meiosis

• meiosis produces gametes
• meiosis includes two divisions, making four daughter cells
• meiosis: divisions

  1. prophase
  2. metaphase
  3. anaphase
  4. telophase
• interphase:

  • chromosomes replicate
  • through semi-conservative replication
  • produces two identical sister chromatids
• prophase 1:

  • homologous chromosomes pair up - bivalents
  • chromosomes condense
  • nuclear envelope breaks down
  • crossing over occurs = at chiasmata
• metaphase 1:

  • homologous chromosomes line up at the equator randomly
• anaphase 1:

  • spindle fibres contract
  • pairs of homologous chromosomes separate
  • to opposite poles of the cell
• telophase 1:

  • two haploid cells are formed
  • spindle fibres breakdown
  • nuclear envelope reforms
• prophase 2:

  • centrioles move to form new poles at right angles to the original poles
  • chromosomes remain coiled
• metaphase 2:

  • chromosomes line up along the equator randomly
  • spindle fibres attach to the centromeres
• anaphase 2:

  • spindle fibres contract
  • non-identical sister chromatids are pulled apart
• telophase 2:

  • spindle fibres break down
  • nuclear envelope reforms
  • four haploid gametes are produced
• after division 1 of meiosis, the two cells produced are haploid as the homologous chromosomes are in different cells
• how to identify anaphase 1:

  • pair of homologous chromosomes separating
  • centromere has not divided
  • sister chromatids visible
• halving the chromosome number in gametes is an advantage as it allows the diploid number to be restored during fertilisation
• meiosis can produce genetic variation
• crossing over of DNA in meiosis can lead to the formation of a new combination of alleles = genetic variation
• two sources of genetic variation in meiosis:

  1. crossing over
  2. independent segregation
• crossing over:

  • occurs in prophase 1 of meiosis
  • homologous chromosomes join together and sections of chromatids from each chromosome are swapped
• chiasmata - points where DNA is swapped between the homologous chromosomes
• multi-enzyme complexes cut and join the DNA at chiasmata
• a pair of homologous chromosomes always has at least one chiasmata
• equal amounts of DNA is crossed over so chromosomes stay the same length
• crossing over: genetic recombination

  • alleles are exchanged between the maternal and paternal homologous chromosomes
  • this produces new combinations of alleles
• its less likely for chiasma to form between two genes that are very close together
• producing new combinations of alleles is less frequent as crossing over is random
• independent segregation:

  • homologous pairs of chromosomes line up on the equator randomly + independent of each other
  • homologous pairs are separated during anaphase 1
  • the combination of alleles that end up in each daughter cell depends on how the homologous chromosomes lined up
• number of different chromosome combinations at the end of meiosis II = 2^n
• n is the haploid number of an organism
• meiosis:

  diploid -> haploid -> haploid
• appearance of a chromosome:

  • two identical chromatids joined together
  • by a centromere
  • forming the chromosome
  • this is during to DNA replication
• allele - a different form of a gene
• importance of meiosis in organisms that reproduce sexually

  1. meiosis halves the chromosome number, producing haploid gametes
  2. when gametes fuse during fertilisation, the diploid number is restored
  3. this keeps the chromosome number constant from one generation to the next -> introducing genetic variation
  4. genetic variation forms from crossing over and independent segregation
• role of independant segregation:

  1. maternal and paternal homologous chromosomes are re-shuffled into random combinations
  2. produces new combinations of alleles
  3. therefore increases genetic variation
• if chromosomes can not pair up -> meiosis can not occur