Making new cells
p. 648A parent cell, often called a mother cell, divides to form two daughter cells. New cells arise only by division of existing cells. Division provides for growth, repair, maintenance and reproduction.
Guiding questions
- How are large numbers of cells with identical genes produced?
- How do eukaryotes make genetically varied cells that can become gametes?
Slides
Notes
Written for this site from the course textbook and class slides, with page references for each statement. The chapter is on pages 648–661 of the course textbook. Image credits are on slide 22.
Why cells divide
What mitosis and meiosis each do
pp. 650–651Mitosis maintains the chromosome number and the genome. Meiosis halves the chromosome number, and generates genetic diversity. The nucleus divides before the cell, so that no anucleate cells are produced.
Why DNA is replicated first
p. 651Replication in interphase gives each chromosome two DNA molecules, called chromatids, held together at a centromere. Each new nucleus can then receive a full set of genes.
Watch out: a replicated chromosome still counts as one chromosome. Once chromatids separate, each is counted as a chromosome.
Condensation and movement of chromosomes
pp. 652–653DNA wraps around proteins called histones, and further coiling, known as supercoiling, gives short, compact chromosomes. Microtubules form a spindle, and microtubule motors use ATP to move the chromosomes. Both mitosis and meiosis share these features.
Watch out: centromere, centrosome and centriole are three different structures.
Mitosis and cytokinesis
The phases of mitosis
pp. 653–654
Mitosis gives two identical nuclei. DNA is copied exactly, and then one chromatid of each chromosome moves to each pole.
Identifying the phases
pp. 655–656- Prophase: threads visible, no clear line
- Metaphase: a line at the centre
- Anaphase: two groups moving apart, in V shapes
- Telophase: two groups at the poles
The mitotic index is the number of cells in mitosis divided by the total number of cells.
Cytokinesis
p. 649In animal cells, a ring of actin and myosin contracts and pinches the membrane together. In plant cells, vesicles gather at the equator and assemble sections of membrane and cell wall.
Equal and unequal cytokinesis
pp. 649–650Cytoplasm is usually shared equally. In oogenesis in humans, the egg keeps most of it, and in yeast budding a small bud forms. Each daughter cell needs at least one mitochondrion, and any other organelle made only by dividing an existing one.
Meiosis
Meiosis halves the chromosome number
pp. 656–658Diploid nuclei have two sets of chromosomes, in homologous pairs, and haploid nuclei have one set. One diploid nucleus gives four haploid nuclei. Sexual life cycles need this, since fertilization doubles the number.
| Meiosis I | Meiosis II | |
|---|---|---|
| Separates | Homologous chromosomes | Sister chromatids |
| Result | Two haploid nuclei | Four haploid nuclei |
Non-disjunction and Down syndrome
pp. 658–659In non-disjunction, chromosomes fail to separate in meiosis. If a gamete with two copies of chromosome 21 is fertilized, the zygote has three copies, and 47 chromosomes in total. This is trisomy 21, or Down syndrome. The risk rises with the age of the mother.
How meiosis generates variation
pp. 660–661Random orientation: each bivalent lines up independently in metaphase I. With a haploid number of n, there are 2 to the power n combinations, which is over 8 million in humans.
Crossing over: in prophase I, non-sister chromatids swap sections, giving new combinations of alleles on a chromosome.
Watch out: neither process creates new alleles. Only mutation does that.
Videos
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Note-taking sheet
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
Quick self-check
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Practice questions
A printable worksheet covering all eleven statements, from quick recall to data questions and a longer answer. It has 48 marks and takes about 50 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Data-based questions
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 9 marks in total. Your teacher has the mark scheme.
A drug that targets mitosis
9 marks
Onion root tips were grown for 24 hours in different concentrations of a drug being tested as an anticancer treatment. Root growth was measured, and cells from the tips were stained and counted.
| Drug / µmol dm−3 | 0 | 10 | 20 | 40 | 80 |
|---|---|---|---|---|---|
| Mitotic index / % | 12 | 15 | 19 | 24 | 27 |
| Cells in anaphase per 100 cells in mitosis | 21 | 14 | 8 | 4 | 2 |
- State the root growth, as a percentage of the control, at 40 µmol dm−3. [1]
- At 80 µmol dm−3, 54 of 200 cells in one field of view were in mitosis. Calculate the mitotic index. [1]
- Describe the relationship between drug concentration and mitotic index. [1]
- Evaluate the hypothesis that the drug stops cells from completing mitosis. [3]
- Suggest how the drug could prevent cells from entering anaphase. [1]
- State one advantage and one disadvantage of using plant root tips to test drugs meant for human cancer. [2]
Practical
Finding the mitotic index of a root or shoot tip
You will need: Prepared slides of Allium root or shoot tip, microscope, tally chart
- Predict how the mitotic index will differ between the tip and further back
- Find the dividing region, then count the cells in each phase of mitosis and in interphase
- Calculate the mitotic index as cells in mitosis divided by the total number of cells
- Compare your result with your prediction, and draw one cell from each phase
Safety: Use prepared slides to avoid acid and stains. Carry microscopes with two hands.
Carry this out with your teacher's supervision and follow your school's lab rules.
Nature of science and TOK
Nature of science
Observations
New staining methods let Walther Flemming describe mitosis in 1882, showing how technique can open up what is visible.
Theory of knowledge
Do our tools reveal reality or shape it?
Stains and fixatives alter cells before we see them, and cells on a slide are dead. How do we know that what we see is not an artefact?
Syllabus statements
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
- D2.1.1Cell division as the way organisms produce new cellsp. 648
- D2.1.2Cytokinesis: dividing a parent cell's cytoplasm between daughter cellsp. 649
- D2.1.3Even and uneven cytokinesisp. 649
- D2.1.4What mitosis and meiosis each do in eukaryotesp. 650
- D2.1.5Why DNA must be replicated before mitosis or meiosisp. 651
- D2.1.6Chromosome condensation and movement, common to mitosis and meiosisp. 652
- D2.1.7The stages of mitosisp. 653
- D2.1.8Recognising the stages of mitosisp. 655
- D2.1.9Meiosis halves the chromosome numberp. 656
- D2.1.10How non-disjunction can cause Down syndromep. 658
- D2.1.11How meiosis generates variationp. 660
HL extension
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