Cell divisionOxford AQA IGCSE Biology: Revision notes
Section 1
What is the cell cycle and what are its main stages?
The cell cycle is the sequence of events that occurs during the life of a cell, from formation to division. It consists of three main stages:
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Growth (G1 phase): The cell increases in size and accumulates nutrients and energy. Organelles are replicated to ensure both daughter cells have sufficient cellular machinery.
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DNA replication (S phase): The cell's DNA is copied so that each chromosome is duplicated. This produces two identical copies of genetic material (sister chromatids) joined at the centromere.
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Mitosis and cytokinesis (M phase): The cell divides to produce two genetically identical daughter cells. DNA condenses into visible chromosomes, and the cell physically splits into two.
The cell spends most of its time in interphase (G1, S, and G2 phases combined), during which growth and DNA replication occur. Mitosis itself is relatively brief.
Examiners want you to explain that DNA replication ensures genetic information is copied before division, so daughter cells receive a complete set of instructions. Always link DNA replication to the purpose of maintaining genetic information across generations.
The cell cycle is like a factory production line: growth is gathering materials, DNA replication is copying the instruction manual, and mitosis is the final assembly and packaging into two identical products.
Section 2
How does mitosis produce genetically identical daughter cells?
Mitosis is a type of cell division that produces two genetically identical daughter cells from one parent cell. Each daughter cell receives an exact copy of the parent cell's genetic material.
Stages of mitosis:
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Prophase: Chromosomes condense and become visible. The nuclear envelope breaks down. Spindle fibres form from the centrosomes.
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Metaphase: Chromosomes line up at the cell's equator (metaphase plate). Spindle fibres attach to the centromere of each chromosome.
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Anaphase: Sister chromatids separate and move to opposite poles of the cell, pulled by spindle fibres. The cell begins to elongate.
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Telophase: Nuclear envelopes reform around the separated chromosome sets. Chromosomes begin to uncoil. Spindle fibres disappear.
Cytokinesis (division of the cytoplasm) follows mitosis, splitting the cell membrane to form two separate daughter cells.
Because DNA has been replicated before mitosis, each daughter cell receives the same number and type of chromosomes as the parent cell, making them genetically identical.
When answering 'describe the stages of mitosis', examiners expect you to mention what happens to chromosomes, the spindle fibres, and the nuclear envelope at each stage. Be precise about the sequence and timing.
A common error is confusing the number of chromosomes before and after mitosis. Remember: mitosis maintains the chromosome number—a diploid cell produces two diploid daughter cells, not haploid cells.
Section 3
What are the functions of mitosis in living organisms?
Mitosis serves three critical functions in multicellular organisms:
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Growth: In developing organisms, mitosis increases the number of cells, allowing the organism to grow larger. A fertilised egg divides repeatedly by mitosis to form billions of cells.
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Repair and replacement: Damaged or worn-out cells are replaced by new cells produced through mitosis. Examples include:
- Skin cells being replaced continuously
- Healing of wounds
- Replacement of red blood cells
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Asexual reproduction: Some organisms reproduce asexually through mitosis, producing offspring that are genetically identical to the parent. Examples include:
- Budding in hydra
- Vegetative reproduction in plants (runners, bulbs)
- Binary fission in bacteria
All these processes rely on mitosis producing genetically identical daughter cells, ensuring that cells maintain the same genetic instructions as the parent cell and other cells in the organism.
If a human child has a skin wound, mitosis in skin cells around the wound produces new identical cells to replace damaged tissue. These new cells are genetically identical to the surrounding skin cells, maintaining the consistency and function of the tissue.
Section 4
How does meiosis differ from mitosis and produce genetic variation?
