Biology · High school and introductory college biology

Mitosis vs Meiosis Study Guide

Mitosis makes two genetically identical cells for growth and repair, while meiosis makes four genetically different sex cells with half the chromosomes. Mitosis keeps the chromosome number the same, and meiosis halves it so that fertilization can restore it. Nearly every comparison question comes down to how many divisions, how many cells and how similar they are.

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Key terms

Cell cycle: The life of a dividing cell: interphase (growth and DNA copying) followed by mitosis and cytokinesis.
Interphase: The longest part of the cell cycle, when the cell grows and copies its DNA (in the S phase) before dividing.
Chromosome: Structure of tightly packed DNA and proteins that carries genes.
Chromatid: One of the two identical copies of a replicated chromosome, joined to its partner at the centromere.
Centromere: The region where two sister chromatids are joined and where spindle fibers attach.
Homologous chromosomes: A pair of chromosomes, one from each parent, that carry the same genes in the same order but may have different versions of them.
Diploid: Having two sets of chromosomes, one from each parent. It is written 2n, and human body cells have 46.
Haploid: Having one set of chromosomes. It is written n, and human gametes have 23.
Gamete: A sex cell (egg or sperm) that is haploid and joins with another gamete in fertilization.
Mitosis: Cell division that produces two genetically identical diploid daughter cells. It is used for growth, repair and asexual reproduction.
Meiosis: Two rounds of cell division that produce four genetically different haploid cells, used to make gametes.
Crossing over: Exchange of DNA between homologous chromosomes during prophase I of meiosis, creating new combinations of genes.
Independent assortment: Random orientation of homologous pairs at metaphase I, so each gamete gets a different mix of maternal and paternal chromosomes.
Cytokinesis: The division of the cytoplasm that splits one cell into two after the nucleus has divided.

Side by side

•Mitosis: one division, two daughter cells, genetically identical, chromosome number stays the same (2n to 2n). Used for growth, repair and asexual reproduction.
•Meiosis: two divisions, four daughter cells, genetically different, chromosome number is halved (2n to n). Used to make gametes.
•Only meiosis has crossing over and independent assortment.

The phases of mitosis, in order

1Prophase: chromosomes condense, the nuclear envelope breaks down and the spindle forms.
2Metaphase: chromosomes line up along the middle of the cell.
3Anaphase: sister chromatids are pulled apart toward opposite ends.
4Telophase: nuclear envelopes re-form and the chromosomes relax; cytokinesis follows.

Remember PMAT: Prophase, Metaphase, Anaphase, Telophase. Interphase comes before and cytokinesis after.

Meiosis I and meiosis II

Meiosis I separates the homologous pairs, so the cells that result are haploid, but each chromosome still has two chromatids. Meiosis II then separates the sister chromatids, much like mitosis. DNA is copied only once, before meiosis I.

Why meiosis creates variety

•Crossing over swaps pieces between homologous chromosomes in prophase I.
•Independent assortment lines up each pair at random in metaphase I.
•Random fertilization combines one of many possible sperm with one of many possible eggs.

Common mistakes

Mistake: Meiosis I separates sister chromatids.
Meiosis I separates homologous chromosomes. Sister chromatids separate in meiosis II.
Mistake: Mitosis makes gametes.
Gametes are made by meiosis. Mitosis makes body cells and copies of a cell.
Mistake: Haploid means one chromosome.
Haploid means one set of chromosomes. A human gamete has 23, one from each pair.
Mistake: DNA is copied again between meiosis I and meiosis II.
DNA is copied once, in interphase before meiosis I. There is no copying between the two divisions.
Mistake: Sister chromatids and homologous chromosomes are the same thing.
Sister chromatids are identical copies of one chromosome. Homologous chromosomes are the two different chromosomes of a pair, one from each parent.

Common questions

What is the main difference between mitosis and meiosis?↓

Mitosis produces two identical diploid cells for growth and repair. Meiosis produces four genetically different haploid cells for reproduction. Mitosis has one division and meiosis has two.

How many chromosomes do human cells have?↓

Human body cells are diploid with 46 chromosomes, or 23 pairs. Gametes (egg and sperm) are haploid with 23. Fertilization joins two gametes and restores 46.

Why is crossing over important?↓

It creates new combinations of genes on a chromosome, so the gametes of one person differ from each other. That variety is a source of the differences between siblings.

What happens in each phase of mitosis?↓

In prophase chromosomes condense and the spindle forms. In metaphase they line up in the middle. In anaphase the sister chromatids are pulled apart. In telophase the nuclei re-form, and cytokinesis splits the cell.

What is the best way to keep mitosis and meiosis straight?↓

Make a two-column comparison from memory: number of divisions, number of cells, identical or different, and chromosome number. Then check it. A matching game on the key terms helps the vocabulary stick.