5.15 · The Mitotic Cell Cycle

Replication & Division of Nuclei & Cells — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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

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5.1 Replication & Division of Nuclei & Cells — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe chromosome structure, the cell cycle, the significance of mitosis, telomeres, stem cells and the formation of tumours.

Sister chromatids
Kinetochores
Interphase
After division
Cytokinesis
Human cells

Part 1 of 8Chromosome structure

Chromosome structure

Chromosome

Chromosome one long linear DNA molecule coiled around histone proteins.
In interphase it is loosely coiled chromatin, and before division it condenses (supercoils) so it becomes visible.

Histones

Histones positively charged proteins around which negatively charged DNA wraps tightly, packaging the DNA to fit the nucleus and condense before division.
- (phosphate groups)

Sister chromatids

Sister chromatids after S phase each chromosome has two genetically identical chromatids held together at the centromere.

Kinetochores

Kinetochores spindle fibres attach at kinetochores on the centromere.

Telomeres

Telomeres protective non-coding repeat sequences seal the chromosome ends.

Part 2 of 8The cell cycle

Stage	Main events	DNA content
G1	Growth, synthesis of proteins, RNA and organelles; checkpoint checks size, nutrients, growth factors and DNA damage	2C
S	DNA replication and synthesis of histones; centrosome duplicated in animal cells	2C to 4C
G2	Further growth, tubulin made, energy stores built up; checkpoint checks replication is complete and error-free	4C
Mitosis (M)	Nuclear division into two identical nuclei	4C to 2C per nucleus
Cytokinesis	Cytoplasm divides to give two cells	2C per cell

C the DNA content of one unreplicated haploid set, so a diploid G1 cell is 2C.

2Interphase

Interphase normally the longest stage, while mitosis takes about 1 h of a roughly 24 h mammalian cycle.

3Control

Control progress is triggered by cyclins, whose concentrations rise and fall during the cycle.

4After division

After division daughter cells re-enter G1 or leave the cycle (G0) to differentiate or become quiescent.

Part 3 of 8Mitosis and cytokinesis

1Mitosis

Mitosis each daughter nucleus receives an exact copy of every chromosome,
so chromosome number stays constant and cells are genetically identical.
- (unless a mutation occurs)

2Stages

Stages prophase, metaphase, anaphase and telophase separate sister chromatids into two nuclei.

3Cytokinesis

Cytokinesis the cytoplasm divides by a cleavage furrow in animal cells and by a cell plate in plant cells.

4Human cells

Human cells a diploid human somatic cell has 46 chromosomes (2n = 46) and each daughter cell also has 46.

5DNA mass

DNA mass it doubles during S phase and halves when the cell divides at cytokinesis.

Part 4 of 8Significance of mitosis

Role	Example
Growth of multicellular organisms	Increase in cell number of a seedling or embryo
Replacement of cells and repair of tissues	Skin epidermis, gut lining, healing wounds
Asexual reproduction	Budding in yeast, vegetative propagation in plants
Clonal expansion in the immune response	Division of selected B and T lymphocytes

Part 5 of 8Telomeres

Telomeres

Structure

Structure repeated non-coding sequences at the end of each chromosome.
- (TTAGGG in humans)

Problem solved

DNA polymerase cannot replicate the very end of the lagging strand, so a little DNA is lost each round.

Function

Function telomeres are a buffer of non-essential DNA, so the loss does not remove genes.

Senescence

Senescence when telomeres become too short the cell stops dividing.

Telomerase

Telomerase it adds telomere repeats and is active in stem cells, germ cells and most cancer cells.

Part 6 of 8Stem cells

Stem cell

Stem cell a cell that divides by mitosis to replace cells that are lost, for example bone marrow stem cells make blood cells and skin stem cells make epidermal cells.

Totipotent

Totipotent cells of the zygote and very early embryo can form any cell type, including the placenta.

Pluripotent

Pluripotent embryonic stem cells can form any body cell but not the placenta.

Multipotent

Multipotent adult stem cells form only a limited range, for example bone marrow cells form only blood cells.

Therapy

Therapy stem cells can replace damaged or lost cells, for example bone marrow transplants for leukaemia and cultured skin cells for burns.

Location

Location adult stem cells are found in bone marrow, skin, gut, heart and brain.

Part 7 of 8How tumours form

How tumours form

Cause

Cause mutations in genes controlling the cell cycle lead to uncontrolled mitosis.
Proto-oncogenes mutate into oncogenes that stimulate division.
Tumour suppressor genes that normally stop division are inactivated.

Why most mutations do not cause cancer

Why most mutations do not cause cancer most mutated cells die early or are destroyed by the immune system and are replaced.
Cancer develops only when a mutated cell escapes both, so its descendants inherit the faulty control;
- a tumour may contain about 10⁹ cells when found.

Carcinogens

Carcinogens agents that increase the chance of mutation, such as tar in cigarette smoke, UV radiation, X-rays, gamma rays and some viruses.

Benign tumours

Benign tumours they stay in one place.

Malignant tumours

Malignant tumours they invade surrounding tissue, and cells spread through blood or lymph to form secondary tumours, which is metastasis.

Part 8 of 8More from the specification

1Chromosome structure

DNA wrapped around histone proteins makes chromatin, which condenses into chromosomes with two sister chromatids joined at the centromere

2Telomeres

Telomeres repeated non-coding DNA sequences at chromosome ends that protect genes and shorten with each division, called telomere shortening

3Mitotic cell cycle

Mitotic cell cycle interphase (G1, S, G2) then the mitotic phases of prophase, metaphase, anaphase and telophase, then cytokinesis

4Result of mitosis

Result of mitosis two genetically identical daughter cells from one parent cell, used for growth and repair

5Stem cells

Stem cells unspecialised cells that divide and then become specialised;
- cancer arises when the control of division is lost

Key terms

Term	Meaning
Cell cycle	The ordered sequence of events between one cell division and the next: interphase, mitosis and cytokinesis.
Interphase	The stage between divisions (G1, S and G2) in which the cell grows and replicates its DNA.
Chromosome	One DNA molecule wound around histone proteins; after replication two sister chromatids joined at a centromere.
Telomere	Repetitive non-coding DNA at the ends of a chromosome that prevents loss of genes during replication.
Stem cell	An unspecialised cell that divides repeatedly by mitosis and can differentiate.
Checkpoint	A control point in the cycle where conditions are verified before proceeding.
Mitotic index	Number of cells in mitosis divided by total number of cells observed.
Metastasis	Spread of cancer cells to form secondary tumours.

Avoid these mistakes

Saying damaged cells are repaired — dead and damaged cells are replaced to repair tissue.

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

Saying telomeres code for proteins — they are non-coding.

Saying all tumours are cancer — benign tumours do not spread.

Last updated: 15 October 2026