AP Biology · Lesson 1 of 6 · Unit 5

Meiosis and its comparison with mitosis

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Meiosis is the bridge between cellular division and genetics: it explains how diploid parents pass on exactly half their chromosomes so that fertilization restores the diploid number across generations. On the AP Biology exam, you will analyze visual models of chromosome movement, calculate DNA mass or chromosome numbers across division stages, and contrast meiotic events with mitosis in both multiple-choice sets and Section II conceptual or model-analysis questions (especially Q4 and Q5).

By the end you’ll be able to

Explain how meiosis ensures the formation of haploid gametes in sexually reproducing diploid organisms.
Describe the events of prophase I (pairing, synapsis and chiasmata), metaphase I, anaphase I and telophase I, including the separation of homologous chromosomes.
Describe the events of meiosis II, including the separation of sister chromatids to form four haploid cells.

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What the exam asks

Track ploidy and DNA content: Given a starting diploid number ( $2n$ ) or DNA mass (often in picograms, $\text{pg}$ ), determine the chromosome count and chromatid state at any stage of meiosis I or II.
Interpret visual representations: Identify whether a diagram shows mitosis, meiosis I, or meiosis II by examining how chromosomes align at the metaphase plate (single file versus homologous pairs).
Compare processes: Explain the shared evolutionary features of mitosis and meiosis alongside their functional differences in daughter cell number, ploidy, and genetic identity.

Core ideas

Ploidy, homologous chromosomes, and sister chromatids

Diploid ( $2n$ ) somatic and germ cells contain two sets of chromosomes: one paternal set and one maternal set. Homologous chromosomes (homologs) are pairs of chromosomes of the same length and centromere position that carry genes for the same traits at corresponding loci, though they may carry different alleles.

Before meiosis begins, DNA replication occurs once during interphase (S phase). Each chromosome now consists of two identical sister chromatids joined at the centromere. Meiosis then uses two consecutive nuclear divisions (meiosis I and meiosis II) without an intervening round of DNA replication to produce four haploid ( $n$ ) gametes or spores.

Meiosis I: Reductive division

Meiosis I separates homologous chromosomes, reducing the ploidy from diploid ( $2n$ ) to haploid ( $n$ ).

Prophase I: Chromatin condenses, the nuclear envelope breaks down, and the spindle forms. Homologous chromosomes pair up gene-for-gene in a process called synapsis to form a tetrad (or bivalent, composed of four chromatids). Non-sister chromatids of homologous chromosomes exchange corresponding DNA segments (crossing over); the X-shaped sites where crossover has occurred and homologs remain physically linked are called chiasmata (singular: chiasma).
Metaphase I: Homologous pairs align in two rows along the metaphase plate. Microtubules from one pole attach to the kinetochore of one homolog, while microtubules from the opposite pole attach to its partner.
Anaphase I: Spindle fibers shorten and chiasmata resolve, pulling homologous chromosomes apart to opposite poles. Sister chromatids remain attached at their centromeres.
Telophase I and Cytokinesis: Two haploid ( $n$ ) daughter cells form. Each chromosome is still duplicated (composed of two sister chromatids).