Meiosis

Meiosis

1. Stages

   • Meiosis 1
     • 1. prophase 1
       • chromatin condenses and undergoes supercoiling
         • chromosomes shorten and thicken
           • can be seen with a light microscope
       • chromosomes come together in homologous pairs
         • forms a bivalent
         • one from father one from mother
       • non sister chromatids wrap around eachother
         • attach at chiasmata
           • swap sections of chromatids
             • crossing over
       • nucleolus disappears
         • nuclear envelop disintergrates
           • spindle forms made of protein microtubules
     • 2. metaphase 1
       • bivalents line up across equator
         • attach to spindle at centromeres
         • attached randomly
           • random assortment
         • each homologous pair facing opposite poles
     • 3. Anaphase 1
       • homologous chromosomes in each bivalent pulled to opposite poles
       • centromeres don't divide
       • chiasmata separate
         • lengths of chromatid which are crossed over remain with chromatid newly attached to
     • 4. Telophase 1
       • animal cells
         • two new nuclear envelopes form
           • cell divides y cytokinesis
         • brief interphase
           • chromosomes uncoil
       • plant cells
         • goes straight from anaphase 1 into meiosis 2
   • Meiosis 2
     • 1. prophase 2
       • nuclear envelop breaks down
       • nucleolus disappears
         • chromosomes condense and spindles form
     • 2. metaphase 2
       • chromosomes arrange at equater
         • attached to spindle by centromeres
       • chromatids of each chromosome are randomly assorted
     • 3. anaphase 2
       • centromeres divide
         • chromatids pulled to opposite poles
       • chromatids randomly segreate
     • 4. telophase 2
       • nuclear envelopes reform around haploid daughter nuclei
       • animals
         • two cells divide to give four haploid cells
       • plants
         • tetrad of four haploid cells is formed
       • haploid nuclei have half chromosome number

2. Genetic variation

   • crossing over
     • during prophase 1
     • homologous chromosomes form bivalents
       • non-sister chromatids wrap around each other
         • attach at chiasmata
           • chromosomes may break
             • broken end of chromatids re-join to end of non-sister chromatids
               • similar sections of non-sister chromatids swap over
     • produces new combinations of alleles on the chromatids
   • genetic reassortment
     • chromosomes
       • metaphase 1
         • random distribution of maternal and paternal chromosomes on the spindle equator
       • anaphase 1
         • subsequent segregation into two daughter nuclei
       • each gamete acquires a different mixture of maternal and paternal chromosomes
     • chromatids
       • metaphase 2
         • random distribution on the spindle equator of sister chromatids
           • determines how they segregate at anaphase 2
       • sister chromatids not identical
         • due to crossing over
   • random mutation
     • DNA mutation may occur in interphase
       • this is where DNA is replicated
     • chromosome mutation may also occur
     • mutation increases genetic variation
     • if occurs in sperm or egg used in fertilisation
       • mutated gene present in every cell of offfspring
   • fertilisation
     • genetic material from two unrelated individuals combined to make a zygote
     • millions of sperm and egg
       • all genetically different

3. before meiosis 1

   • Interphase
     • DNA replicates
       • chromosome consists of identical sister chromatids joined by a centromere