6.4

• Haploid


  Refers to one set of chromosomes (n)
• Diploid


  Refers to two sets of chromosomes (2n)
• In humans, haploid (n) = 23 chromosomes
• Only gametes (sperm and ova) are haploid
• Sexual reproduction


  • Involves the union of male and female sex cells to form a unique individual
  • Most multicellular organisms reproduce sexually; however, some can also reproduce asexually
  • Some unicellular organisms are also capable of sexual reproduction
• Somatic cells


  Body cells that are not sex cells
• Germ cells


  Cells that give rise to gametes (sex cells)
• Gametes


  • Formed through meiosis, in the gonads of the organism
  • Haploids; i.e. they have one of each type of chromosome or half the number of chromosomes that a somatic cell has
  • Somatic cells are diploid (2n) as they get one set of chromosomes (n) from each parent
• Eggs/ova


  Large immobile cells that contain the food stores needed from embryonic development
• Sperm/spermatozoa


  Have limited food reserves and normally have a tail (flagella) for motility and helping them move towards the egg
• Fertilisation


  1. The haploid egg and sperm fuse together forming a diploid zygote

  2. The zygote then divides through mitosis forming a large number of cells that are then able to differentiate and specialize forming the different tissues
• Meiosis produces haploid gametes
• To maintain the chromosomal number of a species, the number of chromosomes in somatic cells and gametes differ
• When 2 gametes fuse, they each contribute 23 chromosomes, resulting in a zygote with 46 chromosomes: 23 from sperm + 23 from egg = 46 in new zygote
• Somatic cells in most animals are diploid as they contain 2 sets of homologous (matching) chromosomes, one from each parent
• Meiosis


  Can be called reduction division because it reduces the number of chromosomes in gametes (daughter cells) to half (n) of that in somatic cells
• Unlike mitosis which produces daughter cells that receive a copy of every chromosome
• Humans have


  • 22 pairs of homologous chromosomes, the autosomes
  • 2 sex determining chromosomes, the X and Y chromosomes
• Males have an X and a Y chromosome, while females have a pair of X chromosomes
• It is the presence of the Y chromosome in males that determines male development
• Males make 4 haploid gametes (sperm) though meiosis, while females make 1 large egg cell (ovum) and 3 smaller ones that degenerate due to uneven cytoplasm division in meiosis
• Meiosis cell division


  1. Prophase 1 - chromosomes condense are each composed of 2 chromatids joined at the centromere. Crossing over at prophase

  2. Metaphase 1 - chromosome pairs line up along the equator, chromatids become apparent, and the nuclear membrane breaks down

  3. Anaphase 1 - the spindle pulls one of each chromosome pair to opposite poles of the cell and the cell membrane begins to pinch

  4. Telophase 1 and cytokinesis - cytoplasm divides and nuclear membrane reforms. Results in 2 diploid daughter cells

  5. Prophase 2 - nuclear membrane breaks down; meiotic spindle is recreated

  6. Metaphase 2 - chromosomes line up single file along the equator

  7. Anaphase 2 - centromeres split separating the 2 chromatids and the now single- strand chromosomes move to opposite ends of the cell

  8. Telophase 2 and cytokinesis - cytoplasm divides, nuclear membrane reforms and 4 haploid daughter cells are created
• Crossing over and recombination occur during meiosis