Key Idea: Every organism starts as a single cell and grows by division — and every division must hand on the right DNA to the new cells. This topic follows that handover from start to finish: the cell cycle prepares a cell to divide (4.4.1), mitosis splits the nucleus into two identical copies for growth and repair (4.4.2), and cytokinesis finally splits the cytoplasm in two (4.4.3). A different division, meiosis, halves the chromosome number to make varied gametes for reproduction (4.4.4) — and a karyogram lets us read a cell's chromosomes to spot errors (4.4.5). D2.1 is one of the most heavily tested topics in the course: quick Paper 1A MCQs, Paper 1B micrograph and karyogram data-questions (identify the phase, calculate a mitotic index, classify a cell), and Paper 2 extended answers (outline how mitosis keeps cells identical; explain how meiosis creates variation).
Mitosis makes two genetically IDENTICAL diploid cells in one division — for growth, repair and asexual reproduction. Meiosis makes four genetically DIFFERENT haploid gametes in two divisions — for sexual reproduction. Almost every exam slip in this topic comes from mixing these two up — pin them down now.
🔁 The cell cycle & interphase (4.4.1)
A cell does not divide the instant it is made — it spends a long time growing and copying its DNA first. This repeating sequence is the cell cycle. It has two parts: a long interphase (preparation) and a short mitotic (M) phase (division). Interphase is split into three stages, always in the order G1 → S → G2.
| Stage | Part of the cycle | What the cell is doing |
|---|---|---|
| G1 (first gap) | Interphase | grows larger; makes new proteins and organelles |
| S (synthesis) | Interphase | replicates its DNA — every chromosome is copied, so the DNA amount doubles |
| G2 (second gap) | Interphase | keeps growing and prepares to divide (final checks) |
| M (mitotic phase) | Division | the nucleus divides (mitosis) and the cell splits in two (cytokinesis) |
The single most important event of interphase is DNA replication in S phase. Before S (at G1) the cell has its normal amount of DNA; after S (at G2) every chromosome has been copied, so the cell has twice as much DNA. This is why a cell at G2 has double the DNA of a cell at G1 — a favourite 'distinguish' point. On a DNA-mass graph, a rising line = S phase, and a sudden drop to half = the cell dividing.
Grow → Synthesise (copy DNA) → Grow again → Mitosis. The two Gaps sit either side of Synthesis, and M (division) comes last. Interphase is the longest, busiest part of the cycle — never a 'resting' phase.
✂️ Mitosis: phases & genetic identity (4.4.2)
Mitosis divides one nucleus into two genetically identical daughter nuclei, each with the same (diploid) chromosome number as the parent. It is used for growth, repair and asexual reproduction. It runs through four phases in a fixed order — remember PMAT.
| Phase (in order) | What the chromosomes do | Memory hook |
|---|---|---|
| Prophase | chromosomes condense (coil up) and become visible; the spindle starts to form | Prophase = Prepare / aPpear |
| Metaphase | chromosomes line up single file along the middle (equator) of the cell | Metaphase = Middle |
| Anaphase | centromeres split; sister chromatids are pulled Apart to opposite poles | Anaphase = Apart |
| Telophase | two new nuclear membranes form around the two groups of chromosomes | Telophase = Two nuclei |
This is the headline Paper 2 point. Two facts working together guarantee it: 1) The DNA was replicated once beforehand (in S phase), so the two sister chromatids of each chromosome are exact copies — the same genes. 2) In anaphase those chromatids separate, with one copy going to each pole. So each daughter receives one complete, identical set of chromosomes — same genes, same chromosome number (still diploid).
People Meet And Talk → Prophase, Metaphase, Anaphase, Telophase. To identify a phase from a micrograph, read the chromosome behaviour: lined up in the middle = metaphase; being pulled apart = anaphase; two separate nuclei forming = telophase. DNA replication, chromosome condensation and spindle formation happen in both mitosis and meiosis.
🧫 Cytokinesis & the mitotic index (4.4.3)
Mitosis divides the nucleus; the cell still has to split its cytoplasm into two separate daughter cells. That final step is cytokinesis — and it happens differently in animal and plant cells, because plant cells have a rigid cell wall.
Animal cytokinesis: A **contractile ring** of filaments forms inside the membrane. The ring **contracts**, pulling the membrane inwards. A **cleavage furrow** deepens until the cell is pinched in two. Division works **inwards**, from the edge to the centre.
Plant cytokinesis: **Vesicles** of wall material gather at the centre. They **fuse** to form a **cell plate**. The cell plate grows **outwards** to the existing walls. A new **wall** and membrane separate the two cells.
