The Mitotic Cell Cycle
How somatic cells produce genetically identical daughter cells. Chromosome architecture, the three-stage cell cycle, telomeres protecting chromosome ends, stem cells in tissue maintenance, and the consequences when control breaks down — tumour formation.
Replication and division of nuclei and cells
Cells reproduce by an ordered cycle of growth, DNA replication, and division. The product — genetically identical daughter cells — underpins growth of multicellular organisms, replacement of damaged cells, tissue repair, and asexual reproduction.
Topic 5 covers only the mitotic cell cycle — the production of two genetically identical diploid daughter cells. Meiosis (the reduction division producing four genetically distinct haploid gametes) is not in Topic 5; it appears in Topic 16 (A2 Inheritance). When answering Topic 5 questions, do not include meiosis.
Chromosome structure
A chromosome is a single long molecule of DNA wound around protein scaffolding, plus its associated proteins. The 9700 syllabus restricts the structural detail required to five components:
Each chromosome contains one continuous double-stranded DNA molecule running from one end to the other. The DNA carries the genes — the genetic information.
DNA is wound around clusters of histone proteins. Each cluster (with the DNA wrapped around it) is a nucleosome — the basic unit of DNA packaging. Histones organise the DNA into a compact structure that fits inside the nucleus.
After DNA replication in S phase, each chromosome consists of two identical sister chromatids — copies of the same DNA molecule, joined at the centromere. They remain joined until anaphase of mitosis.
A specialised constriction along the chromosome where the two sister chromatids are joined together. The centromere is the attachment point for spindle fibres during mitosis (via kinetochore proteins).
Repetitive DNA sequences at each end of the chromosome (in humans: TTAGGG repeats, hundreds to thousands of times). Telomeres do not contain protein-coding genes; their role is to protect the chromosome ends — explained in detail below.
A replicated chromosome (two sister chromatids joined at one centromere) counts as one chromosome, not two. Count by the number of centromeres, not the number of chromatids. Before S phase: one chromatid per chromosome. After S phase: two chromatids per chromosome. In both cases, the chromosome number is the same.
The importance of mitosis
Mitosis produces two daughter cells that are genetically identical to the parent cell — same chromosome number (diploid stays diploid), same DNA sequences, same alleles. This genetic uniformity is essential for four biological roles:
| Role | Why genetic identity matters | Example |
|---|---|---|
| Growth of multicellular organisms | All cells in the body must carry the same genome so they can carry out their specialised roles in a coordinated way | Embryonic development from zygote; growth of a tree from seedling to mature tree |
| Replacement of damaged or dead cells | Replacement cells must be functionally identical to those they replace | Skin epithelial cells (replaced every few weeks); red blood cell production from bone marrow stem cells |
| Repair of tissues by cell replacement | Damaged tissues are reconstituted with cells of the same type | Wound healing; bone repair after fracture; liver regeneration |
| Asexual reproduction | The whole organism is produced by mitosis, so offspring are genetically identical (clones) of the parent | Binary fission in Amoeba; runners and rhizomes in plants; budding in Hydra; vegetative propagation |
The mitotic cell cycle
The cell cycle is the sequence of events from one cell division to the next. It has three main parts:
The longest phase — cells spend approximately 90% of the cell cycle in interphase. The chromosomes are uncondensed (chromatin) and not visible as discrete units. Three sub-phases:
- G1 (first growth): cell grows, synthesises proteins, organelles increase in number, normal cellular activity
- S (synthesis): DNA replication; each chromosome becomes two identical sister chromatids
- G2 (second growth): further growth; checks DNA replication is correct; prepares for division (e.g. synthesises spindle proteins)
Nuclear division — chromosomes are now visible (highly condensed). Mitosis itself has four sub-phases (P, M, A, T) covered in section 5.2. The result of mitosis is two genetically identical nuclei within one cell.
Division of the cytoplasm. In animal cells, an actin ring (the contractile ring) pinches the cell membrane inwards forming a cleavage furrow. In plant cells, vesicles assemble at the equator forming a cell plate, which expands outwards to make a new cell wall. The single cell becomes two separate daughter cells.
For a typical human cell dividing every 24 hours: G1 ≈ 11 hours; S ≈ 8 hours; G2 ≈ 4 hours; M ≈ 1 hour. So interphase is the bulk (~95%) and mitosis is brief. Different cells divide at very different rates — bone marrow stem cells divide every few hours; mature neurons typically do not divide at all.
