How do cells divide, and how does sexual reproduction generate variation?
The cell cycle and mitosis, meiosis and the production of gametes, the sources of genetic variation, and the basics of sexual reproduction in flowering plants and mammals.
A CCEA A-Level Biology answer on the cell cycle and mitosis, meiosis and gamete production, the sources of genetic variation, and the basics of sexual reproduction in flowering plants and mammals.
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What this dot point is asking
CCEA wants you to describe the cell cycle and the stages of mitosis, describe meiosis and how it produces gametes, explain the sources of genetic variation, and outline sexual reproduction in flowering plants and mammals.
The cell cycle and mitosis
DNA is copied in the S phase of interphase, so each chromosome enters mitosis as two identical sister chromatids joined at a centromere. Because the chromatids are exact copies and one goes to each daughter cell, the products are genetically identical. The cell cycle is tightly controlled at checkpoints; uncontrolled mitosis caused by faulty control genes leads to a tumour and can cause cancer.
Meiosis and variation
Genetic variation is generated by crossing over of alleles between homologous chromosomes in prophase I, independent assortment of homologous pairs in metaphase I, and random fertilisation of gametes. In meiosis I, homologous chromosomes (one from each parent) pair as a bivalent, exchange sections of chromatid at chiasmata, then separate. In meiosis II, sister chromatids separate, as in mitosis. Together these processes shuffle alleles so that gametes, and therefore offspring, differ from one another and from their parents.
Sexual reproduction
In flowering plants, pollen carries the male gamete to the stigma; the pollen tube grows down the style and a male gamete fuses with the egg cell, with a second fusion forming the triploid endosperm (double fertilisation). In mammals, a haploid sperm fertilises a haploid egg in the oviduct, restoring the diploid number in the zygote, which then divides by mitosis to form the embryo.
Examples in context
Example 1. Wound healing in the skin. When you cut your skin, cells at the wound edge divide by mitosis to replace the lost cells. Because mitosis produces genetically identical diploid cells, the new skin cells are exact copies of their neighbours, so the tissue is rebuilt correctly. This is the everyday role of mitosis: growth and repair using identical cells.
Example 2. Why siblings differ. Two children of the same parents are rarely alike (unless identical twins) because of meiosis and random fertilisation. Each parent can produce over genetically different gametes through independent assortment alone, and crossing over adds even more combinations. With random fertilisation, the number of possible offspring genotypes is astronomical, which is why sexual reproduction is such a powerful source of variation for natural selection to act on.
Try this
Q1. State two ways meiosis produces genetic variation. [2 marks]
- Cue. Crossing over of alleles; independent assortment of homologous chromosomes.
Q2. Why is it important that gametes are haploid? [2 marks]
- Cue. So that fertilisation restores the diploid number and keeps the chromosome number constant between generations.
Q3. A cell with a diploid number of 8 undergoes meiosis. How many chromosomes are in each gamete, and how many genetically different gametes could independent assortment alone produce? [2 marks]
- Cue. Each gamete has 4 chromosomes; independent assortment alone gives combinations.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20186 marksDescribe the events that occur during the four stages of mitosis and explain why mitosis produces genetically identical daughter cells.Show worked answer →
A 6-mark answer needs the four named stages in order with key events, plus the link to genetic identity.
Prophase: chromosomes condense and become visible as two sister chromatids joined at a centromere; the nuclear envelope breaks down and spindle fibres form.
Metaphase: chromosomes line up along the equator of the cell, attached to spindle fibres by their centromeres.
Anaphase: the centromeres divide and sister chromatids are pulled to opposite poles by the shortening spindle fibres.
Telophase: a nuclear envelope reforms around each set of chromosomes, which then uncoil.
Genetic identity: because DNA was replicated exactly in interphase and sister chromatids are identical copies, each daughter cell receives one copy of every chromosome, so the two cells are genetically identical to each other and to the parent cell.
Markers reward the four stages in order, correct events, and the link to identical sister chromatids made by replication.
CCEA 20204 marksExplain how meiosis and fertilisation together produce genetic variation among offspring.Show worked answer →
A 4-mark answer should give the meiotic sources plus random fertilisation.
Crossing over: in prophase I, homologous chromosomes pair up and exchange sections of chromatid, producing new combinations of alleles.
Independent assortment: in metaphase I, homologous pairs line up randomly, so each gamete gets a random mix of maternal and paternal chromosomes.
Random fertilisation: any one of many genetically different sperm can fuse with any one of many genetically different eggs, multiplying the variation.
Markers reward crossing over, independent assortment and random fertilisation, with the idea that each produces new allele combinations.
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Sources & how we know this
- CCEA GCE Biology specification — CCEA (2016)