Key Idea: In most divisions the cytoplasm (and its organelles) is shared roughly equally between the two daughter cells. The famous exception is egg (gamete) formation: the division is unequal — almost all the cytoplasm goes to one large egg, while the other products are tiny cells (polar bodies). This packs the egg with nutrients for a future embryo.
The mitotic index = (cells in mitosis) ÷ (total cells counted). It is the proportion of cells that are dividing. A high value means many cells are dividing at once — a rapidly growing region (such as a root-tip meristem), or, in a medical context, the uncontrolled division of a tumour. The mark examiners check most is dividing by the TOTAL number of cells, not by the interphase count. Example: 30 cells in mitosis out of 200 → 30 ÷ 200 = 0.15 (15%).
🧬 Meiosis: reduction division & variation (4.4.4)
Meiosis is the special division that makes gametes (sex cells). It takes one diploid (2n) cell and divides it twice into four haploid (n) cells. Because the chromosome number is halved, meiosis is a reduction division — and that halving is essential: it lets fertilisation restore the diploid number without doubling it each generation. The four gametes it makes are all genetically different.
Meiosis at a glance: one diploid (2n) cell divides TWICE. Meiosis I separates the homologous chromosomes (the number halves here), meiosis II separates the sister chromatids — giving four haploid (n) gametes. Contrast this with mitosis, which divides ONCE into two identical diploid cells.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Feature | Meiosis I (first division) | Meiosis II (second division) |
|---|---|---|
| What is separated | homologous chromosomes are pulled apart | sister chromatids are pulled apart |
| Chromosome number | halves: diploid (2n) → haploid (n) | stays haploid (n) |
| This is the… | reduction division (the number halves here) | division that finishes splitting the chromatids |
| Cells made | two haploid cells | four haploid cells (the gametes) |
Key Idea: The four gametes differ because of two processes, each tied to a stage you must name: Crossing over (prophase I): homologous chromosomes pair up and swap matching sections, mixing the alleles along each chromosome into new combinations. Independent assortment (metaphase I): each homologous pair lines up and is sorted to the poles at random, so gametes get a fresh mix of maternal and paternal chromosomes.
Crossing over: Happens in **prophase I**. Homologous chromosomes **pair up** and **swap sections**. Mixes the **alleles** along a chromosome. Creates **new combinations** within each chromosome.
Independent assortment: Happens in **metaphase I**. Each homologous **pair** lines up and sorts **at random**. Shuffles whole **maternal vs paternal** chromosomes. Every gamete gets a **different mix** of chromosomes.
In animals, meiosis occurs in the gonads — testes (sperm) and ovaries (eggs). In a flowering plant, it occurs in the anthers (making pollen / male gametes) and the ovules inside the ovary (making the female gametes / egg cells). 'Two structures where meiosis occurs' = anthers and ovules.
🔬 Karyotypes, karyograms & ploidy (4.4.5)
The number and appearance of all the chromosomes in a cell is its karyotype. To study them, scientists make a karyogram — a photograph in which the chromosomes are cut out and arranged in matching homologous pairs, lined up largest to smallest. Ploidy is a separate idea: it counts how many complete sets of chromosomes a cell has.
| Term | What it is | Think of it as |
|---|---|---|
| Karyotype | the number and appearance of all the chromosomes in a cell | the chromosome PROFILE (the facts) |
| Karyogram | a processed photograph with chromosomes arranged in homologous pairs | the PICTURE you arrange to show it |
| Ploidy | the number of complete chromosome sets in a cell (n, 2n, 3n…) | how many complete SETS the cell carries |
Diploid (2n) — body cell: **Two** sets of chromosomes. Chromosomes appear in **homologous pairs**. Found in **somatic (body) cells**. Human example: **2n = 46**.
Haploid (n) — gamete: **One** set of chromosomes. Chromosomes are **single** (no partner). Found in **gametes** (egg, sperm). Human example: **n = 23**.
Key Idea: A chromosome count tells you the cell type: chromosomes in pairs → diploid → somatic; single chromosomes → haploid → gamete. Partners are matched using three criteria: size (length), centromere position and banding pattern. An extra or missing chromosome (e.g. trisomy 21, three copies of chromosome 21) is caused by non-disjunction — a pair (or sister chromatids) that failed to separate during meiosis.
The karyotype is the information (how many chromosomes and what they look like). The karyogram is the arranged picture. A quick check: a karyogram is a diagram — the image.
