The Cell Cycle and Mitosis

Cell division is essential for growth, repair and replacement of cells in multicellular organisms. The Edexcel A-Level Biology specification (9BI0) requires a detailed understanding of the cell cycle, including interphase and the stages of mitosis, as well as the role of mitosis in organisms and how the cell cycle is controlled.

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The Cell Cycle

The cell cycle is the ordered sequence of events that takes place from the formation of a cell to the point at which it divides into two new daughter cells. In actively dividing human cells, the cell cycle typically lasts around 24 hours, though this varies significantly depending on cell type.

The cell cycle consists of three main phases:

1. Interphase (approximately 90% of the cell cycle)
2. Mitosis (nuclear division)
3. Cytokinesis (division of the cytoplasm)

The following diagram shows the stages of the cell cycle as a continuous loop:

graph LR
    A["G1<br/>Cell growth"] --> B["S Phase<br/>DNA replication"]
    B --> C["G2<br/>Preparation"]
    C --> D["Mitosis<br/>Nuclear division"]
    D --> E["Cytokinesis<br/>Cell division"]
    E --> A

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Interphase

Interphase is the longest phase of the cell cycle, during which the cell grows, carries out its normal functions and prepares for division. It is not a resting phase — the cell is metabolically very active.

Interphase is subdivided into three stages:

G$_1$1​ Phase (First Gap Phase)

• The cell grows in size and produces new organelles and proteins
• The cell carries out its normal metabolic functions
• Checkpoints monitor conditions — the cell assesses whether it has sufficient nutrients, growth factors and is large enough to divide
• If conditions are not favourable, the cell may enter G$_0$0​ phase — a quiescent (resting) state where it exits the cell cycle (some cells, such as neurons and mature red blood cells, remain in G$_0$0​ permanently)

S Phase (Synthesis Phase)

• DNA replication occurs — each chromosome is copied to produce two identical sister chromatids joined at the centromere
• At the end of S phase, the cell has twice the normal amount of DNA (but the chromosome number has not doubled — each chromosome simply consists of two chromatids)
• Centrioles are also duplicated (in animal cells)
• Histones and other chromosomal proteins are synthesised

G$_2$2​ Phase (Second Gap Phase)

• The cell continues to grow and produces proteins required for mitosis (e.g. tubulin for spindle fibres)
• Checkpoints ensure that DNA replication has been completed correctly and that any DNA damage has been repaired
• Organelles such as mitochondria increase in number to provide sufficient energy (ATP) for cell division

Phase	Duration (approx.)	Key events
G$_1$1​	5–12 hours	Cell growth, protein synthesis, organelle production
S	6–8 hours	DNA replication, centriole duplication
G$_2$2​	3–4 hours	Further growth, protein synthesis for mitosis, checkpoint

Exam Tip: A common mistake is to describe interphase as a "resting phase". It is anything but — the cell is growing, replicating DNA and preparing for division. The specification explicitly requires you to understand what happens during each stage of interphase.

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Mitosis

Mitosis is the division of the nucleus to produce two genetically identical daughter nuclei, each with the same number of chromosomes as the parent cell. It is followed by cytokinesis to produce two daughter cells.

Mitosis is a continuous process, but for convenience it is divided into four stages: prophase, metaphase, anaphase and telophase (remembered by the mnemonic PMAT).

Prophase

• Chromatin condenses into visible chromosomes, each consisting of two sister chromatids joined at the centromere
• The centrioles move to opposite poles of the cell and begin to form the spindle apparatus (made of microtubules)
• The nuclear envelope begins to break down
• The nucleolus disappears

Metaphase

• The spindle is fully formed, with spindle fibres extending from each pole
• Chromosomes line up along the metaphase plate (the equator of the cell)
• Each chromosome is attached to spindle fibres at its centromere via protein complexes called kinetochores
• The spindle assembly checkpoint ensures all chromosomes are correctly attached before the cell proceeds to anaphase

