Chapter 11

Cell Division: Mitosis & Meiosis

High School

Growth, repair and reproduction all depend on cells copying and dividing. Two kinds of division serve two purposes: mitosis makes identical copies for growth, while meiosis shuffles and halves the genome to make the gametes of sexual reproduction — the engine of variation.

At a glance
Core ideaMitosis makes identical copies for growth; meiosis halves and shuffles for gametes.
Key termCrossing over — homologous chromosomes swapping segments in prophase I.
You can…Calculate a mitotic index and give daughter-cell chromosome numbers.
Watch outMitosis separates sister chromatids; meiosis I separates whole homologues.
1 Theory

The cell cycle

A dividing cell passes through the cell cycle: interphase (the long growth-and-preparation phase, subdivided into G₁ growth, S phase where DNA is replicated, and G₂ preparation) followed by the mitotic (M) phase where the nucleus and then the cell divide. Checkpoints (G₁/S, G₂/M, spindle) verify that conditions and DNA are correct before proceeding; failure of these controls underlies cancer.

Mitosis — one division, identical cells

Mitosis produces two genetically identical diploid daughter cells. Remember the phases as PMAT:

  • Prophase — chromosomes condense and become visible (each as two sister chromatids joined at a centromere); the nuclear envelope breaks down; the spindle forms.
  • Metaphase — chromosomes line up single-file on the equator (metaphase plate); spindle fibres attach to centromeres.
  • Anaphase — sister chromatids are pulled apart to opposite poles.
  • Telophase — chromosomes decondense; two new nuclear envelopes form.
  • Cytokinesis — the cytoplasm divides, giving two cells (a cleavage furrow in animals; a cell plate in plants).
Phase PProphaseChromosomes condense; nuclear envelope breaks down; spindle forms.
Phase MMetaphaseChromosomes line up single-file on the equator.
Phase AAnaphaseSister chromatids are pulled to opposite poles.
Phase TTelophaseChromosomes decondense; two nuclei re-form.
ThenCytokinesisCytoplasm splits into two identical daughter cells.

Meiosis — two divisions, four varied gametes

Meiosis produces four genetically distinct haploid cells from one diploid cell, through two divisions (meiosis I and II) after a single DNA replication. It halves the chromosome number (diploid 2n → haploid n) so that fertilisation restores the diploid number. Two events generate variation:

  • Crossing over (prophase I) — homologous chromosomes pair up and exchange segments, recombining alleles.
  • Independent assortment (metaphase I) — homologous pairs line up and separate randomly, so each gamete gets a random mix of maternal and paternal chromosomes (2²³ ≈ 8 million combinations in humans, before crossing over).

Mitosis — the photocopier

  • One division after one DNA replication
  • Produces 2 daughter cells
  • Cells are diploid and genetically identical
  • Separates sister chromatids
  • For growth, repair, asexual reproduction

Meiosis — the card-shuffle

  • Two divisions after one DNA replication
  • Produces 4 daughter cells
  • Cells are haploid and genetically varied
  • Meiosis I separates homologous chromosomes
  • Makes gametes; crossing over + independent assortment
2 Explanation

Same machinery, opposite goals

The crucial contrast: mitosis preserves, meiosis diversifies. Mitosis is a photocopier — every daughter cell must be an exact clone for growth and repair to work, so sister chromatids (identical copies) are separated. Meiosis is a card-shuffle — it must produce variety, so it separates homologous chromosomes (the maternal and paternal versions, which carry different alleles) and mixes them by crossing over. The single most important structural difference is that meiosis has two rounds of division after one round of replication, which is exactly what halves the chromosome number.

Why halving is essential

If gametes were diploid, fertilisation would double the chromosome number every generation — 46 → 92 → 184… — quickly catastrophic. Meiosis halves the count to haploid (23 in humans) so that egg + sperm restore the diploid 46. Sexual reproduction is a bargain: you halve, then recombine, buying genetic variation that fuels evolution.

3 Practical

Worked example: calculating the mitotic index

Biologists use the mitotic index — the proportion of cells actively dividing — to judge how fast a tissue is proliferating (important in cancer diagnosis). Work through a root-tip count.

