Mitosis — identical cells, number maintained
Mitosis is nuclear division producing two genetically identical cells. Exact chromosome replication happens first; the copies then separate, one complete set per new nucleus, so the chromosome number is maintained. Its four roles in 0654 are growth (an embryo becoming an adult), repair (healing a cut), replacement of cells that die naturally, and asexual reproduction (a plant grown from a cutting). The named stages are not required.
Meiosis — reduction division making haploid gametes
Meiosis is a reduction division: it halves the chromosome number, taking a diploid parent cell (two full sets) to haploid cells (one of each pair), and the cells produced are genetically different. It is the division that produces gametes. Each gamete gets a different combination of chromosomes, so meiosis is a source of variation. A human body cell has 46 chromosomes (23 pairs); a gamete has 23, and fertilisation restores 46.
Chromosome replication precedes both divisions
Exact chromosome replication occurs before both divisions — what differs is what happens next. Mitosis shares them out so the number stays the same and the cells are identical; meiosis divides so the number is halved and the cells are different. 0654 contrasts the two on exactly three points — chromosome number, genetic identity, and role — and mark schemes expect all three stated distinctly, not one idea reworded. The full table is in the study note.
Drawn from real examiner reports.
Mitosis and meiosis statements swapped
Mark schemes credit three separate ideas per division: chromosome number, genetic identity of the cells produced, and role. Candidates routinely swap them — crediting mitosis with halving the number, or meiosis with producing identical cells — or restate one idea in three sentences, which earns credit only once. Fill in the three-row table mentally before writing anything down.
Flagged s22 P41 Q4b; P42 Q1c · s23 P43 Q7iii
Zygote to embryo is mitosis
After fertilisation the zygote divides repeatedly to build the embryo, and this division is mitosis, not meiosis — despite following reproduction. Every embryo cell needs a full, identical copy of the zygote's genetic information; meiosis would halve it each time. Rule: making gametes = meiosis; growth, repair, replacement or copying a fertilised cell = mitosis.
Flagged w23 P42 Q1b
Meiosis halves — it never doubles
Meiosis is a reduction division, so the gamete (haploid) number is always the body-cell (diploid) number divided by two, never multiplied. A body cell with 32 chromosomes gives gametes with 16, not 64. Sanity-check every answer: a haploid number must always be smaller than the diploid number it came from.
Flagged s22 P41 Q4b
A human body cell has 46, not 23
A human diploid body cell contains 46 chromosomes, arranged in 23 pairs; a human gamete is haploid with 23. Answers offered 23, or 48, or the name of a gamete where a number was asked for. Read which cell type the question names — body cell or gamete — decide diploid or haploid from that, and then give a number.
Flagged s22 P41 Q4c; P42 Q1bii · w23 P32 Q4cii
Haploid means unpaired, not just fewer
Saying a gamete merely has "fewer" chromosomes does not score. Haploid means one full set, unpaired — a sperm nucleus is haploid and its chromosomes are unpaired. Diploid means two full sets, in pairs — a zygote nucleus is diploid and paired. Use the paired/unpaired language, not just a comparison of the two numbers.
Flagged s22 P41 Q1c · s23 P42 Q10a
Only meiosis is a reduction division
Reduction division is the defining description of meiosis — it is called that precisely because it reduces (halves) the chromosome number. Mitosis reduces nothing: the chromosome number is maintained. Calling mitosis a reduction division, or leaving the phrase out when meiosis is being defined, both cost marks.
Answer in three separate points
For any describe or compare question, give three separate, labelled points per division: what happens to the chromosome number, whether the cells are identical or different, and the role. One clear sentence per point — then check you have not reworded the same idea twice.
Never name the stages
0654 does not require the named stages of mitosis or meiosis. Writing prophase, metaphase, anaphase and telophase wastes time and earns nothing — describe the outcomes instead: chromosome number, genetic identity of the cells produced, and role.
Check the direction before you divide
Before any chromosome calculation, state which way you are going. Body cell to gamete (meiosis) means halve; two gametes fusing at fertilisation means add back up to the diploid number. Mitosis changes nothing — each daughter cell carries the same number as the parent.
Link haploid gametes to fertilisation
For "explain why gametes must be haploid", finish the chain: meiosis halves the number, two haploid gametes fuse at fertilisation, the diploid number is restored — so it stays constant from one generation to the next. Stopping at "meiosis halves it" leaves marks unclaimed.
Cambridge 0654 spec reference: Section B16 "Inheritance", sub-topic B16.2 (Extended/Supplement content -- there is no Core statement for cell division in this specification). This leaf covers mitosis as nuclear division producing genetically identical cells, the role of mitosis, chromosome replication before mitosis, and meiosis as a reduction division producing genetically different haploid gametes.
Out of scope for B16.2 (do not use): the named stages of mitosis/meiosis (prophase, metaphase, anaphase, telophase), crossing over, independent assortment, chiasmata, bivalents/tetrads.
| Term | Mark-scheme-precise definition |
|---|---|
| Mitosis | Nuclear division giving rise to two genetically identical cells (diploid to diploid); no chromosome-number change |
| Meiosis | A reduction division: nuclear division that halves the chromosome number (diploid to haploid), giving genetically different cells |
| Diploid | A cell (or nucleus) containing two full sets of chromosomes (chromosomes in homologous pairs) |
| Haploid | A cell (or nucleus) containing one full set of chromosomes (only one of each pair) |
| Gamete | A sex cell (sperm/egg in animals, pollen/egg cell in plants), produced by meiosis, that is haploid |
| Chromosome replication | The process, occurring before both mitosis and meiosis, in which an exact copy of every chromosome is made |
The one idea that unlocks the topic: exact chromosome replication happens first in every division; mitosis then shares the copies out so the chromosome number stays the same and the two new cells are identical; meiosis shares the chromosomes out differently so the chromosome number is halved and the resulting cells are different.
Define mitosis.
Define each of the following terms:
(a) mitosis
(b) meiosis
(4 marks)
