. 6Meiosis II and how meiosis differs from mitosis
  8. 7Asexual reproduction and the cost of sex
  9. 8Genetic diversity and sexual life cycles
  10. 9Chromosome shapes and changes in structure
  11. 10Changes in chromosome number

Meiosis I separates homologs

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Mitosis keeps the chromosome number the same. Gametes need half as many: if a human sperm and egg each carried 46 chromosomes, the fertilized egg would have 92. Meiosis is the process that turns one diploid cell into four haploid cells, each with one complete set of chromosomes. Its name means “to make smaller.”

One copy, two divisions

Meiosis starts after the cell has gone through G₁, S, and G₂, so every chromosome is already a pair of sister chromatids. Then come two divisions in a row, meiosis I and meiosis II, with no DNA replication in between. Each division runs through prophase, prometaphase, metaphase, anaphase, and telophase, numbered I or II.

The two divisions separate different things. Meiosis I separates homologs from each other. Meiosis II separates sister chromatids. The result is that each of the four cells gets exactly one chromosome from every homologous pair, not a random handful.

Pairing up and crossing over

Two events at the start of meiosis I set it apart from mitosis.

First, homologs find each other. Each replicated chromosome lines up side by side with its homolog, so four chromatids lie together: two sisters from one parent’s chromosome and two from the other’s. This four-chromatid group is a bivalent, also called a tetrad, and forming it is synapsis. In most species, a zipper-like protein structure holds the homologs together while they’re paired.

Second, the paired homologs trade pieces. In crossing over, a chromatid of one homolog and a chromatid of the other break at matching points and swap segments. Afterward, the arms of the homologs start to pull apart but stay linked where the swap happened. Each of these X-shaped connections is a chiasma (plural, chiasmata), named for the Greek letter chi (χ). Every chromosome usually has at least one crossover, and large chromosomes can have many.

Homologs Bivalent Crossingover Chiasma
Figure 16.5 After crossing over, two of the four chromatids are mixtures of the two parents’ chromosomes. The chiasma is where the swap happened, and it holds the homologs together until anaphase I.
Check yourselfWhat does a bivalent consist of, and when does it form?

A pair of homologs, each made of two sister chromatids, so four chromatids in all. It forms by synapsis at the start of meiosis I (prophase I) and never in mitosis.

The phases of meiosis I

  1. In prophase I, the replicated chromosomes condense, homologs pair into bivalents, and crossing over occurs. The nuclear envelope starts to break up.
  2. In prometaphase I, the nuclear envelope is gone and the spindle forms. Kinetochore microtubules attach, but each pair of sister chromatids attaches to just one pole, and its homolog attaches to the other.
  3. In metaphase I, the bivalents line up on the metaphase plate in a double row, homolog facing homolog. Which side each parent’s homolog faces is random for every pair.
  4. In anaphase I, the homologs separate and move to opposite poles. Each sister pair stays joined.
  5. In telophase I, the chromosomes reach the poles and loosen, and two nuclei form.

Meiosis I halves the chromosome number, so it’s called a reduction division. With cytokinesis it makes two cells, and each is already haploid: it has one member of each homologous pair, even though each chromosome is still two sister chromatids.

Check yourselfA cell with 2n = 6 finishes meiosis I. How many chromosomes and chromatids does each new cell hold?

Three chromosomes, one from each homologous pair, and six chromatids, because each chromosome is still a pair of sister chromatids.

Practice questions

  1. 1 of 3

    Why must gametes carry half as many chromosomes as body cells?

  2. 2 of 3

    In which phase does crossing over take place?

  3. 3 of 3

    Which division halves the chromosome number and is therefore called the reduction division?