Mitosis and meiosis are the two ways a eukaryotic cell divides its nucleus. Mitosis makes two daughter cells that are genetically identical to the parent and to each other, with the same number of chromosomes. It is used for growth, repair and asexual reproduction. Meiosis makes four daughter cells that are genetically different from each other and have half the chromosome number. It is used to make gametes: sperm and egg cells in animals. Mitosis has one division; meiosis has two. Meiosis creates variation in two ways: crossing over between homologous chromosomes in prophase I, and independent assortment of those pairs in metaphase I. In humans, mitosis keeps 46 chromosomes in each body cell; meiosis produces gametes with 23.

Everything below hangs on one distinction, so get it first: homologous chromosomes are a pair (one from each parent); sister chromatids are two identical copies of one chromosome, joined at the centromere.

The words that do the work

A chromosome after S phase, the familiar X, is still one chromosome. It just has two chromatids. Counting chromosomes means counting centromeres.

The cell cycle

Mitosis is only the short last part of the cell cycle. Most of a cell's life is spent in interphase.

1. G1 (first gap). The cell grows, makes proteins and organelles, and does its normal job.
2. S (synthesis). The DNA replicates. Each chromosome now has two sister chromatids. The chromosome number does not change; the DNA content doubles. DNA replication, explained visually covers how.
3. G2 (second gap). More growth, and checks that replication was complete and correct.
4. M phase. Mitosis (division of the nucleus), followed by cytokinesis (division of the cytoplasm).

Cells that stop dividing, such as most nerve cells, leave the cycle into a resting state called G0. Checkpoints at the end of G1, at the end of G2 and during metaphase stop a damaged cell from dividing. Cancer is, in large part, cells that ignore those checkpoints.

Mitosis, phase by phase

The order is PMAT: prophase, metaphase, anaphase, telophase. AP Biology and some A level courses add prometaphase between the first two.

The key fact: in anaphase of mitosis, sister chromatids separate. Each daughter cell gets one copy of every chromosome, so it has exactly what the parent had.

Meiosis I and II

Meiosis is two divisions after one round of replication. That is the whole trick of halving.

Meiosis I, the reduction division. Homologous chromosomes separate.

• Prophase I. Chromosomes condense and homologous pairs line up side by side (synapsis), forming bivalents. Non-sister chromatids cross over at chiasmata and swap sections of DNA. This is crossing over.
• Metaphase I. Homologous pairs, not single chromosomes, line up on the equator. Which way each pair faces is random. This is independent assortment.
• Anaphase I. Whole chromosomes from each pair are pulled to opposite poles. The sister chromatids stay together.
• Telophase I and cytokinesis. Two cells, each haploid, each chromosome still with two chromatids.

Meiosis II. Sister chromatids separate, much like mitosis, but in haploid cells.

• Prophase II, metaphase II, anaphase II, telophase II. Chromosomes line up singly; centromeres split; chromatids go to opposite poles. Because of crossing over, the two chromatids are no longer identical.
• Result. Four haploid cells, all genetically different.

In human males all four become sperm. In females, the divisions are unequal: one large egg cell and smaller polar bodies that do not develop.

Where the variation comes from

Exam questions usually ask for two sources in meiosis, plus one at fertilisation.

1. Crossing over (prophase I). Swapping sections between non-sister chromatids makes new combinations of alleles on the same chromosome.
2. Independent assortment (metaphase I). Each homologous pair lines up independently of the others. With 23 pairs, one person can make 2 to the power 23, about 8.4 million, different combinations of chromosomes in their gametes, before crossing over is even counted.
3. Random fertilisation. Any sperm can fuse with any egg, multiplying those combinations again.

Mitosis vs meiosis, side by side

Worked example: counting human chromosomes

This is the question that catches people, so follow one human cell through both processes. Count chromosomes by centromeres and chromatids separately.

Notice that the chromosome number halves in meiosis I, not meiosis II. Meiosis II halves the chromatids. And fertilisation, 23 from the sperm plus 23 from the egg, restores 46.

