Meiosis

• • Cells in reproductive organs divide by meiosis to form gametes (sex cells)
  • The number of chromosomes must be halved when the gametes are formed
  • Otherwise, there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg)
  • This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
• • It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell
  • After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete
  • We describe gametes as being haploid – having half the normal number of chromosomes
  • Because of this double division, meiosis produces four haploid cells
• The process of cell division by meiosis to produce haploid gamete cells

  
• Process

  • Each chromosome is duplicated (makes identical copies of itself), forming X-shaped chromosomes
  • First division: the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome
  • Second division: the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart
  • A total of four haploid daughter cells will be produced
• Importance

  • Produces gametes eg. sperm cells and egg cells in animals, pollen grains and ovum cells in plants
  • Increases genetic variation of offspring
  • Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others
• • Gametes join at fertilisation to restore the normal number of chromosomes
  • When gametes fuse, they become a zygote 
  • This contains the full number of chromosomes, half came from the male and half from the female gamete
  • The zygote divides by mitosis to form two new cells, which then continue to divide and after a few days form an embryo
  • Cell division continues and eventually many of the new cells produced become specialised (they differentiate) to perform particular functions and form all the body tissues of the offspring
  • Process of cells becoming specialised is called cell differentiation