Meiosis

Meiosis

1. most organisms are diploid (2n) with 2 copies of each chromosome (homologous pair) one from each parent

   • humans have 23 homologous pairs giving total of 46 chromosomes
     • gametes haploid with only alf number of chromosomes/ DNA present
       • when gametes fuse in sexual reproduction the diploid number is restored

2. Meiotic stages I

   • INTERPHASE I
     • dna content doubled and new cell organelles formed
   • PROPHASE I
     • 1) chromosomes become shorter and thicker 2) homologous chromosomes pair up as a bivalent 3) each chromosome is two chromatids and crossing over between homologous chromosomes with formation of chiasma 4) centrioles move to opposite ends of nucleus, microtubules form asters and then the spindle 5) nuclear envelope breaks and nucleolus disappears
   • METAPHASE I
     • bivalents align on the equator attached to spindle. Maternal and paternal chromosomes are arranged randomly
   • ANAPHASE I
     • whole chromosome (centromere do not split) move to the poles. each one of the random paternal or maternal homologous pair moves to opposite pole by contraction of spindle fibres
   • TELOPHASE I
     • animal cell : cytokinesis may occur or plants: may go straight onto meiosis II

3. Meiotic stages II

   • PROPHASE II
     • new spindles form at right angles to the old spindle
   • METAPHASE II
     • chromosomes line up at spindle equator attached to centromere
   • ANAPHASE II
     • centromeres divide and sister chromatids pulled to opposite poles
   • TELOPHASE II
     • 1) chromosomes lengthen and become indistinct 2) spindle disappears 3) nuclear membrane reforms 4) cytokinesis
       • four haploid cells produced (eg. sperm/eggs)

4. meiosis can generate variation

   • RANDOM ASSORTMENT
     • independent segregation of maternal and paternal homologous chromosomes in metaphase I
   • CROSSING OVER
     • occurs in prophase I with the formation of chiasmata (so get different combinations of alleles when genetic information is exchanges)
   • RANDOM FERTILISATION
     • of many genetically different haploid gametes

5. Meiosis is the reduction division producing gametes in sexually reproducing organisms (diploid 2n to haploid n cells)