Mitosis — identical cells, number maintained
Mitosis is a nuclear division giving rise to genetically identical cells. One parent nucleus divides into two daughter nuclei, each with the same chromosome number and the same genetic information as the parent — the number is maintained, not halved. Every body cell carries an exact copy of the original DNA, so a skin cell and a neurone differ only in which genes are switched on, not in the DNA. Meiosis (17.3) does the opposite: it halves the number and makes variation.
Four roles of mitosis in the syllabus
Growth — one fertilised egg becomes billions of identical cells. Repair of damaged tissues — skin cells divide to heal a cut. Replacement of cells — worn-out cells are replaced routinely, including red blood cells, which have no nucleus and so are supplied by bone marrow. Asexual reproduction — one parent makes offspring by mitosis alone, with no fertilisation (yeast budding, spider-plant plantlets). Those offspring are clones, so the population has no variation to draw on.
Interphase and stem cells — the Extended additions
Interphase comes before mitosis: the DNA replicates and an exact duplication of chromosomes occurs, so each daughter nucleus receives one complete copy of every chromosome. Without it the number would halve at every division. A stem cell is an undifferentiated cell that can divide by mitosis and then differentiate — embryonic ones into any cell type, adult ones (e.g. in bone marrow) into a limited range. Mitosis supplies the identical cells; differentiation specialises them.
Drawn from real examiner reports.
Mitosis does not create variation
Mitosis produces daughter cells genetically identical to the parent and to each other. Variation comes from meiosis and fertilisation. Examiners flag candidates who write that mitosis increases genetic variation — it scores zero. Quick check: if a question mentions variation, gametes or halving, the process is meiosis, not mitosis.
Flagged w22 P12 Q35
Mitosis does not halve chromosomes
Mitosis maintains the chromosome number — a parent cell with 8 chromosomes gives two daughter cells with 8 each. Halving belongs to meiosis, the reduction division that makes gametes. Writing that the DNA is copied so the number is halved contradicts itself: that copy in interphase is exactly what keeps the number constant.
Meitosis is not a word
Examiners report phonetic hybrids — meitosis for mitosis, meosis for meiosis — as too ambiguous to credit. Spell each term out in full, and when you revise pair the name with its function: mitosis for growth, repair and identical cells; meiosis for gametes, halving and variation. Anchoring each name to a function stops the two blurring together.
Flagged s22 P41 Q6a
Right features, wrong name
Some candidates describe both divisions accurately but attach the labels the wrong way round, calling the reduction division mitosis. Fix one anchor before you write: mitosis means identical cells, chromosome number maintained, body cells. Then check the process you have named against that anchor before you move on.
Flagged w22 P31 Q2c
Stem cell needs two ideas, not one
The mark scheme wants two linked ideas: the cell is undifferentiated, and it can divide by mitosis and then differentiate into a specialised type. Saying only that it is a cell that can turn into any other cell omits both mark-carrying elements. Examiners record this definition as poorly known, so write all three parts out.
Flagged w22 P21/22 Q31/Q32
DNA copies before mitosis, not during
The exact duplication of chromosomes happens during interphase, before the nucleus divides. Describing DNA replication as a stage that happens during mitosis loses the Extended mark. Keep the order explicit in your answer: interphase copies the DNA, then mitosis splits the nucleus into two identical nuclei.
Growth ≠ asexual reproduction
Both use mitosis, so the scenario decides which role applies. Growth makes more cells inside one organism, which gets larger. Asexual reproduction makes a new, separate organism — a clone able to live independently, as when a strawberry runner forms a plantlet. Ask whether a new individual has appeared before naming the role.
Answer in the order IDENTICAL, MAINTAINED, ROLE
Build every mitosis answer that way: nuclear division giving genetically identical cells; chromosome number maintained (Extended: the chromosomes were duplicated in interphase); then name which of the four roles the scenario shows.
Name meiosis when you compare
If the question compares the two divisions, write the meiosis side explicitly — reduction division, chromosome number halved, gametes genetically different. Phrases like the other type of division are too vague to secure the comparison marks.
Stem cells: three words, in order
Give undifferentiated, then divides by mitosis, then differentiates. Each one carries credit. Candidates who jump straight to it turns into a bone cell lose the first two marks before they reach the part they knew.
Match the role to the scenario
Healing a cut is repair; a plantlet on a runner is asexual reproduction; replacing worn-out red blood cells is replacement; an embryo enlarging is growth. Name the one role the question describes rather than listing all four.
Mitosis: nuclear division giving rise to genetically identical cells; the chromosome number is maintained.
The four roles of mitosis:
| Role | What happens |
|---|---|
| Growth | An organism increases in size by producing more identical cells |
| Repair of damaged tissues | New cells replace those lost through injury |
| Replacement of cells | Worn-out cells (e.g. red blood cells, skin cells) are continuously replaced |
| Asexual reproduction | A single parent produces genetically identical offspring (clones) by mitosis alone, with no fertilisation |
(Extended) Interphase and DNA replication: before mitosis, the cell passes through interphase, during which the deoxyribonucleic acid (DNA) is copied - an exact duplication of chromosomes occurs. Each chromosome is duplicated so that, when the nucleus divides, each of the two daughter nuclei receives one complete, identical set of chromosomes.
(Extended) Stem cell: an undifferentiated cell that can divide by mitosis and then differentiate into one or more specialised cell types. Stem cells supply new cells for growth, repair and replacement (e.g. bone-marrow stem cells continually replace red and white blood cells).
Define mitosis.
(a) Define mitosis.
(b) State the four roles of mitosis in living organisms.