Telomeres protect chromosome ends
Telomeres are repetitive DNA sequences at the ends of chromosomes. Their main role is to prevent the loss of genes during DNA replication.
DNA polymerase cannot fully replicate the very end of a linear chromosome. With each round of replication, a small section is left uncopied at the chromosome ends — so chromosomes shorten slightly each time they replicate.
Telomeres act as a buffer: they have no protein-coding genes, so the shortening removes only telomere sequence, not vital information. After many divisions the telomere becomes too short, the cell stops dividing (cellular senescence) or self-destructs (apoptosis), so the cell never enters a state where genes are lost.
An enzyme called telomerase can extend telomeres. It is highly active in stem cells (allowing repeated divisions) and in most cancer cells (allowing unlimited division), but is largely switched off in mature somatic cells.
Stem cells
Stem cells are unspecialised cells that retain two key properties: (1) they can divide repeatedly by mitosis to produce more stem cells (self-renewal), and (2) they can give rise to specialised cell types when needed (differentiation).
Stem cells continually divide to replace cells that are lost — e.g. cells lost from the surface of skin and gut lining, red blood cells lost after their 120-day lifespan. Without stem cells, tissues with high turnover would rapidly run out of cells.
After injury, stem cells in or near the damaged tissue divide and differentiate to rebuild the tissue. Examples include haematopoietic stem cells in bone marrow replacing all blood cell types, and stem cells in skin epidermis healing wounds.
| Type | Potential | Source |
|---|---|---|
| Totipotent | Can give rise to any cell type, including extra-embryonic tissues (placenta) | Zygote and very early embryonic cells (first few divisions) |
| Pluripotent | Can give rise to any cell type in the body, but not extra-embryonic tissues | Inner cell mass of blastocyst (embryonic stem cells) |
| Multipotent | Can give rise to a limited range of related cell types | Adult/tissue stem cells (e.g. haematopoietic stem cells in bone marrow) |
Uncontrolled cell division and tumours
The cell cycle is normally tightly regulated: cells divide only when needed, and dysfunctional cells are eliminated by apoptosis. When this control breaks down, cells divide uncontrollably and accumulate as a tumour.
- A cell acquires mutations in genes that control the cell cycle (e.g. proto-oncogenes, tumour suppressor genes)
- Mutated genes no longer regulate division correctly — the cell may divide too rapidly, ignore stop signals, or fail to undergo apoptosis
- The cell divides uncontrollably; daughter cells inherit the same mutations and may acquire further mutations of their own
- A mass of dividing cells — a tumour — forms
- Some tumours remain localised (benign); others invade surrounding tissues and spread to other parts of the body via blood or lymph (malignant) — a malignant tumour is a cancer
Mutations are caused by errors in DNA replication and by damage from mutagens (UV, ionising radiation, certain chemicals). The body has DNA repair systems, but they are not perfect. Over decades of life, mutations gradually accumulate; on average, several mutations in cell-cycle control genes are needed before a tumour develops. This explains why cancer incidence rises sharply with age.
During which phase of the cell cycle is DNA replicated?
- A. G1 phase
- B. S phase
- C. G2 phase
- D. Mitosis
Telomeres are repeat sequences at the ends of linear chromosomes. They have an important role during DNA replication.
(a) Describe the structure of a chromosome, including telomeres and four other named features. [4]
(b) Explain how telomeres protect genes during DNA replication. [3]
(a) Chromosome structure [4 marks — one mark per feature, max 4 from list]
- A chromosome is a single double-stranded DNA molecule with associated proteins [1]
- DNA is wound around histone proteins (organised into nucleosomes), packaging the DNA compactly [1]
- After DNA replication (S phase), each chromosome consists of two identical sister chromatids [1]
- The two sister chromatids are joined at a region called the centromere [1]
- The ends of the chromosome are protected by telomeres — repetitive DNA sequences without protein-coding genes [1]
(b) How telomeres protect genes [3 marks]
- DNA polymerase cannot fully replicate the very end of a linear chromosome — a small region is left uncopied each time [1]
- Without telomeres, this end-shortening would gradually remove protein-coding genes — cells would lose vital genetic information after several divisions [1]
- Telomeres provide a non-coding buffer at chromosome ends; the shortening removes telomere sequence rather than genes; coding regions are protected [1]
Mark scheme guidance: Part (a) needs five named features (DNA, histones, sister chromatids, centromere, telomeres) but the question only asks for four marks — any four are creditable. Part (b) requires the “end-replication problem” idea explicitly.
