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Chapter 16 · Part 6 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 II and how meiosis differs from mitosis

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Meiosis I leaves two haploid cells whose chromosomes are still pairs of sister chromatids. Meiosis II finishes the job by separating those sisters, and it works almost exactly like mitosis.

Meiosis II

After meiosis I and cytokinesis, each cell goes straight into meiosis II. The DNA is not replicated again. Each cell runs through prophase II, prometaphase II, metaphase II, anaphase II, and telophase II, just as in mitosis:

The only difference from mitosis is the starting point. For a cell with 2n = 6, mitosis begins with six pairs of sister chromatids (12 chromatids). Each cell entering meiosis II has only three pairs (six chromatids), one for each homologous pair.

After cytokinesis, the two cells have become four. Each has one complete set of chromosomes, and each chromosome is now a single chromatid.

A diploid cell with two pairs of replicated chromosomes, red and blue, divides in meiosis I into two cells, each holding one replicated chromosome from each pair. Each of those divides in meiosis II into two cells, giving four haploid cells at the bottom, each with two single, unreplicated chromosomes.
Figure 16.6 The diploid cell starts with two pairs of replicated homologs. Meiosis I separates the homologs. Each of the two new cells holds one chromosome from every pair. Meiosis II separates the sister chromatids. Each of the four final cells holds one unreplicated set.
Check yourselfIn anaphase I and anaphase II, what separates in each?

In anaphase I, homologous chromosomes separate, and each sister pair stays together. In anaphase II, the sister chromatids themselves separate.

Mitosis vs. meiosis

Mitosis makes two diploid cells that are genetically identical to the mother cell. Meiosis makes four haploid cells, each with one complete set of chromosomes, that usually differ genetically from each other. A handful of differences in the steps account for those outcomes.

Event Mitosis Meiosis I Meiosis II
DNA replication Before mitosis Before meiosis I None before meiosis II
Synapsis in prophase No Yes, bivalents form No
Crossing over Rare Common Rare
Attachment in prometaphase Each sister pair to both poles Each sister pair to one pole Each sister pair to both poles
Metaphase alignment Sister pairs, single row Bivalents, double row Sister pairs, single row
What separates in anaphase Sister chromatids Homologs Sister chromatids
Result (diploid mother cell) Two diploid cells Two haploid cells Four haploid cells

Naming the phase from a picture

Exam questions often show a dividing cell and ask which phase it’s in. Start from the diploid number, then ask two things: how many replicated chromosomes are lined up, and are they in a single row or paired as bivalents?

Check yourselfAn organism has a diploid number of 10. A cell shows 10 replicated chromosomes in a single row at the metaphase plate. Which phase is it in?

Metaphase of mitosis. Metaphase I would pair those 10 as 5 bivalents in a double row, and metaphase II would show only 5 sister pairs in a single row.

Practice questions

  1. 1 of 3

    Which describes the cells produced when one diploid cell completes meiosis?

  2. 2 of 3

    What happens to the DNA between meiosis I and meiosis II?

  3. 3 of 3

    Looking at a dividing cell, you see each replicated chromosome paired tightly with its homolog, four chromatids side by side. Which phase is the cell in?