IB Biology: Mitosis & the Cell Cycle — FAQ

The cell cycle is the repeating sequence a cell goes through between one division and the next: interphase (G1, S, G2) where the cell grows and copies its DNA, followed by the M phase, mitosis and cytokinesis. According to the current Biology guide (first assessments 2025), it's taught within the D: Continuity and change theme.

Mitosis runs through prophase, metaphase, anaphase and telophase — PMAT. Chromosomes condense and the spindle forms in prophase, line up at the cell's equator in metaphase, sister chromatids get pulled apart in anaphase, then two identical nuclei re-form in telophase before cytokinesis splits the cytoplasm into two cells.

Checkpoint proteins stop a cell moving into the next phase until conditions are right — checking for DNA damage at the G1 and G2 checkpoints, and correct spindle attachment at the M checkpoint. At HL, you also need cyclins and cyclin-dependent kinases (CDKs), whose rising and falling levels drive the cell through each checkpoint.

Cancer develops when mutations disable the checkpoint proteins that normally stop damaged cells dividing, so cells multiply uncontrollably and form a tumour. At HL, you're expected to name mutated tumour-suppressor genes, like p53, and proto-oncogenes as the molecular cause — not just describe "uncontrolled division" in general terms.

Expect short-answer questions on Paper 1 and Paper 2 asking you to identify mitotic stages from micrographs, describe checkpoint control, and calculate mitotic index from a root tip squash photo. Data-based questions comparing cell cycle timing between healthy and cancerous tissue also appear regularly across both SL and HL papers.

SL students need the cell cycle phases, mitosis stages and basic checkpoint control; HL students add cyclins, CDKs and more detail on tumour-suppressor gene mutations in cancer. The core mitosis diagram and mitotic index calculation are identical at both levels — HL simply builds the molecular mechanism on top of the same foundation.

Most schools run a root tip squash practical — staining an onion or garlic root tip, viewing it under a microscope, and counting cells in each mitotic stage to calculate mitotic index. It's a favourite source for internal assessment ideas and Paper 2 data questions because it's cheap, reliable and gives clean numerical results.

"Outline" and "state" ask for the stages in order without detail; "describe" wants what physically happens to chromosomes and the spindle at each stage; "explain" wants you to link checkpoint failure to a cause, like a mutation. "Calculate" appears almost every exam session for mitotic index — practise it until it's automatic.

Mitotic index equals the number of cells visibly in mitosis divided by the total number of cells counted, multiplied by 100 for a percentage. Examiners give you a micrograph and ask you to count and calculate — it's one of the easiest marks on the whole paper if you count carefully and show your working.

Use the mnemonic "PMAT" — Prophase, Metaphase, Anaphase, Telophase — and picture what's physically true at each stage: chromosomes condense (P), line up (M), pull apart (A), two nuclei form (T). I tell students who mix up anaphase and telophase to remember: "A is for Apart."

The biggest one I see every year is writing "chromosomes split" at anaphase instead of "sister chromatids separate" — chromosomes don't split, chromatids that were already joined pull apart. Students also confuse mitosis with meiosis in written answers, and forget that cytokinesis is a distinct stage rather than part of telophase.

Mitosis produces two genetically identical diploid cells for growth and repair; meiosis produces four genetically different haploid gametes for sexual reproduction, using two divisions instead of one. If a question mentions "gametes," "haploid," or "genetic variation," it wants meiosis — if it says "growth," "repair," or "identical," it wants mitosis.

Not conceptually harder, but denser — HL adds cyclins, CDKs and named tumour-suppressor genes on top of the SL content. Most students find the mitosis diagrams themselves straightforward; it's the checkpoint biochemistry and linking a mutation to cancer in an extended response that actually separates SL and HL answers in the exam.