Mitosis and meiosis are the two major ways cells divide. Both begin with one cell and involve copying DNA, but they serve very different purposes.
Mitosis produces new cells for growth, tissue repair, and routine cell replacement. It usually produces two genetically similar daughter cells with the same number of chromosomes as the original cell.
Meiosis produces cells used in sexual reproduction. It produces four genetically different cells, each with half the chromosome number of the original cell.
The key difference is therefore not simply that one process makes two cells and the other makes four. Mitosis preserves the chromosome number and generally preserves genetic identity, while meiosis reduces the chromosome number and deliberately creates genetic variation.
What happens before either type of division?
Before a cell divides, it must copy its DNA. This happens during a stage of the cell cycle called S phase, before the actual nuclear division begins.
After DNA replication, each chromosome consists of two identical copies called sister chromatids, joined at a region called the centromere. The cell can then organize and separate these duplicated chromosomes during division.
This DNA-copying step occurs before both mitosis and meiosis. What follows is where the two processes diverge.
How mitosis works
Mitosis is a single division of the nucleus. Its basic sequence is often described using four stages: prophase, metaphase, anaphase, and telophase, followed by cytokinesis, when the cell’s cytoplasm divides.
During prophase, chromosomes condense so they can be moved efficiently. The nuclear envelope breaks down, and structures called spindle fibers form.
In metaphase, duplicated chromosomes line up near the middle of the cell. In anaphase, the sister chromatids separate and move toward opposite ends of the cell. During telophase, new nuclear envelopes form around the separated chromosome sets.
Cytokinesis then divides the cell, producing two daughter cells.
The important outcome is that each daughter cell receives one copy of every chromosome. In a typical human body cell, the starting cell has 46 chromosomes, and mitosis produces daughter cells that also have 46 chromosomes.
Mitosis is therefore essential for increasing cell number as an organism grows and for replacing cells that are damaged, worn out, or naturally lost.
How meiosis works
Meiosis is more complicated because it involves two rounds of nuclear division, called meiosis I and meiosis II, after only one round of DNA replication.
The first division separates homologous chromosomes. These are matching chromosome pairs: one chromosome of each pair was inherited from the mother and the other from the father.
The second division separates sister chromatids. Because the chromosomes were not replicated again between meiosis I and meiosis II, the process ultimately produces four cells with half the original chromosome number.
In humans, meiosis begins with a cell containing 46 chromosomes and produces cells with 23 chromosomes. These cells can participate in sexual reproduction. In males, meiosis produces sperm; in females, it is involved in producing eggs.
A particularly important feature of meiosis is that its products are genetically different from one another. Two mechanisms are largely responsible: crossing over and independent assortment.
Why meiosis creates genetic variation
During early meiosis I, homologous chromosomes pair with each other. Sections of DNA can be exchanged between homologous chromosomes in a process called crossing over. This produces chromosomes carrying new combinations of genetic material.
Meiosis also randomly distributes maternal and paternal homologous chromosomes into the resulting cells. This is known as independent assortment.
Together, these mechanisms generate many possible combinations of chromosomes and genetic variants. Fertilization adds another layer of variation by combining genetic material from two parents.
Mitosis, by contrast, is designed primarily to maintain genetic continuity. Although mutations can arise in cells produced by mitosis, the process itself does not normally reshuffle chromosomes in the way meiosis does.
The main differences at a glance
| Feature | Mitosis | Meiosis |
|---|---|---|
| Main role | Growth, repair, and cell replacement | Production of cells for sexual reproduction |
| Number of divisions | One | Two |
| DNA replication | Once before division | Once before meiosis I |
| Cells produced | Usually 2 | Usually 4 |
| Chromosome number | Maintained | Reduced by half |
| Genetic similarity | Daughter cells are generally genetically similar to the parent cell and each other | Daughter cells are genetically different |
| Homologous chromosomes pair? | No | Yes, during meiosis I |
| Crossing over | Normally absent | Occurs during meiosis I |
| What separates first? | Sister chromatids | Homologous chromosomes |
| What separates second? | — | Sister chromatids |
The crucial distinction: homologous chromosomes vs. sister chromatids
One of the easiest ways to confuse mitosis and meiosis is to treat homologous chromosomes and sister chromatids as the same thing. They are not.
Homologous chromosomes are a matching pair of chromosomes that carry the same types of genes in corresponding locations, although the versions of those genes can differ. A person inherits one homolog from each parent.
Sister chromatids are the duplicated copies of a single chromosome that result from DNA replication. They are initially essentially identical copies of one another.
In mitosis, sister chromatids separate during the main chromosome-separation step.
In meiosis I, homologous chromosomes separate while sister chromatids remain together. Sister chromatids then separate during meiosis II.
That difference explains much of what makes meiosis distinct from mitosis.
Why meiosis must reduce the chromosome number
Sexual reproduction involves the fusion of two reproductive cells. If those cells carried the full chromosome number, chromosome numbers would double every generation.
Meiosis solves this problem by producing haploid cells, meaning cells with one set of chromosomes. In humans, a haploid cell has 23 chromosomes.
When a sperm and egg fuse during fertilization, their chromosome sets combine to form a diploid cell with two sets of chromosomes—46 chromosomes in humans.
Mitosis then allows that fertilized cell and its descendants to grow while maintaining the organism’s chromosome number.
Are mitosis and meiosis completely separate processes?
They are distinct forms of nuclear division, but they share fundamental cellular machinery and principles. Both depend on DNA replication beforehand, chromosome condensation, spindle fibers, and carefully controlled chromosome movement.
The major difference is what the cell is trying to accomplish.
Mitosis is primarily about maintaining the existing chromosome complement while making additional cells.
Meiosis is about reducing the chromosome complement and generating genetically varied reproductive cells.
That difference in purpose explains why meiosis needs two divisions and why homologous chromosomes pair and exchange DNA during the process.
What happens if chromosome separation goes wrong?
Both mitosis and meiosis require chromosomes to be distributed accurately. Errors can produce cells with abnormal chromosome numbers.
In meiosis, for example, a failure of chromosomes to separate properly is called nondisjunction. It can result in reproductive cells containing an extra chromosome or missing one. If such a cell contributes to fertilization, the resulting embryo can have an abnormal chromosome number.
Errors in mitotic chromosome separation can likewise produce cells with abnormal chromosome numbers. In multicellular organisms, this can have different consequences depending on the affected cell and tissue.
Accurate chromosome segregation is therefore a central requirement of both types of cell division.
A simple way to remember the difference
Think of the two processes in terms of their biological goals:
Mitosis maintains. Meiosis mixes and halves.
Mitosis makes new cells while generally preserving the chromosome number and genetic arrangement of the original cell.
Meiosis first reshuffles genetic material and then reduces the chromosome number by half, creating cells suited for sexual reproduction.
Once you understand what separates during each process—sister chromatids in mitosis, homologous chromosomes first and sister chromatids second in meiosis—the larger differences between the two processes become much easier to follow.
