Mitosis vs Meiosis

The big idea

Your body is built from cells, and cells must make more cells. Sometimes they need to make exact copies for growth and repair. Sometimes they need to make cells with half the chromosome number for reproduction. That is the difference between mitosis and meiosis.

Think of mitosis as photocopying one page. Think of meiosis as copying the page, mixing a few lines, and then splitting it into four smaller pages.

What you'll learn

• Why mitosis makes two identical diploid cells
• How PMAT orders the phases of mitosis
• How meiosis I and meiosis II reduce chromosome number
• Where crossing over happens and why it creates variation

Mitosis: making body cells

Mitosis is the cell's copy-and-paste move. It is used for growth, repair, and replacing worn-out cells. The goal is simple: make two cells that are genetically the same as the original. The stages follow PMAT: prophase, metaphase, anaphase, telophase.

1. Copy first. Before mitosis, the cell goes through interphase — the part of the cell cycle when it grows and copies its DNA. After DNA is copied, each chromosome has two sister chromatids — identical copies joined at the centromere. The cell is getting ready so each new cell can receive a full set.
2. Prophase. Chromosomes condense, so they become easier to move. The nuclear envelope breaks down, and spindle fibers form.
3. Metaphase. Chromosomes line up across the middle of the cell. This middle line helps the spindle attach to each sister chromatid from opposite sides.
4. Anaphase. Sister chromatids separate and move to opposite poles. Once they split, each chromatid counts as its own chromosome.
5. Telophase and cytokinesis. New nuclear envelopes form around the two chromosome sets. Then cytokinesis — the division of the cytoplasm — splits the cell into two.

Mitosis keeps the chromosome number the same, so one diploid cell becomes two identical diploid cells.

Meiosis: making sex cells

Meiosis is different. It makes gametes, such as sperm and eggs, with half the usual chromosome number. Its job is not to make copies. Its job is to cut the chromosome number in half and create variation. Like mitosis, it starts after DNA has already been copied in interphase.

1. Pair up. In prophase I, matching chromosomes from each parent come together. These matching partners are homologous chromosomes — chromosomes with the same genes in the same order, though they may have different versions of those genes. They pair up to form a tetrad — a group of four chromatids.
2. Cross over. Still in prophase I, non-sister chromatids can swap matching DNA segments. This is crossing over — the exchange of DNA between homologous chromosomes. It creates new combinations of genes.
3. Split homologs. In metaphase I, homologous pairs line up in the middle. In anaphase I, the homologous chromosomes separate, but sister chromatids stay together. This is the reduction division, because it changes the cell from diploid to haploid.
4. Shuffle again. The way each pair lines up is random. This is independent assortment — the random separation of homologous chromosome pairs into gametes. Then meiosis II begins, and it looks like mitosis: chromosomes line up again, and anaphase II pulls sister chromatids apart.
5. Finish. After meiosis II, the cell has made four cells, each with one set of chromosomes.

Meiosis makes four genetically different haploid cells.

Side by side: what changes

1. Purpose. Mitosis makes cells for growth, repair, and replacement. Meiosis makes gametes for sexual reproduction.
2. Chromosome count. Diploid means two sets of chromosomes, written $2n$. Haploid means one set, written $n$. Mitosis keeps $2n$ the same. Meiosis changes $2n$ to $n$.
3. Cell count and identity. Mitosis makes 2 identical daughter cells. Meiosis makes 4 daughter cells that are genetically different because of crossing over and independent assortment.

This picture shows the whole difference at a glance.

In the diagram, the top cell is diploid because it has a homologous pair. The left path shows mitosis: the chromosomes are copied and split once, so both daughter cells stay diploid and identical. The right path shows meiosis: crossing over happens first, meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids, leaving four haploid cells.

Putting it together

$1\ \text{diploid cell} \xrightarrow{\text{mitosis}} 2\ \text{diploid cells}$

$1\ \text{diploid cell} \xrightarrow{\text{meiosis}} 4\ \text{haploid cells}$

Read it left to right: one diploid cell divides once to make two identical diploid cells, or divides twice to make four haploid cells.

Example with a simple model cell: if $2n=2$, the cell starts with one homologous pair.

• After mitosis, you get 2 cells, and each one still has 2 chromosomes.
• After meiosis I, you get 2 cells, and each one has 1 chromosome that is still duplicated.
• After meiosis II, you get 4 cells, and each one has 1 chromosome. In humans, the same idea gives body cells $2n=46$ and gametes $n=23$.

Check yourself

1. A skin cell divides by mitosis. How many daughter cells do you get, and are they diploid or haploid? Answer Two identical diploid cells.
2. When does crossing over happen? Answer In prophase I of meiosis.
3. What separates in meiosis I, and what separates in meiosis II? Answer Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II.

Common mistakes

• "Mitosis makes gametes." No. Meiosis makes gametes.
• "Crossing over happens after meiosis I." No. It happens in prophase I, before homologous chromosomes separate.
• "Meiosis is just mitosis twice." No. Meiosis I is different because homologous chromosomes pair, exchange DNA, and separate.

Remember this

• Mitosis: 1 cell → 2 identical diploid cells.
• Meiosis: 1 cell → 4 different haploid cells.
• Mitosis is for growth and repair. Meiosis is for reproduction and variation.