3.3+Meiosis

Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells. (1.8) || One diploid nucleus divides by meiosis to produce four haploid nuclei. The halving of the chromosome number allows a sexual life cycle with fusion of gametes. DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids. The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation. Orientation of pairs of homologous chromosomes prior to separation is random. Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number. Crossing over and random orientation promotes genetic variation. Fusion of gametes from different parents promotes genetic variation.
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 * **Understandings:**

Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities. Application: Studies showing age of parents influences chances of nondisjunction. Application: Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks. Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells. || In 1922 the number of chromosomes counted in a human cell was 48. This remained the established number for 30 years, even though a review of photographic evidence from the time clearly showed that there were 46. For what reasons do existing beliefs carry a certain inertia? ||
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 * **Essential idea:** Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes. ||

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