New cells come from existing cells. Eukaryotes use the cell cycle (interphase to prepare, mitosis to divide) producing two genetically identical daughter cells. Prokaryotes use binary fission. Viruses don't divide at all; they hijack host cells.
Read this topic as an analogy.
Imagine a bakery preparing to split one dough ball into two identical balls. The whole topic falls into place around that single image.
Before the split, the baker prepares. The dough rises and grows in mass (G1). The baker photocopies the recipe card and doubles up every ingredient measurement so there's enough for two balls (S phase, DNA replication). The dough rises once more in final preparation while the baker assembles the cutting board and rolling pin (G2).
The split itself runs in four stages. First, the baker tightens the dough into a coiled roll, ready to be split; the cutting board comes out and the rolling cover comes off (prophase: chromosomes condense, spindle assembles, nuclear envelope breaks down). Second, the baker lays the coiled roll precisely across the centre line of the cutting board (metaphase plate). Third, the baker pulls the two halves apart with both hands; there are two moves to this single pull, releasing the bond holding the sister halves together, then drawing the halves to opposite ends of the board (anaphase: centromeres divide first, then chromatids are pulled to opposite poles). Fourth, the baker smooths each half into a finished ball, drapes a fresh proving cloth over each, and cuts cleanly through the middle with a knife (telophase, then cytokinesis with a cleavage furrow). Plant-cell bakers can't pinch their dough, so they lay down a baking-paper divider across the equator instead (cell plate).
Two identical dough balls. Same recipe, same weight, same future.
A village bakery does the same job more simply. The baker stretches the dough long, lets it grow, and pinches it cleanly in half. No preparation stages. No cutting board, no proving cloth (binary fission has no spindle and no nuclear envelope). The recipe card on the wall is a single circular card (circular DNA). Small backup note-cards in a drawer (plasmids) get distributed between the two halves, but in variable numbers, not one each.
A wedding-stationery courier (a virus) drops by. The courier has no oven of any kind. Instead, it has a plug that fits one particular port on the bakery's machinery (attachment protein binding a specific receptor). Once plugged in, the courier tells the bakery's own ovens to print copies of a recipe insert (viral nucleic acid hijacks the host's machinery). The ovens churn out copies of the insert with the supporting bread (assembly). At the end of the run, the bakery either bursts to release everything at once or the inserts slip out the side throughout the evening (lysis or budding).
The bakery's quality-control board tracks the mitotic index: the proportion of dough balls currently mid-split at any moment. It's a fraction, not a percentage. A reading of 0.05 means one in twenty dough balls is currently being split; the other nineteen are in the preparation phase.
The bakery's aseptic technique is the kitchen hygiene that keeps the dough uncontaminated. Wash hands. Disinfect the bench before and after. Use a sterile pipette to measure liquid volumes. Flame the neck of the milk bottle when pouring. Lift the lid of the proving box at an angle, not fully off. Work close to the warm updraft of the oven, which displaces airborne dust upward (the updraft doesn't kill the dust, it just pushes it away). Drop used utensils into a disinfectant tub afterwards.
Mapping back to formal vocabulary. In an exam answer, the dough ball is the parent cell; the two identical balls are the daughter cells, genetically identical to the parent. The recipe card is DNA; photocopying it is DNA replication in the S phase of interphase, not in mitosis. The four-step split is mitosis in the order prophase, metaphase, anaphase, telophase. The first half of anaphase is centromeres divide; the second half is chromatids pulled to opposite poles. Both are independently marked, and the first is the consistently dropped one. The knife stroke at the equator is cytokinesis: cleavage furrow in animals, cell plate in plants. The village bakery is binary fission: circular DNA, membrane attachment, transverse cell wall, no spindle, no PMAT. The wedding-stationery courier is a virus: attachment proteins on the virus, receptors on the host (the polarity matters), viral nucleic acid enters, host machinery assembles new virions, release by lysis or budding. The proportion of dough balls currently mid-split is the mitotic index: a proportion between 0 and 1, never a percentage. The kitchen hygiene is aseptic technique, judged by named actions, not generic principles. Drop the bakery vocabulary when you write the answer; use these formal terms.
