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AP Biology · Unit 5: Heredity

5.1 Meiosis

Making gametes

Meiosis produces gametes (sperm and eggs). It reduces the chromosome number by half, turning one diploid (2n) cell into four haploid (n) cells. This halving is essential so that fertilization restores the normal diploid number.

Two divisions

Meiosis has two rounds of division. In meiosis I, homologous chromosomes separate (this is the reduction step). In meiosis II, sister chromatids separate — much like mitosis. The end result is four haploid cells, each genetically unique.

Key terms

Meiosis
Cell division that produces four haploid gametes from one diploid cell.
Haploid vs. diploid
Haploid (n) = one set of chromosomes; diploid (2n) = two sets.
Gamete
A haploid reproductive cell (sperm or egg).
Meiosis I
The division where homologous chromosomes separate (reduction division).
Meiosis II
The division where sister chromatids separate, like mitosis.

Try an AP-style question

Data

A diploid cell with 2n = 8 undergoes meiosis.

How many chromosomes are in each cell at the end of meiosis I, and at the end of meiosis II?

  1. 8 (each with one chromatid), then 8 (each with one chromatid)
  2. 4 (each with one chromatid), then 2 (each with one chromatid)
  3. 4 (each with two chromatids), then 4 (each with one chromatid)
  4. 8 (each with two chromatids), then 4 (each with one chromatid)
Show the answer

C. Meiosis I separates homologs, halving the chromosome number (8 → 4, still duplicated). Meiosis II separates sister chromatids (4 unduplicated).

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