Anaphase

• The centromeres divide, separating the two sister chromatids
• The sister chromatids (now called daughter chromosomes) are pulled to opposite poles of the cell by the shortening of the spindle fibres
• The cell begins to elongate
• Each pole receives a complete set of chromosomes

Telophase

• The chromosomes arrive at the poles and begin to decondense (uncoil back into chromatin)
• A nuclear envelope re-forms around each set of chromosomes
• The nucleolus reappears in each new nucleus
• The spindle fibres disassemble

Stage	Key events	Mnemonic aid
Prophase	Chromosomes condense, spindle forms, nuclear envelope breaks down	Prepare
Metaphase	Chromosomes line up at the middle (equator)	Middle
Anaphase	Chromatids pulled apart to opposite poles	Apart
Telophase	Two nuclei form, chromosomes decondense	Two

Exam Tip: In exam questions that ask you to identify the stage of mitosis from a micrograph, look for: condensed chromosomes not yet aligned = prophase; chromosomes at the equator = metaphase; V-shaped chromosomes being pulled apart = anaphase; two groups of chromosomes at the poles with nuclear envelopes forming = telophase.

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Cytokinesis

Cytokinesis is the division of the cytoplasm following mitosis, producing two separate daughter cells.

• In animal cells: a cleavage furrow forms as a ring of actin microfilaments contracts, pinching the cell membrane inwards until the cell divides in two
• In plant cells: a cell plate forms at the equator from vesicles produced by the Golgi apparatus. The vesicles fuse to form a new cell wall and cell membrane between the two daughter cells

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Significance of Mitosis

Mitosis produces two daughter cells that are genetically identical to each other and to the parent cell. This is important for:

Function	Explanation
Growth	Increasing the number of cells in a multicellular organism during development
Repair	Replacing damaged cells (e.g. skin wound healing, liver regeneration)
Replacement	Replacing worn-out cells (e.g. red blood cells are replaced every ~120 days)
Asexual reproduction	Some organisms reproduce by mitosis alone (e.g. binary fission in unicellular eukaryotes, budding in yeast, vegetative propagation in plants)
Genetic consistency	Ensures all cells in a multicellular organism have the same genetic information

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Control of the Cell Cycle

The cell cycle is tightly regulated by a system of checkpoints and signalling molecules. If the cell fails to meet the criteria at a checkpoint, the cycle is halted until the issue is resolved, or the cell is directed to undergo apoptosis (programmed cell death).

Key Checkpoints

Checkpoint	Location in cycle	What is checked
G$_1$1​ checkpoint (restriction point)	End of G$_1$1​	Cell size, nutrients, growth factors, DNA damage
G$_2$2​ checkpoint	End of G$_2$2​	DNA replication complete and accurate? DNA damage?
Spindle assembly checkpoint	Metaphase	All chromosomes correctly attached to spindle fibres?

Cyclins and Cyclin-Dependent Kinases (CDKs)

The progression through the cell cycle is controlled by proteins called cyclins and cyclin-dependent kinases (CDKs):

• Cyclins are regulatory proteins whose concentration rises and falls at specific points in the cell cycle
• CDKs are enzymes that are only active when bound to a cyclin
• The cyclin-CDK complex phosphorylates target proteins, triggering specific events (e.g. DNA replication, chromosome condensation, nuclear envelope breakdown)
• Different cyclins are produced at different stages, ensuring the correct sequence of events

Cancer and Loss of Cell Cycle Control

Cancer results from uncontrolled cell division due to mutations in genes that regulate the cell cycle. Two key types of genes are involved:

• Proto-oncogenes — genes that stimulate cell division. When mutated, they become oncogenes that are permanently active, driving excessive cell proliferation
• Tumour suppressor genes (e.g. p53, Rb) — genes that inhibit cell division or promote apoptosis. When mutated, these brakes are removed, and cells divide uncontrollably

The protein p53 (encoded by the TP53 gene) is often called the "guardian of the genome". It:

• Activates DNA repair mechanisms when DNA damage is detected
• Halts the cell cycle at the G$_1$1​ checkpoint to allow repair
• Triggers apoptosis if the damage is too severe to repair
• Mutations in p53 are found in approximately 50% of all human cancers

Exam Tip: When answering questions about cancer, always link it back to the cell cycle. Explain that cancer involves mutations in genes controlling the cell cycle (oncogenes and tumour suppressor genes), leading to uncontrolled mitosis and tumour formation.

