Mitosis and meiosis (pg. 56 & 57)

• Asexual reproduction involves only one parent. The offspring have identical genes to the parent, so there's no variation between parent and offspring.
• Mitosis is when a cell reproduces itself by splitting to form two cells with identical sets of chromosomes.
• In a cell that's not dividing, the DNA is all spread out in long strings.
• Process of mitosis:

  1. If the cell gets a signal to divide, it needs to duplicate its DNA, so there's one copy for each new cell. The DNA forms X-shaped chromosomes. Each 'arm' of the chromosome is an exact duplicate of the other.
  2. The chromosomes then line up at the center of the cell and cell fibers pull them apart. The two arms of each chromosome go to opposite ends of the cell.
  3. Membranes form around each of the sets of chromosomes. These become the nuclei of the two new cells.
  4. The cytoplasm divides.
• Mitosis also makes new cells for growth and repair of tissues.
• Sexual reproduction involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parents' genes.
• Sexual reproduction process:

  1. The mother and father produce gametes.
  2. At fertilisation, a male gamete fuses with a female gamete to form a zygote.
  3. The zygote undergoes cell division by mitosis and develops into an embryo.
• Gametes are haploid, they have 23 chromosomes - this means they have half the number of chromosomes in a normal cell (23 pairs of chromosomes).
• Meiosis produces four haploid cells where chromosomes are not identical.
• Division 2 in meiosis:

  1. The chromosomes line up again in the center of the cell. The arms of the chromosomes are pulled apart.
  2. You get four haploid gametes. Each gamete only has a single set of chromosomes. The gametes are all genetically different.
• Division 1:

  1. Before the cell starts to divide, it duplicates its DNA so there's enough for each new cell. One arm of each X-shaped chromosome is an exact copy of the other arm.
  2. The chromosomes line up in pairs in the center of the cell. One chromosome in each pair came from the organism's mother and one came from its father.
  3. The pairs are then pulled apart, so each new cell only has one copy of each chromosome.
  4. Each new cell will have a mixture of the mother's and father's chromosomes. This creates genetic variation in the offspring.