5.1.25 · The Mitotic Cell Cycle

Mitosis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 11 parts Lesson 4 of 12 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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5.1.2 Mitosis — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain the purpose of mitosis, outline its stages and describe its importance in growth, repair, asexual reproduction and producing genetically identical cells.

Cytokinesis
Same genes everywhere
Replacement and repair
Centrosomes
Animal cells
Plant cells

Part 1 of 9What mitosis is

Mitosis

Mitosis nuclear division that produces two daughter nuclei with the same number of chromosomes as the parent and, apart from rare mutation, identical genetic information.

Cytokinesis

Cytokinesis it is usually followed by division of the cytoplasm, giving two daughter cells.

Ploidy

Ploidy a diploid (2n) parent gives diploid daughters, and mitosis in a haploid organism gives haploid cells.

Same genes everywhere

Same genes everywhere all cells of an organism have the same genes,
- so specialised cells arise through differential gene expression, not differences in DNA.

Part 2 of 9Importance of mitosis

Growth

Growth daughter cells are genetically identical clones with the same chromosome number, so a zygote can grow into a multicellular organism.
Growth may occur across the body or in regions such as plant meristems.

Replacement and repair

Replacement and repair dead and damaged cells are replaced by genetically identical cells, which repairs tissue.
Replacement is rapid in the skin, gut lining and bone marrow (which makes red blood cells).

Asexual reproduction

Asexual reproduction one parent produces genetically identical offspring.
Examples are budding in yeast and Hydra, strawberry runners and cloning.

Part 3 of 9Preparation for mitosis

1S phase

DNA is replicated semi-conservatively, so each chromosome has two identical sister chromatids.

2Growth

Growth the cell grows, makes extra organelles and proteins, and stores energy as ATP.

3Centrosomes

Centrosomes they are duplicated in animal cells.

4Result

Result the genetic information is in duplicate, so each daughter cell can receive a full copy.

Part 4 of 9Stages in outline

Stage	Main events
Prophase	Chromosomes condense into two chromatids joined at a centromere; the nucleolus disappears; the nuclear envelope breaks down; spindle microtubules form from the centrosomes
Metaphase	Chromosomes line up on the equator; spindle fibres attach to the kinetochores of the centromeres
Anaphase	Centromeres divide; sister chromatids are pulled to opposite poles as spindle fibres shorten, using ATP
Telophase	Chromosomes arrive at the poles and decondense; nuclear envelopes reform; nucleoli reappear; the spindle breaks down

Part 5 of 9Cytokinesis

Timing

Timing it begins in late anaphase or telophase.

Animal cells

Animal cells a ring of actin and myosin filaments contracts and pinches the membrane inwards, forming a cleavage furrow.

Plant cells

Plant cells a cell plate forms at the equator from Golgi vesicles and develops into a new cell wall with middle lamella and plasma membranes.

Part 6 of 9How the spindle moves chromosomes

1Spindle

Spindle it is made of microtubules, protein filaments of tubulin, some attached to kinetochores and others running from pole to pole.

2Anaphase movement

Anaphase movement kinetochore microtubules shorten by losing tubulin subunits,
- and motor proteins use ATP to pull chromatids to the poles, centromere first.

3ATP supply

ATP supply mitochondria are concentrated around the spindle.

4Plant cells

Plant cells they lack centrioles but still form a spindle.

5Accurate separation

Accurate separation the kinetochore of each sister chromatid attaches to microtubules from opposite poles,
so each chromatid is drawn to its own pole and every daughter gets one copy of every chromosome.

Part 7 of 9Control and consequences

1Control

Control cell cycle checkpoints regulate mitosis, and failure leads to uncontrolled division and tumours.

2Clones

Clones asexual reproduction gives organisms with no variation, which are vulnerable to the same diseases.

3Uniformity

Uniformity it benefits a tissue but is a disadvantage for a species facing environmental change.

Part 8 of 9Mitosis compared with binary fission and meiosis

Feature	Mitosis	Meiosis	Binary fission
Organisms	Eukaryotes	Eukaryotes	Prokaryotes
Divisions	One	Two	One, without a spindle
Daughter cells	Two	Four	Two
Chromosome number	Unchanged	Halved	Circular DNA copied
Genetic make-up	Identical	Different	Identical
Homologues	Do not pair	Pair and cross over	Not applicable

Plants
- mitosis happens mainly in meristems, with cell elongation providing most of the increase in size.

Part 9 of 9More from the specification

Mitotic phases and chromosome segregation

Mitosis comprises prophase, metaphase (equatorial alignment), anaphase (sister chromatid separation),
- (chromatin condensation)
and telophase.

Protective role of telomeres

Telomeres are repetitive non-coding DNA sequences at chromosome tips that protect essential genes from progressive shortening during successive replications.

Oncogenesis and uncontrolled cell division

Mutations in proto-oncogenes or tumour suppressor genes cause checkpoints to fail,
resulting in uncontrolled mitotic division and tumour formation.

Malignant tumour

A rapidly growing tumour that invades tissues and may metastasise.

Telomere

Repetitive non-coding DNA at the ends of a chromosome that prevents loss of genes during replication.

Interphase

The stage when the cell grows and DNA is replicated.

Cancer

Disease in which cells divide uncontrollably and spread.

Key terms

Term	Meaning
Mitosis	Nuclear division giving two genetically identical nuclei.
Cytokinesis	Division of the cytoplasm to form two cells.
Spindle	Structure of microtubules that moves chromosomes in cell division.
Sister chromatids	The two identical copies of a replicated chromosome.
Cleavage furrow	Indentation formed in dividing animal cells.
Cell plate	Structure formed at the equator of a dividing plant cell that becomes the new cell wall.
Clone	Genetically identical organisms or cells.
Kinetochore	Protein structure on the centromere where spindle fibres attach.

Avoid these mistakes

Saying mitosis produces gametes — gametes are made by meiosis.

Confusing mitosis (nuclear division) with cytokinesis (cell division).

Saying DNA replicates during prophase — it replicates in S phase.

Writing that homologous chromosomes pair up in mitosis — they do not.

Saying damaged cells are repaired — they are replaced.

Last updated: 15 October 2026