meosis

• Meiosis is the process that produces haploid gamete cells from diploid cells. Meiosis is necessary for sexual reproduction to take place.
• Sexual reproduction

  • Gametes are the sex cells (e.g. sperm cells and egg cells) that fuse together during sexual reproduction.
  • Gametes are haploid (have half the number of chromosomes as diploid cells).
  • When the gametes fuse together, a new diploid organism (an organism with two copies of every chromosome) is created.
  • An individual inherits one copy of a chromosome from each parent.
• Producing gametes

  • Haploid gametes are produced from diploid cells in an organism.
  • The gametes are produced in a process called meiosis.
• Genetic variation

  • Meiosis produces four genetically different daughter cells.
  • Each daughter cell receives a different combination of chromosomes.
  • Meiosis increases genetic diversity in this way.
• Random fertilisation

  • Sexual reproduction involves the random fusion between gametes.
  • Any two gametes could fuse together to produce a random combination of chromosomes.
  • Meiosis and random fertilisation further increase genetic diversity in this way.
• 1. Producing sister chromatids

  • The chromosomes in the cells replicate to produce an identical copy of each other.
  • The replicated chromosomes are called chromatids.
  • The chromatids condense to form a double-armed chromosome. Each arm is made from one chromatid and is joined by a centromere.
  • Each arm is called a sister chromatid.
• 2) Arranging the chromosomes

  • Chromosomes in diploid organisms are found in homologous pairs (chromosomes with identical genes but different alleles).
  • The chromosomes in a pair are arranged in the centre of the cell.
  • The chromosomes are moved by fibres called spindle fibres.
• 3) Separating the chromosomes

  • The homologous chromosomes are separated and pulled to opposite ends of the cell by the spindle fibres.
  • The separation of the chromosomes from each pair is random.
  • Random combinations of chromosomes are pulled to each side of the cell.
• 4) Cell division (meiosis I)

  • As the homologous chromosomes are separated, the cell divides into two cells.
  • One chromosome from a pair is separated into each cell.
  • This division is called meiosis I.
  • Meiosis I produces two genetically different cells.
• 5) Separating sister chromatids

  • The sister chromatids are separated by the spindle fibres and pulled to opposite ends of the cell.
  • The separation of the sister chromatids is random.
• 6) Cell division (meiosis II)

  • As the sister chromatids are separated, the cell divides into two cells.
  • One chromatid from a chromosome is separated into each cell.
  • This division is called meiosis II.
  • Meiosis II produces four genetically different haploid cells.
• Crossing over

  • The chromosomes of a homologous pair are arranged down the middle of the cell before the first cell division in meiosis I.
  • Sections of DNA are swapped between the chromosomes in a pair in a process called crossing over.
  • Crossing over produces new combinations of alleles. This process is a key part of genetic recombination.
  • This increases genetic variation.
• Independent segregation

  • The chromosomes in a homologous pair are separated in meiosis I in a random formation to produce two genetically different daughter cells.
  • The combination of chromosomes in the two daughter cells is random.
  • The process of separating chromosomes into random combinations is called independent segregation.
  • Independent segregation increases genetic variation.