Mitosis vs Meiosis: Differences, Stages, and Purpose
Mitosis makes two identical cells for growth and repair; meiosis makes four genetically different sex cells. Compare their stages, results, and why both matter.
Mitosis and meiosis are the two ways that cells with a nucleus divide. Mitosis produces two genetically identical daughter cells and is used for growth, repair, and replacing worn-out cells. Meiosis produces four genetically different cells, each with half the usual number of chromosomes, and is used to make eggs and sperm in animals and spores in plants and fungi. Both start with a cell that has already copied its DNA, but they diverge in what happens next.
What happens before a cell divides?
Before either process begins, the cell is in interphase. In the G1 phase it grows, in the S phase it copies its DNA (DNA replication), and in the G2 phase it prepares for division. After the S phase every chromosome consists of two identical sister chromatids joined at a point called the centromere. Human body cells have 46 chromosomes, arranged as 23 pairs, with one chromosome of each pair inherited from each parent.
The stages of mitosis
Mitosis is a single division, described in five stages:
- Prophase: chromosomes condense into visible threads and the spindle of protein fibres starts to form.
- Prometaphase: the nuclear envelope breaks down and spindle fibres attach to each chromosome.
- Metaphase: the chromosomes line up along the middle of the cell.
- Anaphase: the sister chromatids separate and are pulled to opposite ends.
- Telophase and cytokinesis: new nuclear envelopes form, the chromosomes uncoil, and the cell pinches in two (in plants, builds a new wall between the halves).
Each daughter cell receives 46 chromosomes identical to the parent's. In cultured human cells, mitosis itself takes roughly an hour, while a whole cell cycle takes about a day. The German biologist Walther Flemming described the process in dividing salamander cells and named it mitosis in 1882.
The stages of meiosis
Meiosis involves one round of DNA copying followed by two rounds of division. In meiosis I, matching chromosomes from the two parents pair up and swap segments in a process called crossing over. They then line up in pairs, in a random orientation, and the two members of each pair are pulled apart, producing two cells that each hold 23 chromosomes, still made of two chromatids. In meiosis II, which resembles mitosis, the sister chromatids are separated, leaving four cells with 23 single chromosomes each.
Mitosis vs meiosis compared
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Making gametes or spores |
| Divisions | One | Two |
| DNA copying rounds | One | One |
| Daughter cells | Two | Four |
| Chromosome number | Same as the parent cell (diploid) | Half of the parent cell (haploid) |
| Genetic makeup | Identical to the parent | Unique combinations |
| Crossing over | Normally absent | Occurs in prophase I |
| Where in humans | Almost all body cells | Ovaries and testes |
Why does meiosis create variety?
Variety comes from two sources. Crossing over exchanges pieces between the maternal and paternal copies of each chromosome. Independent assortment means that each of the 23 pairs lines up independently of the others, so one person can make 223, about 8.4 million, different chromosome combinations even before crossing over is counted. Fertilisation then combines two such cells, so siblings are genetically distinct, which supplies the raw material for natural selection. Meiosis is also the physical explanation of Gregor Mendel's rules of segregation and independent assortment, described in Mendelian inheritance.
When cell division goes wrong
Both processes have checkpoints, but errors still happen. In nondisjunction, chromosomes fail to separate in meiosis, creating eggs or sperm with an extra or missing chromosome. Down syndrome, caused by three copies of chromosome 21, usually arises this way, and the chance rises with the age of the mother. In mitosis, damaged control systems can allow cells to divide without restraint, which is the basis of cancer. Understanding how the two kinds of division differ is the key to understanding growth, inheritance, and many diseases.
Tags
biology cell division chromosomes genetics heredity
Related articles
- Mendelian Inheritance
- DNA Replication
- Tardigrades: The Almost Indestructible Water Bears
- Circadian Rhythm Reset: How the Master Clock Works
- Bioluminescence
- The Deep Ocean