Edexcel GCSE Biology (1BI0)

2.1 Mitosis and the cell cycle

Edexcel GCSE Biology 2.1 is all about mitosis and the cell cycle — how one parent cell becomes two genetically identical daughter cells, and what happens when the controls on that process break down. Unlike AQA, Edexcel keeps stem cells in a separate topic (2.2), so 2.1 zeroes in on the cycle itself: interphase, the named mitosis stages, the role of spindle fibres, and the link between uncontrolled mitosis and cancer. The exam asks the same handful of questions every series — describe what happens in interphase, sequence the stages of mitosis, explain why daughter cells are genetically identical, and explain how uncontrolled mitosis leads to a tumour. Each of these has very precise marking phrases. Learn them here and the marks come easily.

From the 1BI0 specification

Edexcel 1BI0 2.1: The cell cycle is the series of events between one cell division and the next. Most of the cycle is interphase, during which the cell grows, makes more sub-cellular structures (ribosomes, mitochondria), and replicates its DNA so each chromosome has two identical sister chromatids. Mitosis then divides the nucleus: chromosomes condense and become visible, line up at the equator, sister chromatids are pulled to opposite poles by spindle fibres, and a new nuclear envelope forms around each set. Cytokinesis follows — the cytoplasm and cell membrane divide to produce two genetically identical diploid daughter cells. Mitosis is essential for growth, tissue repair, and asexual reproduction. Uncontrolled mitosis produces a tumour, the basis of cancer.

Why this matters

You started life as a single fertilised egg. Every one of your trillions of body cells today is descended from that one cell by mitosis — a kind of cell division that produces two genetically identical 'daughter' cells from one 'parent'. Your body uses mitosis constantly: to grow from a baby into an adult, to repair cuts and broken bones, to replace cells that wear out (skin every few weeks, gut lining every few days). When the controls on mitosis break, cells divide uncontrollably and form a tumour — that's cancer. Mitosis is also the basis of asexual reproduction in some organisms (bacteria, hydra, strawberry runners), and of cloning in agriculture. Edexcel 2.1 expects you to know the cell cycle in detail — interphase, mitosis (with the named stages), cytokinesis — and the consequences when the cycle's control mechanisms fail.

How to learn this topic

Build on what you already know

KS3: cells divide to make new cells; growth requires more cells.
Edexcel 1.1: the nucleus contains DNA / chromosomes.
Edexcel 1.x: humans have 46 chromosomes (23 pairs) in body cells — they are 'diploid'.

Chromosomes first — what they are, where they sit, and why they have to be copied before division.
The cell cycle: a long interphase (growth + DNA replication) followed by a quick mitosis + cytokinesis.
Mitosis stage by stage — prophase, metaphase, anaphase, telophase, cytokinesis. Tie each name to what physically happens.
Why both daughter cells are genetically identical and diploid.
Why mitosis matters in real life: growth, tissue repair, asexual reproduction.
What happens when the cell cycle is not controlled — tumour formation and cancer.

Key terms

chromosome: A long, coiled strand of DNA found in the nucleus. Carries the cell's genetic information in the form of genes. (Examiners want 'chromosomes contain DNA / genes / genetic material'. Saying 'chromosomes ARE genes' is the most common misuse.)
diploid: Having two sets of chromosomes (one from each parent). Human body cells are diploid with 46 chromosomes (23 pairs). (Edexcel uses 'diploid' as a marking phrase for daughter cells after mitosis — they are diploid like the parent.)
mitosis: A type of cell division that produces two daughter cells, each genetically identical and diploid. Used for growth, tissue repair, and asexual reproduction. (Specify 'genetically identical' AND 'daughter cells' — both phrases are usually required for the full mark.)
cell cycle: The controlled sequence of events between one cell division and the next: interphase (growth + DNA replication) followed by mitosis (nuclear division) and cytokinesis (cytoplasmic division). (Edexcel emphasises that the cycle is CONTROLLED — when it's not, you get a tumour.)
interphase: The longest stage of the cell cycle. The cell grows, makes more sub-cellular structures (ribosomes, mitochondria), and replicates its DNA so each chromosome has two sister chromatids. (Don't say mitosis happens during interphase — interphase is the PREPARATION phase before mitosis.)
prophase: The first named stage of mitosis. Chromosomes condense and become visible as X-shaped pairs of sister chromatids. The nuclear envelope breaks down. (Marking phrase: 'chromosomes condense / become visible'.)
metaphase: Stage of mitosis where chromosomes line up at the equator (middle) of the cell, attached to spindle fibres by their centromeres. (Marking phrase: 'chromosomes line up at the equator'.)
anaphase: Stage of mitosis where the spindle fibres contract, pulling sister chromatids apart to opposite poles of the cell. (Marking phrase: 'chromatids pulled to opposite poles' — by spindle fibres.)
telophase: Stage of mitosis where a new nuclear envelope forms around each set of separated chromosomes. The cell now has two complete nuclei. (Frequently combined with cytokinesis in marking schemes.)
cytokinesis: The final stage of cell division: the cytoplasm divides (and in plants a new cell wall forms), producing two separate daughter cells. (Marking phrase: 'cytoplasm divides (cytokinesis) to form two daughter cells'.)
spindle fibre: Protein fibres that attach to chromosomes during mitosis and pull sister chromatids apart to opposite poles in anaphase. (Edexcel awards a mark for 'spindle fibres pull/contract' in mitosis-mechanism questions.)
daughter cell: A new cell produced by cell division. After mitosis, both daughter cells are genetically identical and diploid like the parent cell. (Edexcel often wants 'diploid' as well as 'genetically identical'.)
tumour: A mass of cells produced by uncontrolled mitosis. Can damage surrounding tissues; the basis of cancer. (Marking phrases: 'a tumour forms' and 'surrounding tissues are compressed/damaged'.)

