Meiosis

• Meiosis produces four haploid nuclei from one diploid nucleus.

• Haploid nuclei allow a life cycle with fusion of gametes.

• DNA is replicated before meiosis so that all chromosomes at start of meiosis are 'double stranded' with two sister chromatids.

• The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.

• Random orientation of pairs of homologous chromosomes.

• Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.

• Genetic variation is the result of crossing over and random orientation.

• Different parents providing gametes promotes genetic variation.

SKILLS NEEDED:

• Describe how non-disjunction can cause Downs syndrome and other chromosome abnormalities.

• Remember studies showing age of parents influences chances of non-disjunction.

• Describe methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

• Draw diagrams to show the stages of meiosis (possibly using prepared slides), and resulting in the formation of four haploid cells. (Chiasmata not required)