How does meiosis halve the chromosome number and produce gametes?
Topic 5.1 Meiosis: explain how meiosis produces four haploid cells from one diploid cell, and how it differs from mitosis.
A focused answer to AP Biology Topic 5.1, covering the two divisions of meiosis, homologous chromosomes, the reduction from diploid to haploid, and how meiosis differs from mitosis, with a worked chromosome-count problem.
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What this topic is asking
The College Board (Topic 5.1) wants you to explain how meiosis produces four haploid cells (gametes) from one diploid cell through two rounds of division, and how this differs from mitosis. You should be able to track chromosome number through each stage.
Diploid, haploid and homologous chromosomes
The whole point of meiosis is to take a diploid cell and reduce it to haploid gametes, so that fertilization (the fusion of two haploid gametes) restores the diploid number.
The two divisions
Before meiosis begins, DNA is replicated once (in S phase), so each chromosome is a pair of identical sister chromatids. Then there are two divisions with no DNA replication in between.
So two divisions are needed: the first separates homologues (halving the number), the second separates sister chromatids (cleaning up the duplicated copies). Meiosis is also where the main sources of genetic variation arise, through crossing over and the independent assortment of homologous pairs, which is covered in the next topic.
Meiosis versus mitosis
Try this
Q1. Identify what is separated in anaphase I and in anaphase II. [2 points]
- Cue. Anaphase I separates homologous chromosomes; anaphase II separates sister chromatids.
Q2. Explain why meiosis must halve the chromosome number. [2 points]
- Cue. Fertilization fuses two gametes; if gametes were diploid the offspring would have double the chromosome number, so gametes must be haploid to keep the number constant across generations.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)4 marksSection II (long FRQ excerpt). A diploid organism has a chromosome number of 2n = 8. (a) Calculate the number of chromosomes in each cell at the end of meiosis I and at the end of meiosis II. (b) Explain why meiosis, but not mitosis, requires two rounds of division to produce gametes.Show worked answer →
A 4-point calculate-and-explain FRQ on chromosome number.
(a) Calculate (2 points): (1 point) after meiosis I the homologous pairs are separated, so each cell has chromosomes (still as sister chromatids); (1 point) after meiosis II the sister chromatids separate, so each cell still has 4 chromosomes (now single).
(b) Explain (2 points): (1 point) meiosis must halve the chromosome number from diploid to haploid so that fertilization restores the diploid number; (1 point) the first division separates homologous pairs (reducing the number) and the second separates sister chromatids, so two divisions are needed.
Markers reward the correct counts and linking the two divisions to halving the chromosome number.
AP 2021 (style)1 marksSection I (multiple choice). What is separated during anaphase I of meiosis? (A) Sister chromatids. (B) Homologous chromosomes. (C) Single chromosomes into two nuclei. (D) Centrioles.Show worked answer →
The correct answer is (B).
In anaphase I, homologous chromosomes are pulled to opposite poles, which is what reduces the chromosome number from diploid to haploid. Sister chromatids stay together until anaphase II. This is the key difference from mitosis, where homologues are never paired and separated.
Related dot points
- Topic 5.2 Meiosis and Genetic Diversity: explain how crossing over, independent assortment and random fertilization produce genetic variation.
A focused answer to AP Biology Topic 5.2, covering crossing over, independent assortment and random fertilization as the three sources of genetic variation, with a worked calculation of gamete combinations.
- Topic 5.3 Mendelian Genetics: apply the laws of segregation and independent assortment to predict genotype and phenotype ratios.
A focused answer to AP Biology Topic 5.3, covering the laws of segregation and independent assortment, Punnett squares, monohybrid and dihybrid crosses, and the chi-square test for goodness of fit, with worked calculations.
- Topic 5.6 Chromosomal Inheritance: explain the chromosomal basis of inheritance, including sex determination and the consequences of nondisjunction.
A focused answer to AP Biology Topic 5.6, covering the chromosome theory of inheritance, sex determination, linkage, nondisjunction and aneuploidy, with a worked example of nondisjunction.
- Topic 4.5 Cell Cycle: describe the phases of the cell cycle, including interphase and mitosis, and explain how the events of each phase produce two genetically identical cells.
A focused answer to AP Biology Topic 4.5, covering G1, S, G2, the phases of mitosis, cytokinesis and G0, and how the cycle produces two genetically identical daughter cells, with a worked timing calculation.
- Topic 1.6 Nucleic Acids: describe the structural similarities and differences between DNA and RNA and explain how the directionality and base pairing of nucleic acids support their function.
A focused answer to AP Biology Topic 1.6, covering nucleotide structure, the antiparallel double helix, base pairing, the 5' to 3' directionality, and the structural differences between DNA and RNA.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)