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3.3 Meiosis

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Description

Nature of science:
Making careful observations—meiosis was discovered by microscope examination of dividing germ-line cells. (1.8)
Understandings:
  • 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.
Applications and skills:
  • Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.
  • Application: Studies showing age of parents influences chances of non-disjunction.
  • 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.
Guidance:
  • Preparation of microscope slides showing meiosis is challenging and permanent slides should be available in case no cells in meiosis are visible in temporary mounts.
  • Drawings of the stages of meiosis do not need to include chiasmata.
  • The process of chiasmata formation need not be explained.
 Theory of knowledge:
  • 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?
Utilization:
  • An understanding of karyotypes has allowed diagnoses to be made for the purposes of genetic counselling.
Syllabus and cross-curricular links:
Biology
Topic 1.6 Cell division
Topic 10.1 Meiosis
Topic 11.4 Sexual reproduction

Aims:
  • Aim 8: Pre-natal screening for chromosome abnormalities gives an indication of the sex of the fetus and raises ethical issues over selective abortion of female fetuses in some countries.

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