Chromosome behaviour in mitosis
Mitosis is the nuclear division that produces two genetically identical daughter nuclei. It has four main stages, summarised by the mnemonic PMAT: Prophase → Metaphase → Anaphase → Telophase. The 9700 syllabus expects candidates both to describe chromosome behaviour and to identify the stages from photomicrographs and diagrams.
Prophase
- Chromatin condenses; each chromosome becomes visible as two sister chromatids joined at the centromere
- The nuclear envelope breaks down (begins to disappear)
- The nucleolus disappears
- The spindle apparatus begins to form — microtubules extending from opposite poles of the cell
- In animal cells, centrioles (paired structures) move to opposite poles; plant cells form spindles without centrioles
Metaphase
- The spindle is fully formed
- Spindle microtubules attach to the centromere of each chromosome (via kinetochore proteins on each sister chromatid)
- Each chromosome is pulled by microtubules from both poles — this aligns the chromosomes at the cell's equator (also called the metaphase plate)
- The chromosomes are now in a single line at the centre of the cell, each one ready to be split
Anaphase
- The centromeres divide — each sister chromatid is now an independent chromosome
- Spindle microtubules shorten, pulling the sister chromatids apart toward opposite poles of the cell
- Chromatids are pulled centromere-first; the chromatid arms trail behind, giving a characteristic V-shape under the microscope
- By the end of anaphase, an identical complete set of chromosomes has reached each pole
Telophase
- Chromosomes at each pole begin to de-condense back into chromatin (long, thin, not individually visible)
- A new nuclear envelope forms around each set of chromosomes — producing two nuclei
- Nucleoli reappear in each nucleus
- The spindle breaks down
- Cytokinesis usually begins late in anaphase or early in telophase and completes shortly after
PMAT summary table — recognising stages
| Stage | Chromosomes | Nuclear envelope | Spindle | Position in cell |
|---|---|---|---|---|
| Interphase | Decondensed (chromatin); not individually visible | Intact | Not formed | Diffuse throughout nucleus |
| Prophase | Condensing; visible as 2 sister chromatids per chromosome | Breaking down | Forming at poles | Spread within nucleus |
| Metaphase | Fully condensed; max visibility | Absent | Fully formed; attached to centromeres | Aligned at equator (metaphase plate) |
| Anaphase | Sister chromatids separated; V-shapes pulled apart | Absent | Microtubules shortening | Two groups moving to opposite poles |
| Telophase | De-condensing back to chromatin | Reforming around each set | Breaking down | Two clusters at opposite poles |
9700 examiners frequently show photomicrographs and ask which stage. Use this decision tree:
- Can you see distinct chromosomes? — If no, it's interphase
- Are chromosomes lined up in a single row at the equator? — metaphase
- Are V-shaped groups being pulled toward poles? — anaphase
- Are chromosomes at the poles, with new envelopes forming? — telophase
- Otherwise, with chromosomes visible inside an intact-but-fading nucleus? — prophase
You should also be ready to justify your identification — describe the visible features that match your chosen stage.
Cytokinesis — animal vs plant cells
| Feature | Animal cells | Plant cells |
|---|---|---|
| Mechanism | Cleavage furrow forms; an actin ring contracts and pinches the cell membrane inwards | Cell plate forms at the equator; vesicles from Golgi assemble there and fuse to form a new membrane and cell wall |
| Direction of progression | From outside in (membrane pinching) | From middle outwards (cell plate expanding to existing wall) |
| Reason for difference | No cell wall — can pinch directly | Cell wall present — must build a new wall in the middle |
Genetic significance of mitosis
Throughout mitosis, chromosomes behave in a way that ensures each daughter cell receives an identical complete set of genetic information:
- DNA replication during S phase produces two identical sister chromatids per chromosome — the DNA itself is faithfully copied
- The spindle attaches to both sister chromatids of every chromosome before any are separated — ensuring that one of each goes to each pole
- The sister chromatids of every chromosome separate simultaneously and migrate to opposite poles — never both to the same pole (when this fails, it's a serious error called nondisjunction)
- Two new nuclei reform, each receiving exactly one chromatid from every chromosome — giving identical chromosome sets
This precision is what makes the daughter cells genetically identical to the parent cell — the basis for all the roles of mitosis discussed in 5.1.
During which stage of mitosis do the centromeres divide and sister chromatids separate?