🧠 The big contrast — mitosis vs meiosis
If you remember one table from this whole topic, make it this one — almost every exam error here is a mitosis/meiosis mix-up:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | one | two (meiosis I then II) |
| Daughter cells made | two | four |
| Chromosome number | stays the same (2n → 2n) | halves (2n → n) — reduction division |
| Genetically | identical to parent and to each other | different (variation) |
| What is separated | sister chromatids (in anaphase) | homologous chromosomes (I), then sister chromatids (II) |
| Used for | growth, repair, asexual reproduction | making gametes for sexual reproduction |
✍️ Worked examples
IB-style question — identify the stage from a DNA-mass graph
A graph shows the mass of DNA in a cell over one cell cycle. At point T the DNA mass is steadily rising from its lowest value towards double that value. State which stage of the cell cycle point T represents, and explain your reasoning. [2]
How to score both marks:
Read the data. The DNA mass is increasing (rising from its lowest value towards double), which means the cell is copying its DNA.
Name the stage. DNA is replicated during S phase, so point T is in S phase. (Mark 1: S phase. Mark 2: because the DNA mass is rising / DNA is being replicated.)
Point T is in S phase — the DNA mass is rising because the DNA is being replicated, which only happens in S phase.
IB-style question — outline how mitosis produces identical cells
Outline how the behaviour of chromosomes during mitosis produces two genetically identical daughter cells. [4]
How to score all four marks:
Start with replication. Before mitosis, the DNA is replicated, so each chromosome becomes two identical sister chromatids joined at the centromere.
Line up. In metaphase the chromosomes line up at the equator (middle) of the cell, attached to spindle fibres.
Separate the copies. In anaphase the centromeres split and the sister chromatids are pulled to opposite poles — one identical copy of every chromosome to each end.
Two identical nuclei. Each pole receives a complete, identical set, so two genetically identical diploid nuclei form. (Award 1 mark per distinct point, max 4.)
DNA is replicated into identical sister chromatids; chromosomes line up in the middle; in anaphase the chromatids separate, one copy to each pole; so each daughter nucleus gets an identical, complete set.
IB-style question — calculate and interpret a mitotic index
A student counts 200 cells in an onion root-tip slide, of which 30 cells are in mitosis. Calculate the mitotic index, and explain what this value indicates about the tissue. [3]
How to score all three marks:
Write the rule and substitute. Mitotic index = cells in mitosis ÷ total cells = 30 ÷ 200.
Work it out. 30 ÷ 200 = 0.15 (or 15%). (Mark 1: correct method, dividing by the total. Mark 2: correct value 0.15 / 15%.)
Interpret it. A relatively high value means a large proportion of cells are dividing, so the root tip is an actively growing region (a meristem). (Mark 3: links the index to the fraction of cells dividing / rapid growth.)
Mitotic index = 30 ÷ 200 = 0.15 (15%); a high value means many cells are dividing, so the root tip is a rapidly growing region.
IB-style question — explain how meiosis generates variation
Explain how the stages and processes of meiosis generate genetic variation in the gametes produced. [5]
How to score all five marks:
Set the scene. Meiosis takes one diploid cell and divides it twice to make four haploid gametes.
Crossing over. In prophase I, homologous chromosomes pair up and swap matching sections, mixing the alleles so each chromosome carries a new combination.
Independent assortment. At metaphase I, each homologous pair is sorted to the poles at random, so gametes get different mixes of maternal and paternal chromosomes.
Random combination. Because both processes are random, each of the four gametes ends up with a different, unique combination of alleles and chromosomes. (1 mark per distinct point: two divisions/four gametes; crossing over; prophase I; independent assortment; metaphase I / random — max 5.)
Meiosis divides one diploid cell twice into four haploid gametes; crossing over (prophase I) swaps sections between homologous chromosomes, and independent assortment (metaphase I) sorts each pair at random — so each gamete carries a unique combination of genes.
Why the four gametes differ: crossing over (prophase I) swaps sections between homologous chromosomes, and independent assortment (metaphase I) sorts each pair to the poles at random. Together they make every gamete genetically unique — and a separation that FAILS (non-disjunction) gives a gamete an extra or missing chromosome.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
IB-style question — gamete or somatic? read the error
A human karyogram shows 47 chromosomes, with three copies of chromosome 21 instead of the usual pair. State whether this cell is haploid or diploid, identify the type of error shown, and name the event in meiosis that causes it. [3]
How to score all three marks:
Read the ploidy. The chromosomes are arranged in pairs (two sets), so the cell is diploid — a somatic (body) cell, not a haploid gamete.
Name the error. There is an extra chromosome 21 — three copies instead of two — which is a trisomy (trisomy 21, Down syndrome).