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Observing Mitosis: The Root Tip Squash

The Edexcel specification includes observing the stages of mitosis in a root tip squash preparation.

Method

1. Cut the tip (~1 cm) from a growing root (e.g. onion or garlic)
2. Place in acetic orcein (or toluidine blue) stain and warm gently — the stain binds to DNA and makes chromosomes visible
3. Place the stained root tip on a microscope slide, add a drop of acetic acid
4. Apply a coverslip and squash firmly by pressing down with a mounted needle or the flat side of a pencil (this spreads the cells into a single layer)
5. Observe under a light microscope at high magnification

Calculating the Mitotic Index

The mitotic index is the proportion of cells in a sample that are undergoing mitosis:

$\text{Mitotic index} = \frac{\text{Number of cells in mitosis}}{\text{Total number of cells observed}}$Mitotic index=Total number of cells observedNumber of cells in mitosis​

A high mitotic index indicates rapid cell division (e.g. in meristematic tissue, or in cancerous tissue).

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Summary

• The cell cycle consists of interphase (G$_1$1​, S, G$_2$2​) and mitosis (PMAT) followed by cytokinesis
• Interphase is the phase of growth, DNA replication and preparation
• Mitosis produces two genetically identical daughter cells and is important for growth, repair and asexual reproduction
• The cell cycle is controlled by checkpoints, cyclins and CDKs
• Loss of cell cycle control leads to cancer — mutations in oncogenes and tumour suppressor genes
• The mitotic index can be calculated by observing cells in a root tip squash

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A-Level Deep Dive: The Cell Cycle and Mitosis

Spec mapping

The Edexcel 9BI0 specification places the cell cycle and mitosis within Topic 2 (Cells, Viruses and Reproduction). Candidates must: describe the cell cycle as G$_1$1​ $\to$→ S $\to$→ G$_2$2​ $\to$→ M (PMAT) $\to$→ cytokinesis, recognising that interphase is by far the longest phase in actively dividing cells; identify the four stages of nuclear division (prophase, metaphase, anaphase, telophase) from light micrographs; explain the biological role of mitosis in growth, tissue repair, replacement of cells and asexual reproduction; describe how the cycle is regulated at the G$_1$1​/S, G$_2$2​/M and spindle-assembly checkpoints via cyclins and cyclin-dependent kinases (CDKs); and link checkpoint failure to uncontrolled proliferation and cancer. Synoptic links: Topic 2 (next lesson, meiosis as a contrast), Topic 5 (DNA replication in S phase, semi-conservative model), Topic 6 (cancer pathology — mutations in p53, Rb, oncogenes), Topic 8 (tissue regeneration and wound repair). Refer to the official Pearson Edexcel 9BI0 specification document for the exact wording.

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Worked example with full mark scheme

Question (8 marks):

A student examines a stained root-tip squash of Allium cepa (onion) under the high-power objective of a light microscope. From a field of view of 240 cells, she counts 12 cells in prophase, 6 in metaphase, 5 in anaphase and 7 in telophase.

(a) Calculate the mitotic index for this field of view, expressing your answer to 3 significant figures. (2)

(b) Suggest, with reasoning, why the count of cells in anaphase is the lowest. (2)

(c) Explain how a loss-of-function mutation in the p53 tumour-suppressor gene at the G$_1$1​/S checkpoint could result in tumour formation. (4)

Solution with mark scheme:

(a) Step 1 — total dividing cells. $12 + 6 + 5 + 7 = 30$12+6+5+7=30 cells in mitosis.

