Chapter Review — Cell Cycle, Mitosis, Meiosis, Gametes & Fertilisation
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Use the word bank to complete the passage below.
The cell cycle consists of three main stages. The first and longest stage is , during which DNA is replicated in the . The nucleus then divides by , and the cytoplasm splits during . This produces two genetically identical daughter cells.
To produce sex cells called , cells divide by . This produces genetically different cells. In prophase I, chromosomes pair up and exchange genetic material in a process called . The random alignment of chromosome pairs during metaphase I is known as .
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Click on each phase of the cell cycle to reveal a description.
Click a phase in the diagram above to learn about it.
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| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | Click to reveal | Click to reveal |
| Number of daughter cells | Click to reveal | Click to reveal |
| Chromosome number of daughters | Click to reveal | Click to reveal |
| Genetic variation? | Click to reveal | Click to reveal |
| Where it occurs | Click to reveal | Click to reveal |
| Purpose | Click to reveal | Click to reveal |
| Homologous pairing? | Click to reveal | Click to reveal |
| Crossing over? | Click to reveal | Click to reveal |
Click the stages in the correct order (1st to 8th). Click “Reset” to start again.
Use these sentence starters to structure your answer:
Mitosis produces two genetically identical daughter cells with the same diploid number of chromosomes as the parent cell. This is important for growth because the organism needs to increase its cell number while ensuring every new cell contains the complete set of genetic instructions needed to function correctly. Because the daughter cells are genetically identical, the same proteins can be produced, allowing the new cells to carry out the same functions as the original cell.
During prophase I of meiosis, homologous chromosomes pair up to form bivalents. Non-sister chromatids wrap around each other and exchange sections of DNA at points called chiasmata. This means the chromatids now have new combinations of alleles that were not present in either parent chromosome. The resulting gametes are therefore genetically different from each other and from the parent cell, increasing genetic variation in the offspring.
The cutting grows into a new plant by mitosis, because new cells are needed for growth and development. Mitosis produces daughter cells that are genetically identical to the parent cell, as the DNA is replicated exactly during the S phase and then separated equally during nuclear division. Since only one parent is involved and no gametes or fertilisation occur, this is asexual reproduction, and the offspring is a genetic clone of the parent plant.
Mitosis is used because the replacement cells must be genetically identical to the original skin cells so they can carry out the same functions (e.g. producing the same structural proteins such as keratin). Mitosis produces diploid cells with the full set of 46 chromosomes. Meiosis would produce haploid cells with only 23 chromosomes, which would not have all the genes necessary to function as skin cells. Additionally, meiosis introduces genetic variation, which is not desirable when replacing specific tissue cells.
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Meiosis is a form of nuclear division that produces four genetically different haploid gametes from a single diploid parent cell. It is the primary source of genetic variation in sexually reproducing organisms.
During prophase I, homologous chromosomes pair up to form bivalents and crossing over occurs. Non-sister chromatids exchange sections of DNA at chiasmata, producing recombinant chromatids with new combinations of alleles. This means each gamete can carry a unique set of alleles that was not present in the parent.
During metaphase I, homologous pairs align randomly on the metaphase plate in a process called independent assortment. Because each pair can orient in two ways, a human cell (with 23 pairs) can produce 223 (over 8 million) different combinations of maternal and paternal chromosomes in the gametes.
When combined with random fertilisation — any sperm can fuse with any egg — the total number of possible genetic combinations becomes enormous (223 × 223 = over 70 trillion), and this is before accounting for crossing over.
This genetic variation is critically important for the survival of a population. It provides the raw material for natural selection: in a changing environment, some individuals will have allele combinations that confer a selective advantage, enabling them to survive and reproduce. Without meiosis, offspring would be genetically identical to their parents, and a population could be wiped out by a single disease or environmental change.
In conclusion, meiosis is fundamentally important for maintaining genetic diversity. Through crossing over, independent assortment and its link to random fertilisation, it ensures that each generation contains genetically unique individuals, enabling populations to adapt and evolve over time.
Advantages of asexual reproduction: It is faster as no mate is needed; all individuals can reproduce; offspring are genetically identical to the parent, which is advantageous in stable environments where the parent is already well-adapted; less energy is expended as there is no need to produce gametes or find a mate.
Disadvantages of asexual reproduction: No genetic variation is produced (no crossing over, independent assortment or random fertilisation). This means the population is vulnerable to environmental change — a single disease or change in conditions could eliminate the entire population. There is no raw material for natural selection to act upon, so the population cannot evolve and adapt over time.
