Mitosis gets all the attention, but it is a small slice of a cell’s life. Most of the time a cell is in interphase, growing, working and quietly copying its DNA. And the decision to divide at all is not casual — it is controlled by a chemical timer that has to be satisfied at three separate checkpoints.
📘 What you need to know
The cell cycle is the regulated sequence of events between one cell division and the next. Rapid increase in cell number is proliferation.
It has three parts in order: interphase (G1, S, G2), nuclear division (mitosis) and cell division (cytokinesis).
Interphase is the longest and most active phase. The cell grows, makes proteins, increases its number of mitochondria and chloroplasts, and replicates its DNA.
G1: growth — RNA, enzymes and other proteins are made. A signal here tells the cell to divide; cells that never receive it enter G0.
S: DNA replicates, so each chromosome ends up with two sister chromatids. G2: growth continues, the new DNA is checked and errors repaired, and tubulin is made for the spindle.
The cycle is controlled by cyclins (proteins) and cyclin-dependent kinases (enzymes). There are three checkpoints: G1, G2 and M.
Plant growth happens at meristems: shoot apical, root apical, axillary buds and lateral meristems (cambium) in the vascular bundles.
Where proliferation happens
Plants and animals both begin as a zygote that divides repeatedly to form an embryo. That rapid increase in cell number is called proliferation, and it continues in particular places throughout life.
In plants: meristems
Meristem cells are undifferentiated and actively divide by mitosis to make new plant tissue.
The shoot apical meristem sits at the tip of a shoot; the root apical meristem at the tip of a root.
Axillary buds contain meristem tissue that can form new side branches.
Lateral meristems run parallel to the sides of the stem, within the vascular bundles that contain xylem and phloem. They let stems grow in diameter, and this tissue is called cambium.
In animals: embryos, replacement and repair
After fertilisation, the ovum divides by mitosis into a two-cell embryo, then four, and eventually a hollow ball called a blastocyst containing blastomeres, which are undifferentiated and will form the foetus.
Skin cells in the epidermis are lost daily and replaced by proliferation of stem cells in the basal layer.
In wound healing, once a clot has sealed the wound, blood vessels dilate so macrophages and fibroblasts can reach it. Macrophages engulf pathogens; fibroblasts proliferate, break down the fibrin and lay down a new matrix of collagen fibres to close the wound.
The shape of the cycle
The proportions matter as much as the order. A cell spends most of its life in interphase; mitosis and cytokinesis are brief by comparison.
The widths are roughly to scale. This is why, on any slide, most of the cells you can see are in interphase.
The length of the cycle varies with environmental conditions, cell type and organism. Onion root tip cells divide roughly once every 20 hours; human intestinal epithelial cells roughly every 10 hours.
Inside interphase
Phase
What “G” or “S” stands for
What happens
G1
Growth
Cell makes RNA, enzymes and other proteins needed for growth; a signal to divide may be received here
S
Synthesis of DNA
DNA in the nucleus replicates, so each chromosome consists of two identical sister chromatids
G2
Growth
Cell continues to grow, the new DNA is checked and errors repaired, and tubulin is made for the spindle
G0. Some cells never receive the signal to divide again. They leave the cycle at G1 and enter G0, where they continue to function but no longer divide. Mature nerve cells are a familiar example.
Control by cyclins
Movement from one phase to the next is triggered by chemical signals called cyclins. The cycle has three checkpoints, at G1, G2 and M, which must be passed before the next stage can begin.
There are four cyclins, D, E, A and B, whose concentrations rise and fall over the cycle, each triggering specific events. The mechanism works like this:
A cyclin’s concentration rises until it reaches a threshold level.
At that concentration it binds to a cyclin-dependent kinase (CDK), forming an activated complex.
The complex phosphorylates a target protein — attaches a phosphate group — which activates it.
The activated target protein triggers a specific function, such as DNA replication.
Once that job is complete the phosphate is released, the cyclin breaks down, and the CDK becomes inactive again.
The point of all this is timing: it ensures DNA replication, organelle multiplication and protein synthesis happen in the right order and at the right moment.
You are not expected to know what each individual cyclin does — only the mechanism and why timing matters.
Worked examples
WORKED EXAMPLE
Proportion of the cycle spent in mitosis
A cell cycle lasts 20 hours, of which mitosis takes 1.5 hours. Calculate the percentage of the cycle spent in mitosis, and comment on what this means for a stained slide.
Step 1: Set up the calculation
Percentage = (time in mitosis ÷ total cycle time) × 100
Step 2: Substitute(1.5 ÷ 20) × 100 = 7.5%Step 3: Interpret
Only about 7.5% of the cells on a slide would be caught in mitosis; the rest would be in interphase.
7.5%Proportion of cells in a stage is proportional to the time that stage takes.
WORKED EXAMPLE
Which phase makes the most mRNA?
Identify the phase of the cell cycle in which a cell would be producing the greatest number of mRNA molecules, and explain your answer.
Step 1: Link mRNA to a process
mRNA is made by transcription, the first stage of protein synthesis.
Step 2: Find where protein synthesis peaks
In G1 the cell makes RNA, enzymes and other proteins needed for growth.
Step 3: Rule out the rest
S phase is dominated by DNA replication, and during mitosis the chromosomes are condensed and not being transcribed.
G1 phaseCondensed chromosomes cannot be transcribed — a useful point to add.
WORKED EXAMPLE
Blocking a kinase
A drug prevents cyclins from binding to cyclin-dependent kinases. Predict the effect on a population of dividing cells.
Step 1: What the binding does
Binding forms an activated complex that phosphorylates target proteins.
Step 2: What is lost
Without it, no target protein is activated, so the trigger for the next stage never arrives.
Step 3: Predict the outcome
Cells would be held at a checkpoint and would not progress into division.
The cells stop advancing through the cycle and cease dividingDrugs of this general type are used to slow rapidly dividing cancer cells.
💡 Exam tip
Know what happens in each part of interphase, not just the names. G1 growth, S replication, G2 checking.
Say the cycle is interphase, mitosis, cytokinesis in that order — and that interphase is the longest.
Name both control molecules: cyclins and cyclin-dependent kinases, and use the word phosphorylates.
Give the three checkpoints: G1, G2 and M.
For plants, be specific about meristem type: shoot apical, root apical, axillary bud, lateral (cambium).
Link wound healing to fibroblast proliferation and collagen, not just “the skin repairs itself”.
⚠ Common mix-up
Calling interphase a resting phase. It is the most metabolically active part of the cycle.
Treating interphase as part of mitosis. Mitosis is only prophase to telophase.
Putting DNA replication in prophase. It happens in S phase, well before.
Confusing G0 with G1. G0 is leaving the cycle; G1 is part of it.
Saying cyclins are enzymes. Cyclins are proteins; the kinases are the enzymes.
Thinking meristems are only at the tips. Lateral meristems in the vascular bundles let stems thicken.
Up next: Uncontrolled Division & Tumours — you have just seen how carefully division is regulated. Now we look at what happens when the genes running that control system are the ones that mutate.
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