Name the cause. It is caused by non-disjunction: chromosome 21 failed to separate during meiosis, so a gamete received an extra copy. (Mark 1: diploid. Mark 2: trisomy / extra chromosome 21. Mark 3: non-disjunction / failure to separate.)
Diploid (chromosomes in pairs); a trisomy (three copies of chromosome 21, total 47), caused by non-disjunction — chromosome 21 failed to separate during meiosis.
✅ Quick self-check
Tap each card to check yourself.
What are the stages of the cell cycle, and when does DNA double? Interphase = G1 (growth), S (DNA replication) and G2 (growth + prep), then the short M phase (division). DNA is copied in S phase, so a G2 cell has twice the DNA of a G1 cell. The M phase is NOT part of interphase.
Why are the two daughter cells of mitosis genetically identical? The DNA was replicated once into two identical sister chromatids per chromosome; in anaphase these separate so each daughter receives one complete, identical, diploid set. Order of phases: PMAT.
How does cytokinesis differ in animal vs plant cells? Animal cells pinch inwards — a contractile ring forms a cleavage furrow. Plant cells build outwards — vesicles fuse into a cell plate that grows to the walls. Cytoplasm is usually shared equally; egg formation is the unequal exception.
What is the mitotic index, and what does a high value mean? Cells in mitosis ÷ total cells counted — the proportion dividing. A high value means a large fraction of cells are dividing: rapidly growing tissue (a meristem), or uncontrolled division in a tumour.
Why is meiosis a reduction division, and how does it create variation? It halves the chromosome number (2n → n) so fertilisation can restore the diploid number. Crossing over (prophase I) and independent assortment (metaphase I) make all four haploid gametes genetically different.
How do you read ploidy and errors from a karyogram? Chromosomes in homologous pairs = diploid (somatic); single chromosomes = haploid (gamete). Classify partners by size, centromere position and banding pattern. An extra/missing chromosome (e.g. trisomy 21) is caused by non-disjunction.
Exam Tips
- Interphase = G1, S and G2 — never include the M phase. DNA doubles in S, so a G2 cell has TWICE the DNA of a G1 cell.
- On a DNA-mass graph: a rising line = S phase; a sudden drop to half = the cell dividing (mitosis sharing DNA out).
- Identify a mitotic phase from a micrograph by chromosome behaviour: lined up in the middle = metaphase; pulled apart = anaphase; two nuclei forming = telophase. Remember PMAT.
- For 'outline how mitosis gives identical cells [4]', give the CHAIN: DNA replicated → identical chromatids → separate in anaphase → each cell gets one identical complete set.
- Describe ANIMAL cytokinesis as a contractile ring + cleavage furrow pinching inwards — never a cell plate (that is plant only).
- Mitotic index = cells in mitosis ÷ TOTAL cells. Dividing by the total (not the interphase count) is the key marking point.
- Meiosis I separates HOMOLOGOUS chromosomes (number halves here); meiosis II separates sister chromatids. Don't swap them.
- An 'explain how meiosis generates variation' answer needs BOTH crossing over (prophase I) AND independent assortment (metaphase I), each tied to its stage.
- Gamete vs somatic from a count: chromosomes in pairs = diploid (somatic); single chromosomes = haploid (gamete) — and say WHY.
- An extra/missing whole chromosome on a karyogram (e.g. trisomy 21) means non-disjunction — a pair that failed to separate in meiosis, never a change to DNA bases.
Key Idea: The cell cycle (4.4.1) prepares a cell to divide: interphase G1 → S → G2 (DNA doubles in S), then the short M phase. Mitosis (4.4.2) splits the nucleus in one division — PMAT (prophase, metaphase, anaphase, telophase) — giving two genetically identical diploid cells, because replicated sister chromatids separate one copy to each pole. Cytokinesis (4.4.3) then splits the cytoplasm: animal cells pinch in (contractile ring → cleavage furrow), plant cells build out (cell plate); cytoplasm is usually equal except in the egg. The mitotic index = cells in mitosis ÷ total cells (a high value = rapid growth or a tumour). Meiosis (4.4.4) is the reduction division: one diploid cell divides twice into four genetically different haploid gametes — meiosis I separates homologous chromosomes, meiosis II the sister chromatids, with crossing over (prophase I) and independent assortment (metaphase I) creating variation. A karyogram (4.4.5) arranges chromosomes into homologous pairs by size, centromere position and banding: pairs → diploid (somatic), singles → haploid (gamete), and an extra/missing chromosome (e.g. trisomy 21) reveals non-disjunction.