Sexual reproduction, meiosis and gamete formation

• Meiosis

  The process by which gametes (sperm and egg cells) are produced, with half the normal number of chromosomes
• Normal human cells

  • Contain 23 pairs of chromosomes
• Gametes (sperm and egg cells)

  • Contain 23 single chromosomes
• Meiosis

  1. Chromosomes are copied

  2. Cell divides into two

  3. Cells divide one more time forming gametes
• Meiosis produces four gametes from one original cell, and each gamete is genetically different
• Fertilization

  The joining of a male gamete (sperm) and a female gamete (egg) to form a new cell with the full number of chromosomes
• Meiosis

  Halves the number of chromosomes
• Fertilization

  Restores the full number of chromosomes
• Embryo

  The clump of identical cells that forms after fertilization, which then differentiates into different cell types as the organism develops
• You'll find plenty of questions on meiosis and fertilization in the revision workbook
• Meiosis

  Cell division that produces gametes (sperm, eggs, pollen)
• Meiosis

  • Only takes place in sex organs
  • Produces haploid gametes (not diploid)
• Fertilization

  Gametes fuse to produce a diploid zygote
• Gametes produced by meiosis are genetically different
• Homologous chromosomes

  Chromosome pairs where one chromosome comes from each parent and have the same genes
• Homologous chromosomes can have different alleles
• Crossing over in meiosis

  1. Homologous chromosomes come together

  2. Chromatids wrap around each other

  3. Parts of chromatids can break off and exchange

  4. Results in recombinant chromosomes
• Crossing over in meiosis is a major source of genetic variation