Notes

Chromosomes — the unit of inheritance

The nucleus contains your chromosomes — long, tightly-coiled strands of DNA. The DNA is divided into sections called genes that each code for a specific protein. Humans have 46 chromosomes (23 pairs) in every body cell — we say body cells are diploid. Sperm and egg cells have only 23 (they are 'haploid') — so when they fuse at fertilisation, the new cell has the normal diploid 46.

Before a cell can divide, every chromosome must be copied — otherwise the two daughter cells would end up with only half the genetic information each.

The cell cycle

The cell cycle is the series of events between one cell division and the next. Most of the cycle is interphase — a long preparation phase. During interphase:

The cell grows in size.
The cell makes more sub-cellular structures: ribosomes, mitochondria, etc.
The DNA is replicated so each chromosome now exists as two identical sister chromatids, joined at a centromere.

Only after interphase does the cell enter mitosis — the actual nuclear division phase — followed by cytokinesis, when the cytoplasm divides.

Without interphase, mitosis could not produce genetically identical cells, because there would be no copied DNA to share between the daughters.

Mitosis — the named stages

Mitosis is one continuous process, but Edexcel expects you to know the named stages and what happens in each:

Prophase — chromosomes condense / become visible as X-shaped pairs of sister chromatids. The nuclear envelope breaks down and spindle fibres begin to form.
Metaphase — chromosomes line up at the equator (middle) of the cell. Spindle fibres attach to the centromeres.
Anaphase — sister chromatids are pulled to opposite poles of the cell by the spindle fibres (the fibres contract).
Telophase — a new nuclear envelope forms around each set of chromosomes. The cell now has two complete nuclei.
Cytokinesis — the cytoplasm divides (the cell membrane pinches in two; in plants a new cell wall forms), producing two separate daughter cells.

The result: two genetically identical, diploid daughter cells.

Why the daughter cells are genetically identical

This is one of the most-asked Edexcel questions. The full marking-phrase chain is:

DNA replicated during interphase — so each chromosome has two identical sister chromatids.
In anaphase, chromatids pulled to opposite poles by spindle fibres — one of each pair goes to each daughter cell.
Because each daughter receives one chromatid from every chromosome, the daughter cells are diploid / have the same number of chromosomes as the parent.
The cell cycle ensures DNA replication before every division, so the pattern repeats reliably.

If any step fails — e.g. DNA not replicated, or chromatids not separated correctly — the daughter cells would have the wrong number of chromosomes.

Why mitosis matters

Mitosis is used wherever the body needs more of the same kind of cell:

Growth — turning a fertilised egg into a baby, then a baby into an adult.
Tissue repair — fixing a cut, knitting a broken bone, replacing damaged tissue.
Replacement of worn-out cells — your skin, gut lining, and blood cells are constantly being replaced.
Asexual reproduction — some organisms (e.g. bacteria, hydra, strawberry plants via runners) reproduce without a partner, using mitosis to produce genetically identical offspring (clones).

In each of these, the key feature is that the new cells are genetically identical to the parent. That's what makes mitosis the right tool for growth and repair — and the wrong tool for making sex cells (that's meiosis, a different process).

When mitosis is not controlled — cancer

Normal mitosis is tightly regulated. Cells have built-in checks that decide WHEN to divide and WHEN to stop. The cell cycle is therefore not just a sequence of events but a controlled sequence.

When those controls fail (usually because of mutations in particular genes), there is uncontrolled growth and division. The cell cycle is not controlled — cells divide over and over again without stopping. The growing mass of cells is called a tumour.

A tumour causes harm because:

Surrounding tissues are compressed/damaged as the tumour grows and takes up space.
A tumour uses up nutrients and oxygen that healthy cells need.
Cells from a malignant tumour can break off and spread (metastasis) to other parts of the body.

So cancer is not 'mitosis is bad'. Mitosis is essential and normal. Cancer is uncontrolled mitosis — the brakes on the cell cycle have failed.