Meiosis is a type of cell division that produces four genetically different haploid cells (gametes), each with half the chromosome number of the parent cell. This is fundamentally different from mitosis.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cells produced | Two | Four |
| Chromosome number | Maintained (diploid → diploid) | Halved (diploid → haploid) |
| Genetic variation | Daughter cells identical | Daughter cells genetically different |
| Type of cells produced | Somatic (body) cells | Gametes (sex cells) |
| Where it occurs | All body cells | Reproductive organs only |
Meiosis I separates homologous chromosome pairs, reducing the chromosome number by half. Meiosis II resembles mitosis, separating sister chromatids. The result is four non-identical haploid cells.
Sources of genetic variation in meiosis:
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Independent assortment: During meiosis I, homologous chromosomes are randomly distributed to daughter cells. Each gamete receives a random mix of maternal and paternal chromosomes, creating different combinations.
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Crossing over (recombination) [Higher Tier]: Before meiosis I, homologous chromosomes pair up and exchange sections of DNA. This shuffles alleles between chromosomes, creating new genetic combinations that did not exist in the parent cell.
To explain genetic variation, always mention both independent assortment (which applies to all meiosis) and crossing over (Higher Tier only). State explicitly that these processes create new combinations of alleles that differ from the parent cell.
If mitosis is photocopying a document four times, meiosis is like shuffling a deck of cards containing genetic information—each shuffle produces a different hand, ensuring no two are identical.
Section 5
Why are mitosis and meiosis significant for chromosome number and genetic variation?
Significance of mitosis:
- Maintains chromosome number: Mitosis preserves the diploid number in body cells. This ensures all somatic cells contain the correct genetic instructions for the organism.
- Prevents genetic variation: Genetically identical daughter cells mean genetic information is stable across growth and repair, maintaining the consistency of tissues and organs.
- Essential for growth and survival: Without mitosis, organisms could not grow, repair damage, or maintain tissues throughout life.
Significance of meiosis:
- Halves chromosome number: Meiosis produces haploid gametes (n), which is essential for sexual reproduction. When gametes fuse during fertilisation, the diploid number (2n) is restored in the offspring.
- Creates genetic variation: Independent assortment and crossing over ensure that each gamete is unique. This variation produces genetic diversity in offspring, allowing populations to adapt to environmental changes and resist diseases.
- Essential for sexual reproduction: Without meiosis, sexual reproduction would be impossible, and offspring would be identical clones of their parents.
The balance between the two:
Mitosis maintains genetic stability within an organism during its lifetime, while meiosis creates genetic diversity across generations. Together, they allow organisms to grow, repair themselves, and reproduce successfully—either asexually (through mitosis) or sexually (through meiosis). The reduction in chromosome number during meiosis is critical: without it, chromosome numbers would double with each generation, which would be lethal.
When answering 'explain the significance of mitosis and meiosis', structure your answer around three points: chromosome maintenance, genetic variation, and biological function. Link each process explicitly to its role in organism survival and reproduction.
In humans, meiosis in males produces four different sperm cells (each with 23 chromosomes), while mitosis in skin cells produces two identical cells (each with 46 chromosomes). This allows sexual reproduction to create variation whilst maintaining stable body tissues.
Must Know
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Mitosis produces two genetically identical daughter cells with the same chromosome number as the parent; it is essential for growth, repair, and asexual reproduction.
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The cell cycle consists of growth (G1), DNA replication (S phase), and mitosis/cytokinesis (M phase). DNA replication must occur before mitosis to ensure each daughter cell receives a complete set of genetic instructions.
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Meiosis produces four genetically different haploid gametes from one diploid parent cell. The chromosome number is halved, making sexual reproduction and fertilisation possible.
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Genetic variation in meiosis arises from independent assortment (random distribution of homologous chromosome pairs) and crossing over (exchange of genetic material between homologous chromosomes), both of which shuffle alleles to create unique combinations.
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Mitosis maintains genetic stability (identical cells with diploid chromosome number), while meiosis creates genetic diversity (different cells with haploid chromosome number). Together, they enable organisms to grow, repair tissue, and reproduce—maintaining both individual stability and population variation.
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