The cell cycle: interphase prepares the cell, then mitosis divides the nucleus.
The cell cycle has two major phases: interphase, where the cell prepares for division (about 90% of the total cycle time), and mitosis, where the nucleus divides. Cytokinesis then divides the cytoplasm. The result is two daughter cells genetically identical to the parent and to each other.
The three sub-phases of interphase.
| Sub-phase | What happens |
|---|---|
| G1 | The cell grows in volume. Protein synthesis is active and the number of organelles increases, so each future daughter cell will inherit a full complement. |
| S | DNA replication. Each chromosome is copied by semi-conservative replication into two identical sister chromatids joined at the centromere. |
| G2 | Further growth. Tubulin is synthesised for spindle fibre assembly. Chromosomes begin to condense in late interphase (an AQA-specific nuance, not in prophase). A final replication check happens before mitosis starts. |
DNA replication happens in S phase. Don't write that
DNA replicates during mitosis. Replication is complete before mitosis begins; by the time prophase starts, every chromosome is already two sister chromatids.
Mitosis serves three functions in multicellular organisms. Growth: repeated rounds of mitosis develop an organism from a single fertilised egg. Repair: worn-out or damaged cells are replaced. Asexual reproduction: some organisms (yeast, certain plants) produce identical offspring by mitotic division. Cancer is the failure of cell-cycle regulation: mutations bypass checkpoint controls, cells divide without restraint, and a tumour forms. The AQA-level link is that cancer is uncontrolled mitosis, not a distinct process.
Mitosis runs in four stages, with anaphase as two independently credited marks.
Mitosis divides the nucleus in four sequential stages: prophase, metaphase, anaphase, telophase (PMAT). Each stage has specific structural events that mark schemes credit by name. Adding a meiotic qualifier ("anaphase I") imports the wrong vocabulary and is rejected.
- Prophase. Chromosomes finish condensing into discrete bodies that become visible under a light microscope. Centrioles migrate to opposite poles and assemble the spindle apparatus. The nuclear envelope breaks down in late prophase, releasing the chromosomes into the cytoplasm.
- Metaphase. The condensed chromosomes line up along the metaphase plate at the cell's equator, each attached to spindle fibres at the centromere.
- Anaphase. Two independently credited marks in fixed order. First, centromeres divide and sister chromatids are released from each other. Second, the chromatids are pulled to opposite poles by contracting spindle fibres. Each chromatid is now a chromosome in its own right.
- Telophase. Chromosomes arrive at opposite poles. The spindle disassembles. Nuclear envelopes re-form around each set, producing two nuclei. Chromosomes decondense and nucleoli reappear.
Write
condense,shorten and thicken, orcoilsfor prophase chromosome behaviour. Don't writecontracted. AQA rejects "contracted" as the wrong physical process.
Write
centromeres dividefirst, thenchromatids pulled to opposite polessecond. Both halves are independently marked. The first is the consistently dropped half on two-mark anaphase questions.
Write
chromosomesorchromatidsfor what moves at anaphase. Don't writehomologous chromosomes. Homologous is meiotic vocabulary; crossing over and bivalents are also meiotic rejects in any mitosis description.
Pitfall — Anaphase two-mark structure
Anaphase carries two independent marks in fixed order.
Mark schemes credit centromere division separately from chromatid movement. Centromere division is the cause; chromatid movement is the effect. Naming only the effect ("chromatids pulled apart") earns one mark out of two. The cause mark ("centromeres divide") is the consistently dropped one across examiner reports from 2021, 2022, and 2024.
Cytokinesis splits the cytoplasm after telophase. In animal cells, a contractile ring of actin filaments at the equator pulls the membrane inward, forming a cleavage furrow that pinches the cell in two. In plant cells, the rigid cell wall blocks pinching; instead, a cell plate of membrane and wall material is laid down across the equator and grows outward to fuse with the existing cell wall.
Binary fission divides prokaryotes simply, with no spindle and no PMAT stages.
