ANSWERS

1. What is mitosis?
   A method of multiplication of cells by a specific process of division.
2. What is meiosis?
   A stage of reduction cell division when the chromosomes of a gamete are halved in number ready for union at fertilisation (ie. sexual cell division).
3. Describe the 4 phases of the "cell cycle".
   
   • Interphase - the cell carries out its normal functions (non-reproductive phase).
   • Synthesis (S) - the cell prepares for replication by synthesising new genetic material.
   • Gap (G2) - resting phase.
   • Nuclear division (M) - The new genetic material separates into the 2 new cells.
4. Describe cell division by mitosis.
   
   • The nuclear membrane disappears.
   • Each chromosome splits into 2 identical fibres called chromatids held together at the centromere.
   • The chromosomes arrange themselves so that their centromeres are in a line along the centre of the cell.
   • The chromatids then pull apart from their partners until the centromere splits.
   • The separated chromatids move to opposite sides of the cell.
   • The cytoplasm begins to divide.
   • The nuclear membrane re-forms around the chromosomes and the cell is completely divided.
   • The result is 2 new cells with the same number of chromosomes as the original cell.
5. Describe what happens at each of the following stages:
   
   • Prophase - Nuclear membrane breaks down and the chromatin contracts.
   • Metaphase - Nuclear spindle formed.
   • Anaphase - Chromatids separate.
   • Telophase - Cytoplasm divides.
   • Interphase - The non-dividing state of a cell.
6. At which stage of mitosis can chromosomes be seen most easily under the light microscope?
   Metaphase.
7. Where does meiosis occur?
   In the testes and ovaries when the germ cells or gametes (spermatozoa/ova) are formed.
8. Describe cell division by meiosis.
   
   • The chromatids form.
   • Each chromosome aligns with its homologous partner.
   • "Crossing over" occurs - this is where the 4 chromatids become entwined and exchange segments.
   • The chromosomes move apart and the cytoplasm divides.
   • Each cell immediately divides again by mitosis.
   • The result is 4 new cells with only half the number of chromosomes of the original cell.
9. What is a gamete?
   A germ cell; a sex cell which combines with another to form a zygote from which a complete organism develops (a spermatozoon or an ovum).
10. What is a zygote?
    A zygote is formed when a sperm fertilises an ovum. A zygote has the full number of chromosomes, and in this way the new individual inherits genetic information from each parent, but with some variation due to the crossing over. This explains why offspring resemble their parents yet may be quite different.