meiosis

• prophase 1


  -chromatins condense- super coiling- become visible

  -chromosomes come together in homologous pairs to form a bivalent

  -non-sister chromatids wrap around each other and attach at at chiasmata

  -sections of the chromatids cross over

  -nuclear envelope disintegrates

  -centriole migrates to poles and spindle fibres form
• metaphase 1


  -bivalents line up at equator

  -spindle attach to centromeres

  -bivalents are arranged randomly (independent assortment) with each member of a homologous pair facing opposite poles

  -allows chromosomes to independently segregate when pulled apart
• anaphase 1 


  -homologous pairs in each bivalent are pulled to opposite poles by spindle fibres

  -centromeres dont divide

  -chiasmata separate and lengths that have been crossed over stay with their newly attached chromatid
• telophase 1 animals

  -nuclear envelopes reform and cell divides by cytokinesis

  -brief interphase- no dna replication- chromosomes uncoil and become invisible again
• telophase 1- plants


  -no telophase 1

  -cell progresses straight from anaphase 1 to meiosis 2
• prophase 2


  -nuclear envelope breaks down again if reformed

  -nucleolus disappears, chromatin condenses and spindles form
• metaphase 2 


  -chromosomes line up at equator and spindle fibres attach at centromeres

  -chromatids are arranged randomly- independent assortment
• anaphase 2 


  -centromeres divide

  -chromatids are pulled to opposite poles by spindle fibres

  -chromatids randomly segregate

  -1 arm pulled apart- could be maternal or paternal
• telophase 2

  -nuclear envelope forms around haploid daughter nuclei

  -animal cells- 2 cells divide to give 4 haploid cells

  -plants cells- tetrad of 4 haploid cells formed
• meiosis increases genetic variation by

  -crossing over (prophase 1)

  -assortment of chromosomes (metaphase 1)

  -assortment of chromatids (metaphase 2 )

  -random mutations
• why is genetic variation so important?

  -natural selection- some variants are better adapted so more likely to survive

  -without variation, evolution would not occur

  -changing conditions may lead to a variant being better adapted- more likely to survive
• chiasmata

  -point where non- sister chromatids join and cross over
• crossing over

  -where lengths of DNA are swapped from one chromatid to another
• bivalent

  -pair of joined (synapsed) homologous chromosomes during prophase and metaphase of meiosis 1
• sources of variation in meiosis

  -random mutations can occur in interphase when DNA replicates

  -crossing over

  -independent assortment of chromosomes (metaphase 1 and 2)
• independent assortment

  -inheritance of one chromosome is independent from the inheritance of another

  -either chromosome from a homologous pair can be passed into the gamete

  -each gamete receives a different combination of maternal and paternal chromosomes
• fertilisation

  -randomly combining 2 sets of chromosomes

  -one from each genetically unrelated individuals
• crossing over

  -results in new combination of alleles

  -lead to variation within a population