Example 1: mitotic index
Out of 400 cells in a root-tip squash, 36 are dividing. Calculate the mitotic index.
- Mitotic index = dividing cells ÷ total cells × 100.
- 36 ÷ 400 × 100 = 9%.
This page needs the IB Demystified site-wide snippets. Ask the site administrator to install them.
IB DemystifiedMYP Sciences
Cell division: mitosis and meiosis
You began as a single cell. Billions of divisions later, every one of your body cells carries the same 46 chromosomes, yet your brother or sister is not your copy. Two kinds of cell division explain both facts.
Recommended for MYP 4 · eAssessment priority · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding cell division
- Mitosis, meiosis and stem cells
- Cell division in the real world
- Worked examples
- In the eAssessment
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- describe chromosomes and the difference between diploid and haploid cells
- explain mitosis and its role in growth, repair and asexual reproduction
- explain meiosis and how it produces genetically different gametes
- compare mitosis and meiosis
- explain stem cells and cancer in terms of cell division
- calculate and interpret the mitotic index
Before you start
You will use these skills. If any feel shaky, review them first.
- cell structure and the nucleus (see Cells and microscopy)
- DNA, genes and chromosomes (see DNA, genes and inheritance)
- using a light microscope
Key vocabulary
- Chromosome
- A long DNA molecule carrying many genes; in body cells they come in pairs, one from each parent.
- Diploid / haploid
- Having two sets of chromosomes (46 in human body cells) / one set (23 in human gametes).
- Mitosis
- Division producing two genetically identical diploid cells.
- Meiosis
- Two divisions producing four genetically different haploid gametes.
- Cell cycle
- The sequence of growth, DNA copying (interphase) and division that a cell goes through.
- Mitotic index
- The percentage of cells in a sample that are dividing.
- Stem cell
- An unspecialised cell that can divide and differentiate into specialised cells.
Understanding cell division
-
What is it?
Before any division, a cell copies its DNA during interphase. In mitosis, the copies are separated so each of two new cells gets a full, identical set. In meiosis, two divisions share out one chromosome of each pair, producing four haploid gametes.
-
Why are there two kinds?
Growth and repair need new cells identical to the old ones, so they can do the same jobs: mitosis. Sexual reproduction needs gametes with half the chromosomes, so fertilisation restores the full number, and it needs variation, which meiosis provides by shuffling which chromosome of each pair goes into each gamete.
-
How do we know?
In the 1880s Walther Flemming used stains to see chromosomes separating in dividing cells. Today, stained root-tip squashes let students see every stage, and counting cells in each stage reveals how long each takes.
-
Why does it matter?
Faulty control of mitosis causes cancer, and many cancer treatments target dividing cells. Stem cells, which divide and then specialise, are used in bone marrow transplants and are being researched for repairing damaged organs.
-
What does it connect to?
Cell division links to DNA and inheritance, variation and evolution, growth, cancer and cloning in biology; and to radiation and mutations in physics.
Mitosis, meiosis and stem cells
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | One | Two |
| Cells produced | Two | Four |
| Chromosomes (human) | 46 (diploid) | 23 (haploid) |
| Genetically | Identical to parent | Different from each other |
| Used for | Growth, repair, asexual reproduction | Making gametes |
Stem cells in embryos can become almost any cell; adult stem cells (in bone marrow or skin) form a narrower range. Cancer starts when mutations in genes that control the cell cycle let cells divide uncontrollably.
Cell division in the real world
Bone marrow transplants replace a patient's blood-forming stem cells and are a standard treatment for some leukaemias. Gardeners and farmers clone plants from cuttings, relying on mitosis. Checking the chromosomes of cells from an unborn baby can reveal conditions caused by errors in meiosis, such as Down syndrome (an extra copy of chromosome 21).
Worked examples
Example 2: chromosome numbers
A horse body cell has 64 chromosomes. How many are in a horse sperm cell and in a horse zygote?
- Sperm are made by meiosis, which halves the number: 64 ÷ 2 = 32.
- At fertilisation, sperm (32) + egg (32) = 64 in the zygote.
In the eAssessment
Cell division questions test whether you can move between diagrams, numbers and explanations. Expect:
- Interpret diagrams of dividing cells, and count chromosomes and cells.
- Calculate chromosome numbers, mitotic index and time spent in each stage.
- Compare mitosis and meiosis precisely, describing both in each point.
- Discuss stem cell research or cancer prevention, weighing ethics and evidence.
Common ways to lose marks: mixing up mitosis and meiosis; saying gametes have 46 chromosomes; giving differences that describe only one process; and treating the mitotic index as a speed of division.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: how long does each stage of mitosis take?
Partially guided investigation · about 60 minutes · pairs
- Research question
- What proportion of the cell cycle do onion root-tip cells spend in interphase and in each stage of mitosis?
- Scientific background
- At any moment, the number of cells in a stage is proportional to how long that stage lasts. Counting many cells in a stained root-tip squash therefore lets you estimate the time spent in each stage, if you know the length of the whole cycle.
- Hypothesis
- Predict which stage will contain the most cells and which the fewest, with a justification.
- Variables
- Identify what you must keep the same when preparing and counting slides, and explain why.
- Apparatus
- Onion roots about 1–2 cm long, warm 1 mol/dm³ hydrochloric acid, stain (such as toluidine blue), microscope slides and cover slips, scalpel, mounted needle, filter paper, light microscope.
- Method
- Cut the last 5 mm of a root tip and soften it in warm acid for 5 minutes, as your teacher shows you.
- Rinse, stain, then press the tip gently under a cover slip to spread the cells in a single layer.
- At high power, record the stage of at least 200 cells, moving across several fields of view.
- Calculate the percentage in each stage and, using a 24-hour cycle, the time for each stage.
Safety. Wear eye protection. Hydrochloric acid is an irritant and the stain can stain skin; use small volumes and wipe up spills. Cut away from your fingers on a tile. Take care with glass slides and never direct sunlight into a microscope mirror.
Then evaluate: which stages were hardest to identify, and how might that affect your results?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
|---|