Prokaryotes do not undergo mitosis. They lack the nuclear envelope and the linear chromosomes that mitosis requires. Instead they divide by binary fission, a simpler process with no spindle, no PMAT stages, and no chromosome condensation.
- The single circular DNA chromosome replicates. Both copies attach to separate points on the cell-surface membrane.
- The bacterium grows. The membrane extends inward between the two attachment points, drawing the chromosomes apart as the cell elongates.
- A transverse cell wall forms across the middle, dividing the cell into two daughters, each with one copy of the circular chromosome. Plasmids also replicate, but distribute between the two daughters in variable numbers, not one each.
Write
circular DNAin any binary fission answer. "DNA replicates" alone does not distinguish binary fission from mitosis. Plasmid distribution is variable; do not write that plasmids split equally.
Viral replication hijacks the host's machinery in five steps.
Viruses cannot divide. They are not cells and have no metabolic machinery of their own. They produce new copies only by commandeering a living host cell's biosynthetic apparatus. The credited skeleton is five steps in fixed order.
- Attachment proteins on the virus bind to complementary receptors on the host cell surface. The polarity is fixed.
- The viral nucleic acid (DNA or RNA, depending on the virus) enters the host cell, either by injection through the membrane or by envelope-membrane fusion.
- The viral nucleic acid directs the host's ribosomes, polymerases, and energy-producing machinery to replicate the viral genome and synthesise viral proteins.
- New viral nucleic acids and capsid proteins assemble into complete virus particles inside the host cell.
- The host cell lyses, releasing all virions at once, or new virions bud continuously from the host membrane.
Attachment proteins are on the virus. Receptors are on the host cell. Reversing the polarity ("receptor protein on the virus") is an explicit reject.
Write
viral nucleic acidorgenetic material. Don't writemRNA injected. The viral genome may be DNA or RNA depending on the virus and is not specifically mRNA.
Mitotic index is a proportion; aseptic technique is named actions.
Two practical tools support the study of cell division. The mitotic index quantifies what proportion of cells are in mitosis at a given moment; the rest are in interphase, since the cell cycle has two phases and the index measures only one. Aseptic technique is the procedural hygiene that lets us culture bacteria without contamination.
mitotic index = (number of cells in mitosis) ÷ (total cells in the field of view). Three precision rules apply. The unit of count is cells, not chromosomes or chromatids. The denominator is total cells in the field of view, nothing else. The result is a proportion between 0 and 1, never a percentage; multiplying by 100 is an explicit reject.
Mitotic index is a proportion between 0 and 1. Don't multiply by 100 to give a percentage. AQA rejects the percentage form on every mitotic index question.
When the mitotic index decreases, write that
more cells are in interphase. Don't write thatfewer cells are dividing. The cell cycle has two phases; both must be named on Conclude questions, because mitotic index measures only one of them.
- Wash your hands. Disinfect the work surface before starting and again afterwards.
- Use a sterile pipette or syringe to transfer measured liquid volumes. Loops can only streak solid agar; they cannot transfer a measured volume.
- Flame the neck of the bottle when pouring. Never flame the agar plate itself or the Petri dish lid.
- Lift the Petri dish lid at an angle, just enough to allow access. Do not lift the lid fully off.
- Work close to the upward convection current produced by a Bunsen burner flame. The mechanism is displacement of airborne microbes by the rising air, not killing them.
- Use a sterile spreader to distribute culture across the agar.
- Place all used equipment in disinfectant after use.
Pitfall — Statistical conclusion grammar
The subject of a statistical conclusion is
the differenceorthe correlation, neverthe results."The results are significant" and "the results are due to chance" are both rejected. The credited construction is "the difference between [groups] is/is not significant" or "the correlation between [variables] is strong/significant". For standard-deviation overlap questions, the criterion is mean ± 2 standard deviations, not ± 1; calculate four bounds and compare. For log-scale axes, an axis value of n means 10ⁿ, not n.
Key terms
- mitosis
- mitotic index
- anaphase
- prophase
- binary fission
- circular DNA
- condense
- centromere
- DNA replication
- nucleic acid
- receptors
- sterile