The mitotic cell cycle

• S phase doubles the DNA, not the chromosome number: count chromosomes by their centromeres, and four chromosomes in the parent means four in each daughter.
• Count the centromeres. S phase doubles the DNA, not the chromosomes. Centromeres divide, sister chromatids to opposite poles, four in, four out.

• 5.1.1 I can describe — describe the structure of a chromosome, limited to: • DNA • histone proteins • sister chromatids • centromere • telomeres
• 5.1.2 I can explain — explain the importance of mitosis in the production of genetically identical daughter cells during: • growth of multicellular organisms • replacement of damaged or dead cells • repair of tissues by cell replacement • asexual reproduction
• 5.1.3 I can outline — outline the mitotic cell cycle, including: • interphase (growth in G₁ and G₂ phases and DNA replication in S phase) • mitosis • cytokinesis
• 5.1.4 I can outline — outline the role of telomeres in preventing the loss of genes from the ends of chromosomes during DNA replication
• 5.1.5 I can outline — outline the role of stem cells in cell replacement and tissue repair by mitosis
• 5.1.6 I can explain — explain how uncontrolled cell division can result in the formation of a tumour
• 5.2.1 I can describe — describe the behaviour of chromosomes in plant and animal cells during the mitotic cell cycle and the associated behaviour of the nuclear envelope, the cell surface membrane and the spindle (names of the main stages of mitosis are expected: prophase, metaphase, anaphase and telophase)
• 5.2.2 I can interpret — interpret photomicrographs, diagrams and microscope slides of cells in different stages of the mitotic cell cycle and identify the main stages of mitosis

Precise vocabulary is part of the biology. Water moves down a water potential gradient; an active site is complementary to its substrate; enzymes are denatured, not killed; ATP releases energy when it is hydrolysed, and respiration never produces energy. Give a calculated answer to the same number of significant figures as the least precise data, or one more, with its unit. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as variables, method, recording, graphs and evaluation rather than as theory.

• “After S phase the cell has twice as many chromosomes.” Correct S phase doubles the DNA, not the chromosome number. Each chromosome now has two sister chromatids, but it has one centromere and is still one chromosome. Count chromosomes by centromeres.
• “In anaphase the chromosomes separate.” Correct The centromeres divide and the sister chromatids of each chromosome separate to opposite poles. Name the chromatids; “chromosomes separate” does not say what splits.
• “In metaphase the chromosomes line up in pairs.” Correct In mitosis every chromosome lines up separately on the equator, with spindle fibres from both poles attached at its centromere.
• “Plant cells divide by forming a cleavage furrow.” Correct A plant cell has a rigid wall and forms a cell plate from vesicles at the equator; an animal cell forms a cleavage furrow. Plant cells also have no centrioles.
• “Telomeres are genes that protect the chromosome.” Correct Telomeres are repeated, non-coding base sequences at the ends. What is lost at each replication is telomere, so no gene is lost.
• “A tumour forms because a mutation makes cells grow bigger.” Correct The mutation is in a gene that controls cell division; the cell divides by mitosis without control, every daughter inherits the mutation, and the number of cells rises until a mass forms.
• “Most cells on the slide are in prophase because it is the longest stage.” Correct Most cells are in interphase, the longest part of the cycle. Prophase is only the longest stage of mitosis.
• “After S phase the cell has twice as many chromosomes.” Repair The DNA doubles; each chromosome now has two chromatids, but it is still one chromosome with one centromere.
• “Nothing happens in interphase; the cell is just waiting to divide.” Repair In interphase the cell grows, makes proteins, RNA and organelles, replicates its DNA (S phase) and makes the proteins of the spindle (G2).
• “Sister chromatids are two different chromosomes.” Repair They are two identical copies of one chromosome, joined at the centromere.
• “In metaphase the chromosomes line up in pairs.” Repair In mitosis each chromosome lines up separately on the equator.
• “In anaphase the chromosomes separate.” Repair The centromeres divide and the sister chromatids separate to opposite poles.
• “The spindle fibres attach to the ends of the chromosomes.” Repair They attach at the centromere, which is why the centromere leads in anaphase.
• “Plant cells form a cleavage furrow.” Repair Plant cells form a cell plate from vesicles at the equator; animal cells form a cleavage furrow.
• “Plant cells use centrioles to make the spindle.” Repair Plant cells have no centrioles; they make a spindle without them.
• “Mitosis halves the chromosome number.” Repair Each daughter cell has the same number as the parent (4 → 4; 46 → 46).
• “Telomeres contain genes that protect the chromosome.” Repair Telomeres are repeated non-coding sequences; the length lost at each replication is telomere, so no gene is lost.
• “Stem cells are specialised cells that repair tissue.” Repair Stem cells are unspecialised; they divide by mitosis and their daughters can remain stem cells or differentiate.
• “A tumour forms because a mutation makes the cells grow bigger.” Repair The mutation is in a gene that controls cell division; the cells divide by mitosis without control, so the number of cells increases.
• “Most cells on the slide are in prophase because it is the longest stage.” Repair Most cells are in interphase, the longest part of the cycle; prophase is only the longest stage of mitosis.
• “Cytokinesis is the last stage of mitosis.” Repair Mitosis is nuclear division, in four stages; cytokinesis is the separate division of the cytoplasm.
• “The ring of chromosomes in cell G is prophase.” Repair A ring around a clear centre is a metaphase plate seen end-on, from a pole. Prophase threads fill a nucleus-shaped area.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.
• Interleave with the chapters that use this one. Topic 6 explains S phase: when you reach 6.1.4, re-answer “why is the end of a strand not copied, and what do telomeres do about it?”. Topic 11 uses mitosis in the division of lymphocytes: re-answer “why are the daughter cells of a lymphocyte genetically identical?”. Topic 16 builds on these stages: before starting it, re-draw the storyboard. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 5 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• A multiple-choice item turns on precise counts and sequences: chromosomes, chromatids and DNA molecules at a named stage, the stage in which the centromeres divide, or which feature belongs to plant or animal cytokinesis. A structured question asks you to describe the behaviour of the chromosomes, nuclear envelope, membrane and spindle, to explain why the daughter cells are identical, or to explain a tumour as cause, mechanism and outcome.
• A DNA-content graph to read phase by phase; a photomicrograph or drawing of a root-tip squash to identify stages in; tables of cell counts. The numerical skills are percentages and pie-chart angles, actual size from magnification (both recall), and a mitotic index or a stage length from a relationship the question states.
• A Paper 3 slide task: prepare and stain a root-tip squash, find the meristem, identify and draw stages, measure with a calibrated graticule and count cells. As an investigation, the independent variable is distance from the tip or a treatment, the dependent variable is the mitotic index from a fixed number of cells, and the main error is classifying cells at the boundaries between stages. Paper 5 can ask you to plan such an investigation.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.