When this goes wrong and a pair fails to separate, which is called non-disjunction, a gamete ends up with 24 or 22 chromosomes. A zygote with three copies of chromosome 21 has Down syndrome.

The mistakes that lose marks

• Saying replication happens in mitosis. It happens in S phase of interphase, before mitosis starts.
• Counting an X-shaped chromosome as two. It is one chromosome with two chromatids.
• Saying meiosis II halves the chromosome number. Meiosis I does. Meiosis II separates chromatids.
• Mixing up homologous chromosomes and sister chromatids. Homologues are a pair from two parents with possibly different alleles; sister chromatids are copies.
• Putting crossing over in the wrong phase. Prophase I only. Independent assortment is metaphase I.
• Calling interphase a resting phase. The cell is growing and copying its DNA. G0 is the resting state.
• Forgetting cytokinesis. Mitosis divides the nucleus. Cytokinesis divides the cell.

A scene plan for a four minute video

The whole topic is a comparison, so build the video as two lanes running side by side, mitosis on the left and meiosis on the right, with the same starting cell at the top of both. How to explain a comparison in a video covers keeping two lanes readable.

1. One body cell, and the question: why do skin cells copy themselves exactly, but sperm and eggs do not?
2. A colour code: maternal chromosomes red, paternal blue. Show just two pairs, not 23.
3. S phase: each chromosome becomes an X.
4. Left lane, mitosis: chromosomes in a single line, chromatids pulled apart, two identical cells.
5. Right lane, meiosis I: pairs side by side, a red and blue section swapping at a chiasma.
6. Metaphase I: the pairs lining up, then flipping to show a second random arrangement.
7. Anaphase I: whole chromosomes separating; two cells with half the number.
8. Meiosis II: chromatids separating; four cells, each a different mix of red and blue.
9. The counting table, filling row by row from 46 to 23.
10. Fertilisation: 23 plus 23 back to 46, and the loop closes.

Using only two pairs on screen is a deliberate simplification. Say so in the narration, then give the real number.

Make your own version

1. 1

   Check your specification

   GCSE needs the outcome and purpose of each process; A level and AP Biology need the phases, chiasmata and independent assortment. Cut what your course does not test.

2. 2

   Fix a colour code and a pair count

   Maternal one colour, paternal another, two homologous pairs on screen. Decide this before you write, so every scene matches.

3. 3

   Write one paragraph per scene

   Follow the scene plan above, using your course's exact terms. About 600 words fills four minutes at 150 spoken words a minute.

4. 4

   Paste the script into openCanviz

   Choose Keep my wording so every term is spoken as written, set a target length, and pick the whiteboard style so the chromosomes are drawn as you describe them.

5. 5

   Count the chromosomes in every scene

   Pause on each scene and count centromeres. Drafted drawings can show the wrong number, join homologues at a centromere or put crossing over in mitosis. Fix any scene in the editor.

For a wider biology project, how to make a biology explainer video for a school assignment covers planning and marking criteria. Watching is not revising: pause before each scene and say what separates next.

Common questions

What is the main difference between mitosis and meiosis? Mitosis makes two genetically identical diploid cells for growth and repair. Meiosis makes four genetically different haploid cells for sexual reproduction.

Why does meiosis need two divisions? Because the DNA is replicated once, before meiosis begins. The first division separates homologous pairs and halves the chromosome number; the second separates the sister chromatids, so each gamete gets one copy of each chromosome.

Where do mitosis and meiosis happen in the human body? Mitosis happens in body cells throughout the body, for example in skin, bone marrow and the lining of the gut. Meiosis happens only in the testes and ovaries.

How many chromosomes are in a human gamete? 23, one from each homologous pair. Fertilisation combines two gametes to restore the diploid number of 46.

Is crossing over the same as independent assortment? No. Crossing over swaps sections between chromatids of a homologous pair in prophase I. Independent assortment is the random orientation of whole pairs at the equator in metaphase I. Both create variation, in different ways.