5.1 Meiosis
- Syllabus
- 2025
- Topic
- 5.1
- Level
- —
Learning objectives
5.1A—Explain how meiosis results in the transmission of chromosomes from one generation to the nextExplain how meiosis results in the transmission of chromosomes from one generation to the next.• Meiosis is a process that ensures the formation of haploid gamete cells, sometimes referred to as daughter cells, in sexually reproducing diploid organisms.• Meiosis I involves the following steps:- i. Prophase I: Homologous chromosomes pair up and condense, synapsis occurs and then chiasmata may form, meiotic spindle begins to form, centrosomes move to opposite poles of the cell, and the nuclear envelope breaks down.- ii. Metaphase I: Meiotic spindle fibers align homologous pairs of chromosomes along the equator of the cell at the metaphase plate.- iii. Anaphase I: Homologous chromosomes separate, while sister chromatids remain attached, as meiotic spindle fibers pull chromosomes toward poles.- iv. T elophase I: Meiotic spindle breaks down, a new nuclear envelope develops, a cleavage furrow (animal cell) or cell plate (plant cell) forms, and cytokinesis occurs. Two haploid daughter cells are formed (at the end of meiosis I).• Meiosis II involves the following steps:- i. Prophase II: Meiotic spindle forms; sister chromatids connected at the centromere attach to meiotic spindle.- ii. Metaphase II: Chromosomes align along the metaphase plate; the kinetochore of each chromatid is attached to a microtubule extending from the poles.- iii. Anaphase II: Proteins at the centromeres break down, and sister chromatids are pulled apart and toward opposite poles in the cell.- iv. T elophase II: Meiotic spindle breaks down, a new nuclear envelope develops, a cleavage furrow (animal cell) or a cell plate (plant cell) forms, chromatids begin to decondense, and cytokinesis occurs. Four haploid daughter cells are formed, each with an unduplicated chromatid.5.1B—Describe similarities and differences between the phases and outcomes of mitosis and meiosisDescribe similarities and differences between the phases and outcomes of mitosis and meiosis.• Mitosis and meiosis are similar in the use of a spindle apparatus to move chromosomes but differ in the number of cells produced and the genetic content of the daughter cells.
How Meiosis Produces Haploid Cells
Meiosis forms haploid cells from a diploid starting cell through two divisions. Meiosis I separates homologous chromosome pairs, reducing the chromosome set; meiosis II separates sister chromatids, producing four haploid cells that can transmit chromosomes to the next generation.
| Meiosis I phase | Defining event | Chromosome consequence |
|---|---|---|
| Prophase I | Homologous chromosomes pair by synapsis; chiasmata may form; chromosomes condense and the spindle forms | Each homologous pair is organized for separation |
| Metaphase I | Homologous pairs align at the metaphase plate | Each homolog faces a different pole |
| Anaphase I | Homologous chromosomes move to opposite poles while sister chromatids remain joined | Each pole receives one homolog from every pair |
| Telophase I and cytokinesis | Nuclei may reform and the cell divides | Two haploid cells form, but each chromosome still has two sister chromatids |
| Meiosis II phase | Defining event | Chromosome consequence |
|---|---|---|
| Prophase II | A spindle forms and sister chromatids attach to spindle microtubules | Chromatids are prepared for separation |
| Metaphase II | Chromosomes align at the metaphase plate | Sister chromatids face opposite poles |
| Anaphase II | Centromere proteins break down and sister chromatids separate | Each pole receives one chromatid from each chromosome |
| Telophase II and cytokinesis | Nuclei reform, chromosomes decondense, and cells divide | Four haploid cells form with unduplicated chromosomes |
Sister chromatids do not separate in anaphase I. The reduction to haploid chromosome sets occurs because homologous chromosomes separate first; chromatids separate only in anaphase II.
Mitosis and Meiosis: Same Machinery, Different Outcomes
Mitosis and meiosis both use spindle fibers to align and move chromosomes, followed by nuclear division and usually cytokinesis. Their chromosome behavior and outcomes differ because mitosis has one division, whereas meiosis has two.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Homolog pairing | Homologous chromosomes do not pair for separation | Homologs pair during prophase I |
| What separates first | Sister chromatids | Homologous chromosomes in meiosis I |
| Number of divisions | One | Two |
| Final products | Two daughter cells | Four daughter cells |
| Chromosome set | Preserves the parent cell's chromosome number | Produces haploid cells from a diploid starting cell |
| Genetic content | Daughter cells are genetically identical to each other, barring mutation | Products are not generally genetically identical |
| Main biological role | Growth, tissue repair, and asexual reproduction | Formation of cells used in sexual reproduction |
In both processes, accurate transmission depends on spindle attachment, alignment at a metaphase plate, movement toward opposite poles, and division of the cell. The key comparison is therefore not whether chromosomes move, but which chromosome structures separate and what products result.
Meiosis II resembles mitosis because sister chromatids separate, but the overall outcomes are not the same: meiosis began with homolog pairing and a reduction division in meiosis I.