IB Biology · Theme D: Continuity and change · SL and HL
D2.1 Cell and nuclear division
A one-page summary of D2.1 Cell and nuclear division, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How can large numbers of genetically identical cells be produced?
How do eukaryotes produce genetically varied cells that can develop into gametes?
What D2.1 covers
Dividing a cell
- D2.1.1One mother cell, two daughter cells
- D2.1.2Pinch or build: two ways to split the cytoplasm
- D2.1.3Splitting the cytoplasm unevenly
- D2.1.4Two kinds of nuclear division
Chromosomes: copied, coiled and moved
- D2.1.5Copy the DNA first
- D2.1.6Coil it up: condensation
- D2.1.6Pulled apart by microtubules
Mitosis
- D2.1.7Four phases, two identical nuclei
- D2.1.7Count chromosomes and chromatids
- D2.1.8Which phase is this?
Meiosis and genetic variation
- D2.1.9One diploid nucleus, four haploid nuclei
- D2.1.9Two rounds of segregation
- D2.1.9Why sex needs meiosis
- D2.1.10When chromosomes fail to separate
- D2.1.10Seeing trisomy 21 in a karyotype
- D2.1.11Random orientation shuffles the parents
- D2.1.11Crossing over: swapping segments
HL — the cell cycle, its control and cancer
- D2.1.12 · HLProliferation for growth, replacement and repair
- D2.1.13 · HLThe cell cycle: grow, copy, divide
- D2.1.14 · HLInterphase is not resting
- D2.1.15 · HLCyclins decide whether the cycle goes on
- D2.1.16 · HLWhen the accelerator sticks or the brake fails
- D2.1.17 · HLBenign or malignant: what a tumor does
- D2.1.17 · HLThe mitotic index: counting dividing cells
D2.1 Cell and nuclear division: summary
Cell division
- A mother cell divides into two daughter cells; nuclear division comes first, then cytokinesis.
- Animal cells pinch with an actin-myosin ring; plant cells build a cell plate. Yeast budding and oogenesis are unequal.
Chromosomes
- DNA replication comes first: each chromosome has two sister chromatids until anaphase.
- Histones and supercoiling condense chromosomes; microtubules and motor proteins move them.
Mitosis
- Prophase, metaphase, anaphase, telophase: two genetically identical nuclei with the parent's chromosome number.
- Human cell: 46 chromosomes, 92 chromatids after replication, 92 chromosomes in anaphase.
Meiosis
- One diploid nucleus gives four haploid nuclei. Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.
- Needed so that fertilization restores 2n. Non-disjunction can cause trisomy 21 (Down syndrome).
Variation
- Random orientation of bivalents: 2n combinations (223 in humans).
- Crossing over at chiasmata swaps segments between non-sister chromatids.
HL · Cycle and cancer
- G1, S, G2, mitosis, cytokinesis; cyclin levels reach thresholds to pass checkpoints.
- Oncogenes (stuck accelerator) and lost tumor suppressors (failed brake); benign versus malignant; mitotic index = dividing cells ÷ all cells.
Key terms
- Cytokinesis
- The splitting of the cytoplasm of a parent cell between its daughter cells.
- Sister chromatids
- The two identical DNA molecules of a replicated chromosome, held together until anaphase.
- Histone
- A protein that DNA winds around; histones and supercoiling condense DNA into chromosomes.
- Mitosis
- Nuclear division that gives two genetically identical nuclei with the same chromosome number as the parent.
- Diploid (2n) and haploid (n)
- Diploid: chromosomes in homologous pairs. Haploid: one chromosome of each pair.
- Bivalent
- A pair of homologous chromosomes lined up together in meiosis I.
- Crossing over
- The exchange of segments between non-sister chromatids of homologous chromosomes in prophase I.
- Non-disjunction
- The failure of chromosomes or chromatids to separate in meiosis, giving gametes with too many or too few chromosomes.
- Meiosis
- A reduction division: two nuclear divisions that turn one diploid nucleus into four genetically varied haploid nuclei.
- Cell proliferation
- The production of many new cells by repeated cell division, for growth, replacement and repair.
- Interphase
- The stages G1, S and G2 between divisions, when the cell grows and replicates its DNA.
- Checkpoint
- A point in the cell cycle where the cycle continues only if conditions are satisfactory.
- Cyclin
- A protein whose rising and falling concentration controls passage through the cell cycle checkpoints.
- Proto-oncogene and oncogene
- A normal gene that promotes division; an overactive mutated form of it is an oncogene.
- Tumor suppressor gene
- A gene that restrains cell division; a mutation that disables it removes the brake.
- Malignant tumor
- A tumor that invades neighbouring tissue and can metastasize; a malignant tumor is cancer.
- Metastasis
- The spread of cells from a primary tumor through blood or lymph to form secondary tumors elsewhere.
- Mitotic index
- The number of cells in mitosis divided by the total number of cells in a sample.
Sample exam questions
Three of the 54 multiple-choice questions for D2.1. Try each one before opening the answer.
Question 1. In all living organisms, cell division produces new cells from a single parent cell. What is the parent cell in this process commonly called?
- Stem cell
- Mother cell
- Zygote
- Gamete
Show the answer
Answer: B. The parent cell that divides to produce two daughter cells is called the mother cell.
Question 2. How does cytokinesis occur in a plant cell, given that a rigid cell wall prevents the membrane from pinching inward?
- A contractile ring of actin and myosin pulls the wall apart
- The nucleus itself splits the cell wall in two
- Vesicles assemble sections of new membrane and cell wall at the cell's midline
- Microtubule motors physically tear the cell wall
Show the answer
Answer: C. In plant cells, Golgi-derived vesicles fuse at the equator to build a new cell plate of membrane and cell wall material.
Question 3. If non-disjunction of chromosome 21 occurs in meiosis I, what is the immediate consequence for the resulting gametes?
- All gametes get exactly one chromosome 21, as normal
- Some gametes get both homologous chromosome 21s, others get neither
- All gametes become diploid
- The gametes are unaffected because meiosis II corrects the error
Show the answer
Answer: B. If a homologous pair fails to separate in meiosis I, one resulting cell line gets both copies of chromosome 21 and the other gets none.
Practise D2.1
Study notes, every question and full markschemes for D2.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.