Evaluation: Overall, while asexual reproduction is advantageous in stable environments for rapid population growth, sexual reproduction (involving meiosis) provides a significant long-term advantage because the genetic variation produced allows populations to adapt to changing conditions and resist pathogens. This is why the vast majority of complex organisms use sexual reproduction.
| Organism | Diploid number (2n) | Haploid number (n) |
|---|---|---|
| Human | 46 | 23 |
| Fruit fly | 8 | 4 |
| Potato | 48 | 24 |
| Dog | 78 | 39 |
| Wheat | 42 | 21 |
(a) Calculate the number of possible chromosome combinations in the gametes of a fruit fly due to independent assortment alone. Show your working.
Number of pairs (n) = 4. Possible combinations = 2n = 24 = 16 different combinations.
(b) A dog has a diploid number of 78. If two dogs mate, calculate the total number of possible zygote combinations due to independent assortment alone (ignoring crossing over).
n = 39. Each parent can produce 239 different gametes. Total combinations = 239 × 239 = 278 ≈ 3.0 × 1023 possible combinations. This is an astronomically large number, illustrating the enormous genetic diversity possible from independent assortment and random fertilisation alone.
(c) Explain why the actual number of genetically different gametes produced by an organism is even greater than the value calculated from independent assortment alone.
The calculation 2n only accounts for independent assortment — the random orientation of homologous pairs. Crossing over during prophase I creates additional variation by exchanging sections of DNA between non-sister chromatids, producing recombinant chromosomes with new allele combinations. Since crossing over can occur at different points along the chromosomes and involves different chromatids, the number of genetically different gametes is far greater than 2n.
Question: DNA replication occurs during the S phase of interphase before both mitosis and meiosis. Explain why accurate DNA replication is essential before mitosis, and discuss what could happen if errors (mutations) occur during this process.
During the S phase, DNA replication occurs by the semi-conservative mechanism: the double helix unwinds, hydrogen bonds between complementary base pairs break, and each strand acts as a template. DNA polymerase adds free nucleotides by complementary base pairing (A-T, C-G), producing two identical copies of each DNA molecule.
Accurate replication is essential before mitosis because the purpose of mitosis is to produce genetically identical daughter cells. If replication errors occur and are not corrected by proofreading enzymes, mutations may be introduced. These mutations will be passed to both daughter cells and all subsequent cells produced from them.
If a mutation affects a gene controlling the cell cycle (e.g. a proto-oncogene or tumour suppressor gene), the cell may divide uncontrollably, leading to tumour formation and potentially cancer. This illustrates how the accuracy of DNA replication is fundamentally linked to the proper regulation of cell division.
Question: Explain the role of the centrioles and the spindle fibres (microtubules) during mitosis. Why are centrioles absent in most plant cells, and how do plant cells still manage to divide?
In animal cells, centrioles move to opposite poles during prophase and organise the spindle fibres (composed of microtubules made of the protein tubulin). Spindle fibres attach to centromeres of chromosomes at the kinetochore and are responsible for moving chromosomes during anaphase.
Most plant cells lack centrioles but can still form a spindle. They use microtubule organising centres (MTOCs) at each pole to nucleate spindle fibres. The process of chromosome separation is functionally the same. Additionally, during cytokinesis, plant cells cannot pinch inward due to their rigid cell wall; instead, vesicles from the Golgi apparatus fuse at the equator to form a cell plate, which develops into a new cell wall.
State two differences between mitosis and meiosis. (2 marks)
Accept: crossing over occurs in meiosis but not mitosis; homologous pairing occurs in meiosis but not mitosis; mitosis involves one division, meiosis involves two.
Describe the process of double fertilisation in flowering plants. (4 marks)
Describe and explain how the structure of a mammalian sperm cell and egg cell are adapted for their roles in fertilisation. (6 marks)
Plan your answer:
Sperm cell adaptations:
The sperm cell has a streamlined shape to reduce friction as it swims through the female reproductive tract. It possesses a long flagellum (tail) which enables it to swim towards the egg. The mid-piece contains many mitochondria arranged in a spiral, which carry out aerobic respiration to release the ATP needed to power the movement of the flagellum. The acrosome at the head contains hydrolytic enzymes (e.g. hyaluronidase and acrosin) that digest the protective layers (zona pellucida and follicle cells) surrounding the egg, allowing the sperm to penetrate. The nucleus is haploid (n) so that when it fuses with the haploid egg, the diploid number (2n) is restored.
Egg cell adaptations:
The egg cell is much larger than the sperm because it contains a large volume of cytoplasm rich in nutrients (lipid droplets, proteins) to nourish the early embryo after fertilisation. It has a haploid nucleus to ensure the correct diploid number upon fertilisation. The egg is surrounded by a zona pellucida (glycoprotein layer) which contains receptors for sperm binding and prevents polyspermy. After fertilisation, cortical granules release their contents, causing the zona pellucida to harden (the cortical reaction), blocking further sperm entry and ensuring only one sperm fertilises the egg.
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