- A. Prophase
- B. Metaphase
- C. Anaphase
- D. Telophase
A student examines a slide of root tip cells undergoing mitosis. They identify a cell in which the chromosomes are lined up in a single row across the centre of the cell, with spindle fibres attached to each centromere.
(a) Identify the stage of mitosis and justify your answer using TWO observable features. [3]
(b) Describe what happens during the next stage of mitosis. [3]
(a) Stage and justification [3 marks]
Stage: Metaphase [1]
- Chromosomes are aligned at the equator / metaphase plate / centre of the cell [1]
- Spindle fibres are attached to centromeres of each chromosome [1]
- Chromosomes are at maximum condensation / clearly visible as discrete units [1]
- The nuclear envelope is absent [1]
(b) Next stage — anaphase [3 marks]
- The centromeres divide — each sister chromatid becomes an independent chromosome [1]
- Spindle microtubules shorten, pulling the sister chromatids apart towards opposite poles of the cell [1]
- Chromatids are pulled centromere-first, with arms trailing — giving the characteristic V-shape [1]
- By the end of anaphase, an identical complete set of chromosomes has reached each pole (also acceptable for the third mark)
Mark scheme guidance: The justification marks in (a) require observable features visible on the slide, not general statements about metaphase. “Chromosomes are at the equator” is a feature; “DNA has been replicated” is not (you can't see this on the slide).
Topic 5 Practice — Comprehensive
Mixed practice covering both sub-sections in 9700 P1/P2 style. Try each before revealing the answer.
Which sequence correctly orders the events of the cell cycle?
- A. G1 → G2 → S → M → cytokinesis
- B. G1 → S → G2 → M → cytokinesis
- C. M → G1 → G2 → S → cytokinesis
- D. S → G1 → G2 → M → cytokinesis
A diploid human cell has 46 chromosomes. After DNA replication in S phase, how many chromosomes does it contain?
- A. 46 chromosomes, each with two sister chromatids
- B. 92 chromosomes, each with one chromatid
- C. 23 chromosomes, each with two sister chromatids
- D. 92 chromosomes, each with two sister chromatids
Which property best distinguishes a stem cell from a fully differentiated cell?
- A. Stem cells have more genes than other cells.
- B. Stem cells contain unique organelles not found in other cells.
- C. Stem cells can both self-renew by mitosis and differentiate into specialised cell types.
- D. Stem cells lack a nucleus.
Bone marrow contains haematopoietic stem cells, which divide continuously to produce all types of blood cell. A patient with leukaemia has uncontrolled mitosis in bone marrow stem cells.
(a) Explain how stem cells contribute to the daily replacement of red blood cells. [3]
(b) Describe how a mutation in a gene that controls the cell cycle could lead to leukaemia. [3]
(c) Suggest why bone marrow stem cells are particularly likely to develop cancer-causing mutations compared with mature, non-dividing cells. [2]
(a) Stem cells in red blood cell replacement [3 marks]
- Mature red blood cells live for around 120 days and lose function over time, so must be continually replaced [1]
- Bone marrow contains haematopoietic stem cells which can self-renew (divide by mitosis to produce more stem cells) [1]
- Some daughter cells differentiate into specialised cells — eventually becoming mature red blood cells with haemoglobin (and the other blood cell types) [1]
(b) Mutation leading to leukaemia [3 marks]
- A mutation in a gene that controls the cell cycle (e.g. a proto-oncogene or tumour suppressor gene) alters cell-cycle regulation [1]
- The mutated cell may divide too rapidly, fail to respond to stop signals, or fail to undergo apoptosis when it should [1]
- Uncontrolled mitosis produces a mass of abnormal cells (a tumour) in bone marrow — leukaemia is cancer of blood-forming cells [1]
(c) Why dividing cells are more vulnerable [2 marks]
- Stem cells divide frequently, and DNA replication itself is a source of mutations (small error rate per replication) [1]
- Mature, non-dividing cells do not replicate their DNA, so they accumulate fewer replication-induced mutations; mutations are also unlikely to be passed on if the cell does not divide [1]
- Alternative point: stem cells live a long time and accumulate damage from mutagens (UV, radiation, chemicals) across many divisions [1]
Synoptic note: This question links Topic 5 (cell cycle, stem cells, tumours) with Topic 1 (specialised cells: red blood cells without nuclei). Mature red blood cells cannot divide and cannot be the source of leukaemia — only the stem cells in bone marrow can.
