• Before meiosis occurs, all of the DNA inside the nucleus of the 'parent' cell is?replicated
  • This occurs during a period of the cell cycle known as?interphase
• Once this has occurred, each chromosome now consists of?two genetically identical sister?chromatids, which are joined together by a?centromere
  • The sister chromatids are genetically identical because DNA replication is a very accurate process and only a very small number of mistakes occur when DNA is being copied
• The two DNA molecules formed by DNA replication prior to meiosis are considered to be?sister chromatids?until the?splitting of the centromere?at the start of?anaphase?(a stage during meiosis II, during which the sister chromatids are?pulled apart)
• After this, they are once again considered as?individual chromosomes

DNA replication before meiosis

• At the start of meiosis,?homologous chromosomes?pair up?with each other
  • As DNA replication has already occurred, each chromosome is made up of?two sister chromatids
  • This means that a pair of homologous chromosomes is made up of?four DNA molecules
• A pair of homologous chromosomes is known as a?bivalent
• The pairing process resulting in the formation of a bivalent is known as?synapsis
• After synapsis has occurred, a process known as?crossing over?may occur
• During crossing over, two?non-sister chromatids?(i.e. one chromatid from each of the homologous chromosomes) form a?junction
• At this junction, the two chromatids?break?and?rejoin?with each other
• As these crossover events occur at exactly the same position on the two non-sister chromatids, this allows?genes?to?exchange?between the chromatids
• Non-sister chromatids are?homologous?but are?not genetically identical?and this means that some of the?alleles?of the exchanged genes will be?different
• This process, therefore, produces chromatids with?completely new combinations of alleles?(that were not previously present in the DNA of the 'parent' cell)
• As these chromatids will eventually be split up into?different gametes, crossing over is of great?importance?because it is a significant source of?genetic variation?between gametes
  • This ensures there is genetic variation in?populations?of?sexually-reproducing species, which is key to a species' ability to?evolve?and?adapt?to changes in its environment over time

Crossing over of non-sister chromatids leading to the exchange of genetic material

• At metaphase, during meiosis I,?homologous chromosomes line up at the cell equator?as they prepare to?separate
• Spindle microtubules?grow out from the?poles?of the cell and attach to the?centromeres?of the chromosomes
• Each of the two homologous chromosomes in a bivalent is attached to a?different pole
• The?orientation of the bivalents?when they line up at the cell equator determines which pole each chromosome gets attached to (and eventually pulled towards)
• The orientation of the bivalents is?completely random
• In addition, the bivalents also?assort independently of one another?(i.e. the orientation of one bivalent never affects the orientation of another)

The orientation of bivalents lining up at the cell equator is random

• During meiosis, the homologous chromosomes forming a bivalent separate in a process known as?disjunction
• The homologous chromosomes then move to?opposite poles?of the cell
• As?one chromosome of each type?moves to each pole, the two separate nuclei formed by the first division of meiosis (meiosis I) now only contain?one of each type of chromosome, making the two new cells?haploid
  • Essentially, the chromosome number of the cells has been?halved
• This is why the first division of meiosis is known as a?reduction division
  • The chromosome number has been?reduced?(halved) from?diploid?to?haploid