  1. Prepare a slide. Squash a stained garlic or onion root tip (a region of rapid growth) so cells spread in one layer.
  2. Count under the microscope. In one field of view you count 200 cells total. Of these, 34 show visible condensed chromosomes (i.e. are in prophase, metaphase, anaphase or telophase — undergoing mitosis).
  3. Apply the formula. mitotic index = cells in mitosis ÷ total cells.
  4. Compute. mitotic index = 34 ÷ 200 = 0.17 (or 17%).
  5. Interpret time in each phase. The fraction of cells in a phase estimates the fraction of cycle time spent there. If the whole cycle is 24 hours, mitosis lasts ≈ 0.17 × 24 ≈ 4.1 hours. Most cells are in interphase, so its index would be much higher.
  6. Apply the idea. A tumour biopsy with an unusually high mitotic index indicates rapid, uncontrolled division — one marker pathologists use to grade cancers.
4 Q&A

Test yourself

Q1 A human skin cell has 46 chromosomes. State the chromosome number in the daughter cells after (a) mitosis and (b) meiosis, and explain the difference.

(a) After mitosis: 46 in each of two daughter cells — mitosis conserves the diploid number by separating identical sister chromatids. (b) After meiosis: 23 in each of four daughter cells — meiosis halves the number to haploid by separating homologous chromosomes across two divisions with only one replication. Meiosis makes gametes; halving is what lets fertilisation restore 46.

Q2 Explain two distinct ways meiosis generates genetic variation.

(1) Crossing over in prophase I: homologous chromosomes pair and swap corresponding segments, producing new combinations of alleles on a single chromosome (recombination). (2) Independent assortment in metaphase I: each homologous pair aligns and separates independently of the others, so gametes receive a random mixture of maternal and paternal chromosomes. Together these produce enormous variety; random fertilisation then multiplies it further.

Q3 Why is it critical that DNA is replicated before mitosis, and what happens if replication is incomplete?

Replication in S phase makes an identical copy of every chromosome (as sister chromatids) so that when they separate in anaphase, each daughter cell receives a complete, identical genome. If replication is incomplete or faulty, daughter cells may inherit missing or damaged genes. The G₂/M checkpoint normally halts the cycle until replication is verified; when checkpoints fail, cells divide with damaged DNA, accumulating mutations — a route to cancer.

Q4 During anaphase I of meiosis, what separates — and how does this differ from anaphase of mitosis?

In anaphase I of meiosis, whole homologous chromosomes (each still consisting of two sister chromatids) are pulled to opposite poles; the homologues separate. In anaphase of mitosis (and anaphase II of meiosis), sister chromatids separate. This difference is exactly why meiosis I reduces chromosome number (homologues part) while mitosis keeps it constant (identical chromatids part).

Q5 Non-disjunction in meiosis can produce a gamete with an extra chromosome. Trace how this leads to a condition such as Down syndrome.

Non-disjunction is the failure of chromosomes (or chromatids) to separate properly during meiosis. If chromosome 21 fails to separate, a gamete may carry two copies of chromosome 21 instead of one. When this gamete is fertilised by a normal gamete (one copy), the zygote has three copies — trisomy 21, causing Down syndrome. Every body cell then carries 47 chromosomes. Non-disjunction risk rises with maternal age, reflecting the long arrest of human eggs in meiosis.

Concept mind map

How the ideas connect

Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.

interphaseprophasemetaphaseanaphasetelophasecrossing overgametesMitosis & Meiosis
Infographic

The process, step by step

Step 1InterphaseCell grows and DNA replicates so each chromosome has two chromatids.
Step 2ProphaseChromosomes condense and become visible; the spindle forms.
Step 3MetaphaseChromosomes line up on the cell equator.
Step 4AnaphaseSister chromatids are pulled to opposite poles.
Step 5TelophaseNuclei reform and the cytoplasm divides (cytokinesis).
Solved examples

Worked problems, step by step

Follow each solution line by line, then try to reproduce it on paper before moving on.

Example 1In a field of 500 cells, 40 are in mitosis. Find the mitotic index.

  1. Mitotic index = dividing cells / total
  2. 40 / 500 = 0.08

Example 2A body cell has 46 chromosomes. How many are in each gamete after meiosis?

  1. Meiosis halves the chromosome number
  2. 46 / 2 = 23
Practice problem set

Now you try

Work each one out first, then tap to reveal the worked answer.

1What is the main purpose of mitosis?
Mitosis produces two genetically identical cells for growth, repair and asexual reproduction.
2How does meiosis create genetic variation?
Crossing over and independent assortment shuffle alleles, so each gamete is genetically unique.
3How many divisions occur in meiosis and how many cells result?
Meiosis has two divisions and produces four haploid cells.
4What does diploid mean?
Diploid means a cell has two complete sets of chromosomes, one from each parent.
5When during the cell cycle is DNA copied?
DNA is replicated during the S phase of interphase, before division begins.
6Why must gametes be haploid?
So that fertilisation restores the normal diploid number rather than doubling it each generation.