Describe the differences between metaphase and anaphase of mitosis. Refer to chromosome behaviour, the spindle, and the position of structures in the cell. [6]
Six creditable points (any six):
- Chromosomes: in metaphase, each chromosome consists of two sister chromatids joined at the centromere; in anaphase, the sister chromatids have separated and become independent chromosomes [1]
- Centromeres: in metaphase, the centromeres are intact; in anaphase, the centromeres have divided [1]
- Position: in metaphase, all chromosomes are aligned at the equator (metaphase plate); in anaphase, two groups of chromatids move toward opposite poles [1]
- Spindle: in metaphase, the spindle is fully formed and attached to centromeres; in anaphase, microtubules shorten, pulling chromatids apart [1]
- Shape: in metaphase, chromosomes are X-shaped (or paired) at the equator; in anaphase, chromatids are pulled centromere-first with arms trailing — characteristic V-shape [1]
- Movement: in metaphase, chromosomes are stationary at the equator; in anaphase, chromatids are actively moving toward poles [1]
Topic 5 — Common Mistakes
- 🧬Including meiosis in Topic 5 answersMeiosis is not in Topic 5 of the 9700 syllabus — it appears in Topic 16 (A2 Inheritance). Topic 5 covers only the mitotic cell cycle. Confusing the two costs marks even when the meiosis content itself is correct.
- 📊Counting sister chromatids as separate chromosomesA replicated chromosome (two sister chromatids joined at one centromere) is one chromosome, not two. Always count by centromeres. After S phase a human cell has 46 chromosomes (each with two chromatids), not 92.
- ♻Saying mitosis halves the chromosome numberWrong. Mitosis preserves chromosome number — diploid cells produce diploid daughter cells. The halving (reduction division) is meiosis, which is in Topic 16.
- ⚠Forgetting that interphase is part of the cell cycleInterphase is the longest phase but is not part of mitosis itself. Some students describe a “5-stage mitosis” including interphase — wrong. Mitosis = PMAT only. Interphase + mitosis + cytokinesis = the complete cell cycle.
- 🌉Mixing up what happens in S phase vs M phaseS phase: DNA replication only. Chromosomes are not visible as discrete units, and there is no division. Mitosis (M): chromosomes are highly condensed and visible, and they are physically separated. Don't conflate the two.
- 🔒Saying telomeres “cap and protect” without explaining the end-replication problemThe 9700 mark scheme is specific: telomeres prevent the loss of genes during DNA replication. The mechanism is that DNA polymerase cannot fully replicate chromosome ends, so end sequences are lost each time; telomeres are non-coding sacrificial sequences. Vague “protective cap” statements typically score one mark out of three.
- 🧬Treating “stem cell” and “embryonic cell” as synonymsEmbryonic cells are one type of stem cell (pluripotent), but stem cells also exist in adult tissues (multipotent — e.g. haematopoietic in bone marrow). Saying stem cells exist only in embryos is incorrect.
- 🐷Saying cancer cells “multiply faster” than normal cellsNot always true — some cancer cells divide more slowly than normal stem cells. The defining feature is that cancer cells divide uncontrollably: they fail to respond to normal stop signals, do not undergo apoptosis, and do not exit the cell cycle when they should. Speed is variable; loss of regulation is universal.
- 🌿Plant cells use cleavage furrows for cytokinesisWrong. Plant cells form a cell plate at the equator, which expands outwards to form a new cell wall and membrane. Cleavage furrows are an animal-cell mechanism — impossible in plants because of the rigid existing cell wall.
- 🔮Identifying mitosis stages without justifying with observable features9700 examiners ask candidates to identify and explain why. Saying “this is metaphase” without describing a visible feature (chromosomes at equator, spindle attached, etc.) typically scores zero or one mark. Always pair the answer with the visible evidence.
- 🎯Saying chromosomes “line up” in anaphaseAnaphase is the opposite of lining up — chromatids are being pulled apart from the equator toward poles. Lining up at the equator happens in metaphase. Mixing them up is one of the most common identification errors on photomicrograph questions.
Topic 5 connects forward to Topic 6 (DNA structure and replication mechanism), Topic 11 (clonal selection during immune response), and Topic 16 (meiosis vs mitosis comparisons in inheritance). Highest-yield items: chromosome structure with all five named features, the three-stage cell cycle with what happens in each, telomeres and the end-replication problem, the four PMAT stages with observable features, and the link from cell-cycle mutations to tumour formation. Practical 5 (root tip squash) makes mitotic stage identification a high-frequency Paper 3 question — learn to justify identifications from photomicrographs.