Plant vs animal cytokinesis (brief)

In animal cells, cytokinesis happens by the cell membrane pinching inward (a 'cleavage furrow'). In plant cells the rigid cell wall can't pinch, so a new cell plate forms across the middle and develops into a new cell wall. The end result is the same — two daughter cells, each genetically identical and diploid.

Quick summary — Edexcel exam phrases

If you remember nothing else, remember these five sets of phrases. They are the Edexcel mark-scheme go-tos for 2.1:

Interphase importance: cell grows; DNA is replicated/copied; each daughter cell gets a full set of chromosomes; without interphase mitosis could not produce genetically identical cells.
Mitosis sequence: chromosomes condense / become visible; chromosomes line up at the equator; chromatids pulled to opposite poles; cytoplasm divides (cytokinesis) to form two daughter cells.
Why daughter cells identical: DNA replicated during interphase; chromatids pulled to opposite poles by spindle fibres; daughter cells diploid / have same number of chromosomes; cell cycle ensures DNA replication before every division.
Cancer from uncontrolled mitosis: uncontrolled growth and division; cell cycle is not controlled; a tumour forms; surrounding tissues are compressed/damaged.
DNA replication + anaphase link: each daughter cell receives a full set of chromosomes; otherwise wrong number of chromosomes; chromatids pulled to opposite ends/poles; spindle fibres pull/contract.

Exam tips

Always specify that daughter cells are 'genetically identical' AND 'diploid' — Edexcel awards both phrases separately.
DNA / chromosomes MUST be replicated BEFORE mitosis, in interphase. Don't say 'during mitosis'.
When describing mitosis stages, use the marking phrases verbatim: 'chromosomes condense', 'line up at the equator', 'chromatids pulled to opposite poles', 'cytoplasm divides'.
For cancer questions, write 'uncontrolled mitosis' or 'cell cycle is not controlled' — NOT just 'mitosis causes cancer'.
Always mention 'spindle fibres' when explaining how chromatids separate — Edexcel awards a mark for the mechanism.
If asked why daughter cells have the same number of chromosomes, link DNA replication in interphase to chromatid separation in anaphase.
Edexcel keeps stem cells in topic 2.2 — for 2.1 stick to the cell cycle, mitosis, and cancer.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

What's the difference between mitosis and meiosis?

Mitosis is the cell division used for growth, tissue repair, and asexual reproduction. It produces TWO daughter cells that are genetically IDENTICAL to the parent and diploid (full chromosome number). Meiosis is the cell division used to make sex cells (sperm and eggs). It produces FOUR daughter cells that are all GENETICALLY DIFFERENT and haploid (half the chromosome number — 23 in humans). Memory trick: MItosis MAKES IDENTICAL; MEiosis MAKES EXTRA (and different).

Why is interphase the longest stage of the cell cycle?

Interphase is when the cell does almost all its work: it grows, makes more ribosomes and mitochondria, and replicates all of its DNA. Mitosis itself is comparatively quick — it takes about an hour, while interphase can take 20+ hours. If you imagine a cell as someone packing for a long trip, interphase is the packing, mitosis is the trip.

What's the difference between a chromosome and a chromatid?

Before DNA replication, a chromosome is a single strand of DNA. After replication in interphase, each chromosome consists of TWO identical sister chromatids joined at a centromere — that's the familiar X shape. In anaphase the chromatids separate; once separated, each former chromatid is again called a chromosome in the new daughter cell. So 'chromatid' is just the name for half of an X-shaped duplicated chromosome.

Why does Edexcel describe cancer as 'uncontrolled mitosis'?

Mitosis is a normal, essential process — without it you couldn't grow or heal. Each cell has tight controls that decide WHEN to divide and WHEN to stop. Cancer happens when those controls fail (usually due to mutations in particular genes) and a cell divides over and over again without stopping. The result is a tumour — an ever-growing mass of cells. So cancer isn't 'mitosis is bad', it's 'the brakes on mitosis have failed'. The Edexcel marking phrases are 'uncontrolled growth and division' and 'cell cycle is not controlled'.

How do spindle fibres actually separate chromatids?

Spindle fibres are protein threads that form during prophase and attach to the centromere of each chromosome by metaphase. In anaphase, the fibres CONTRACT — they get shorter — pulling the two sister chromatids apart toward opposite poles of the cell. You can think of them as fishing lines reeling in chromatids to the cell's two ends. Edexcel awards a mark for naming the spindle fibres and saying they 'pull' or 'contract'.

What happens if mitosis goes wrong and a chromatid doesn't separate?

If sister chromatids don't separate properly (a failure called non-disjunction), one daughter cell ends up with an extra copy of that chromosome and the other ends up with one fewer. This can be lethal for the cell, or in early development it leads to conditions like Down syndrome (an extra copy of chromosome 21). The Edexcel marking phrase is 'wrong number of chromosomes'.