meiosis

• A cell spends most of its time in interphase, carrying out cellular functions and preparing for cell division.

  There are 3 stages (G1, S, G2) with checkpoints to check for damaged DNA. However, it only occurs before meiosis I.
• Meiosis is a type of nuclear division that ensures each daughter cell receives half the number of chromosomes as the parent cell. Also, known as a reduction division.
• It is two cellular divisions occurring within germ cells in the ovaries or testes and involves a diploid parent cell dividing into 4 genetically different haploid daughter cells.
• Important role for:

  • formation of gametes in the reproductive organs of plants and animals
  • involved in the passing on of genetic material
  • introduces genetic variation
  • ensures the chromosome number of each species is maintained, so each parent only contributes half of their chromosomes to the offspring
• There are 2 sequential rounds of division:

  • meiosis I
  • meiosis IIEach division has 4 sub-phases:
  • prophase
  • metaphase
  • anaphase
  • telophase
  • Cytokinesis occurs after each division.
• crossing over – non-sister chromatids of homologous chromosomes may exchange portions of their genetic information resulting in recombination (producing chromosomes with new combinations of genetic information)
• Independent assortment – maternal and paternal homologous pairs ofchromosomes are separated and randomly assorted into haploid cells
• Therefore, the gametes produced havedifferent genetic combinations
• prophase 1

  • chromosomes condense (shorten and thicken) and become visible
  • homologous chromosomes pair up and crossing over occurs
  • nuclear membrane breaks down
  • centrioles move to opposite ends of the cell and form spindle fibres
• metaphase 1

  • spindle fibres attach to the centromeres of chromosomes
  • pairs of homologous chromosomes align randomly along the equator in the middle of the cell
• anaphase 1

  • spindle fibres contract,  separating the pairs of homologous chromosomes
  • separated chromosomes are pulled to opposite ends of the cell
• telophase 1

  • nuclear membrane reforms around the 2 sets of chromosomes forming 2 nuclei
  • spindle fibres break down and disappear
  • chromosomes become longer and thinner
• cytokinesis 1

  Cytokinesis is the division of the cytoplasm. As the cytoplasm divides it separates the two nuclei and other organelles into two daughter cells with half the number of chromosomes.
• prophase 2

  • chromosomes condense (shorten and thicken) and become visible
  • nuclear membrane breaks down
  • centrioles move to opposite ends of the cell and form spindle fibres
• metaphase ii

  • spindle fibres attach to the centromeres of chromosomes
  • chromosomes align along the equator in the middle of the cell
• anaphase ii

  • spindle fibres attach to the centromeres of chromosomes
  • chromosomes align along the equator in the middle of the cell
• cytokinesis ii

  Cytokinesis is the division of the cytoplasm. As the cytoplasm divides it separates the four nuclei and other organelles into four unique daughter cells.