Meiosis

Nature of science:

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.

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.

• 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.

• 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.