Meiosis

Prophase I -> each chromosome duplicates and remains closely associated - sister chromatids. Crossing over occurs late in this stage

Metaphase I ->homologous chromosomes align at the equatorial plate - chiasmata remain in place holding the pairs together on the spindle so that in Anaphase, one member goes to each pole

Anaphase I -> homologous pairs separate with sister chromatids remaining together

Telophase I -> Two daughter cells are formed - each contains one chromosome of the homologous pair 

Prophase II -> Centrioles replicate but DNA does not replicate

Metaphase II -> Chromosomes align at the equatorial plate

Anaphase II -> Centromeres divide and sister chromatids migrate separately to each pole

Telophase II -> Cell division is complete. Four haploid daughter cells are obtained - they are all genetically different

• each chromosome is divided into two chromatids
• homologous chromosomes twist around each other
• This tension causes breaks 
• cross overs are called chiasmata
• fragments may attach to other homologous chromosome
• creates a new genetic combination and viariation in gametes

• Crossing over shuffles alleles
• Genetic reassortment due to the random distribution and subsequent segregation of the chromosomes in the homologous pairs during meiosis I
• Genetic reassortment due to random distribution and segregation of sister chromatids at meisos II
• Random mutation

Fertilisation increases genetic variation by randomly combining chromosomes at the two chromosomes are unrelated - 1 of 300 million sperm will fertilise the egg

• Random distribution of maternal and paternal chromosomes on the spindle equator that then segregate in Anaphase 1
• Gamete acquires a different mix of maternal and paternal chromosomes

Re-assortment of Chromatids

• Random distribution of the spindle equator of sister chromatids at Metaphase II
• Crossing over prevents sister chromatids being genetically identical
• the alignment at metaphase II determines how they segregate at Anaphase II

Mutation

DNA mutation may occur during interphase during replication - chromosome mutation may also occur and these increase genetic variation

Advantage - they increase variation

Disadvantage - Requires a lot of energy, Organisms more vulnerable when pregnant