Step 2 — mitotic index. $\text{MI} = 30 / 240 = 0.125$MI=30/240=0.125.

M1 (AO2.1) — correct sum (30) and division by total (240). A1 (AO2.1) — quoted to 3 s.f. as 0.125 (or 12.5%).

(b) M1 (AO2.1) — anaphase is the shortest stage of mitosis (chromatid separation by spindle shortening occurs in only a few minutes), so the probability of catching a fixed cell in anaphase is correspondingly low. A1 (AO3.1a) — links the count to stage duration: in a fixed snapshot, the proportion of cells at any stage is approximately proportional to the time the cell spends there, so anaphase brevity gives the lowest count. A common pitfall is to write "anaphase is rare" without the duration argument — examiners credit the time-proportionality reasoning.

(c) M1 (AO1.2) — p53 is a transcription factor that, in response to DNA damage detected at the G$_1$1​/S checkpoint, upregulates p21 (a CDK inhibitor) and arrests the cycle so that DNA can be repaired before replication. M1 (AO2.1) — a loss-of-function p53 mutation removes this brake; cells with damaged DNA pass G$_1$1​/S and enter S phase, where mutations are replicated into both daughter cells. A1 (AO3.1a) — repeated cycles without checkpoint arrest cause accumulated mutations (including in further checkpoint genes and in oncogenes such as Ras), driving the multi-step progression characteristic of cancer. A1 (AO3.2a) — concludes that the resulting clone exhibits uncontrolled proliferation, evades apoptosis, and forms a tumour; p53 is mutated in roughly half of all human cancers, illustrating its role as the "guardian of the genome".

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Compare and contrast the events of interphase and mitosis, and explain why interphase typically occupies the majority of the cell cycle in dividing cells.

Mark scheme decomposition by AO:

Marking point	AO	Credit-worthy content
1	AO1.1	Interphase is subdivided into G$_1$1​ (cell growth, organelle synthesis), S (semi-conservative DNA replication; each chromosome acquires a sister chromatid) and G$_2$2​ (further growth, checkpoint and spindle-protein synthesis).
2	AO1.2	Mitosis comprises prophase (chromatin condenses, nuclear envelope breaks down, centrosomes migrate), metaphase (chromosomes align on the metaphase plate), anaphase (sister chromatids separate to opposite poles) and telophase (chromosomes decondense, nuclear envelope reforms); cytokinesis follows.
3	AO2.1	Compares the chromosomal state: in interphase chromosomes are decondensed chromatin (transcriptionally active and being replicated); in mitosis they are condensed and individually visible, transcriptionally silent.
4	AO2.1	Applies the duration logic: a typical mammalian cell with a 24 h cycle spends $\sim 11$∼11 h in G$_1$1​, $\sim 8$∼8 h in S, $\sim 4$∼4 h in G$_2$2​ and only $\sim 1$∼1 h in M — so interphase is $\sim 23/24$∼23/24 of the cycle.
5	AO3.1a	Justifies the imbalance: the cell must double its content (DNA, organelles, cytoplasm) before division, and must perform DNA replication accurately with proofreading and mismatch repair — both demand far more time than the mechanical task of segregating already-replicated chromosomes.
6	AO3.2a	Concludes that the disproportionate length of interphase is adaptive: it permits checkpoint arrest for damage repair (G$_1$1​/S, G$_2$2​/M), so prolonging interphase preserves genome integrity, whereas mitosis itself is a brief mechanical event once spindle attachment is verified.

Total: 6 marks split AO1 = 2, AO2 = 2, AO3 = 2. This is a typical Section A "compare and contrast" item — Edexcel rewards candidates who explain the imbalance (AO3) rather than simply list the stages (AO1).

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