AP BiologyUnit 5: Heredity5.1
AP Biology · Unit 5 · Topic 5.1

Meiosis

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Copy once, divide twice. Meiosis I splits pairs and halves the count. Meiosis II splits chromatids.

Meiosis is a halving machine. It copies DNA once and divides twice, so each gamete gets one set of chromosomes. Then fertilization restores two sets instead of doubling the count every generation.

Key terms

  • Homologous chromosomes — a matched pair, one from each parent. Same genes in the same order, but the alleles can differ.
  • Sister chromatids — the two identical copies of one chromosome made in S phase, joined at the centromere.
  • Diploid (2n) vs haploid (n) — two chromosome sets vs one. Your body cells: 2n = 46. Your gametes: n = 23.
  • Synapsis — homologs pairing up along their length in prophase I. The paired unit, four chromatids in all, is a tetrad.
  • Crossing over — in prophase I, one chromatid from each homolog trades a matching segment. Afterward, sisters can carry different alleles.

How it works

  1. Before meiosis, DNA is copied once. Each chromosome now has two sister chromatids. There's no copying between the two divisions.
  2. Prophase I: homologs find each other, pair along their length, and cross over.
  3. Metaphase I: homologs line up at the cell's middle as pairs, not single file as in mitosis.
  4. Anaphase I: homologs separate to opposite poles. Sister chromatids stay joined. Each new cell is haploid, but each chromosome still has two chromatids.
  5. Meiosis II: no DNA copying first. Sister chromatids split at the centromere and go to opposite poles, as in mitosis.
  6. Result: four haploid cells with one chromatid per chromosome. Thanks to crossing over, all four can differ genetically.

Why it matters

Homologs splitting in anaphase I is the physical basis of Mendel's law of segregation: each gamete gets one allele for each gene. That makes meiosis the bridge to the rest of Unit 5. Expect counting questions. Count chromosomes by centromeres. A chromosome keeps two chromatids from S phase until they split in anaphase II.

Exam traps

TrapActually
Meiosis II is mitosis all over again.Both split sister chromatids. But meiosis II starts with one set of chromosomes, and no DNA copying comes before it.
DNA is copied before each meiotic division.Once only, before meiosis I. One copy, two divisions: that mismatch is what halves the count.
Anaphase I separates sister chromatids.It separates homologs. If pairs move apart and each chromosome still has two chromatids, it's meiosis I.
Gametes made by mitosis would work fine, with less variety.They'd carry two sets each. Fertilization would then double the chromosome number every generation.
Worked example

An animal species has 2n = 24, and each of its cells in G1 holds 20 picograms (pg) of DNA. For one cell going through meiosis, give the chromosome number and the DNA content per cell (a) in prophase I, (b) at the end of meiosis I, and (c) at the end of meiosis II.

  1. S phase doubles the DNA but not the chromosome count, because two sister chromatids share one centromere. Prophase I: 24 chromosomes, 40 pg.
  2. Anaphase I sends one member of each pair to each pole. Each cell: 12 chromosomes, each still two chromatids, so 20 pg.
  3. Anaphase II splits the chromatids, and each one becomes a chromosome in its own right. Each cell: 12 chromosomes, 10 pg.

(a) 24 chromosomes, 40 pg. (b) 12 chromosomes, 20 pg. (c) 12 chromosomes, 10 pg.

Meiosis I halves the chromosome number. Meiosis II halves the DNA again but leaves the count at 12. The near miss is 6 chromosomes at the end: students halve the count twice, forgetting that each separated chromatid now counts as a chromosome.

Now try it on real exam-style questions

Ten questions on Meiosis, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 5.1 Meiosis

What is the main role of meiosis in the life of an animal?

  1. A Making haploid eggs and sperm
  2. B Growing and repairing body tissues
  3. C Copying DNA before a cell divides
  4. D Fusing sperm and egg into a zygote
Show the answer

A — Meiosis makes haploid gametes, so that when a sperm and an egg fuse the zygote gets back the normal diploid number. Growth and repair of tissues use mitosis instead.

Question 2 · 5.1 Meiosis

After DNA replication, a chromosome consists of two identical copies joined at the centromere. What are these copies called?

  1. A Homologous chromosomes
  2. B Sister chromatids
  3. C Nonsister chromatids
  4. D Alleles of each gene
Show the answer

B — Replication makes two identical sister chromatids held together at the centromere. Homologous chromosomes are different: one comes from each parent, and they can carry different alleles of the same genes.

Question 3 · 5.1 Meiosis

During which stage do the two members of each chromosome pair lie side by side along their full lengths, forming a structure built from four chromatids?

  1. A Prophase of the first meiotic division
  2. B Prophase of the second meiotic division
  3. C Metaphase of the first meiotic division
  4. D Telophase of the first meiotic division
Show the answer

A — Partner chromosomes find each other and zip together along their lengths at the very start of meiosis, in the first prophase. Because each partner is already replicated, the paired structure contains four chromatids. This close alignment is what allows matching segments to be exchanged, and it is unique to meiosis: no stage of mitosis brings partners together in this way.