Essential Idea:  Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.

• Outline answer to each objective statement for topic 3.3 (coming soon)
• Quizlet study set for this topic

At SHS, Topic 3.3 is taught in the following class unit(s):​​​

• Chromosomes (unit 15)
• Meiosis (unit 16)

3.3.U1  One of diploid nucleus divides by meiosis to produce four haploid nuclei.

• Compare divisions of meiosis I and meiosis II.​

3.3.U2  The halving of the chromosomes number allows a sexual life cycle with fusion of gametes.

• Compare sexual and asexual life cycles.
• Explain why meiosis must occur as part of a sexual life cycle.​

3.3.U3  DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.

• State that DNA is replicated in interphase before meiosis.
• Given a diploid number (for example 2n=4), outline the movement and structure of DNA through the stages of meiosis.​

3.3.U4  The early stages of meiosis involves pairing of homologous chromosomes and crossing over followed by condensation.

• ​List three events that occur in prophase 1 of meiosis.  
• Define bivalent and synapsis.
• Outline the process and result of crossing over.​

3.3.U5  Orientation of pairs of homologous chromosomes prior to separation is random.

• Describe the attachment of spindle microtubules to chromosomes during meiosis I.
• Describe random orientation of chromosomes during meiosis I.

3.3.U6  Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.

• Explain why meiosis I is a reductive division.
• State that cells are haploid at the end of meiosis I.

3.3.U7  Crossing over and random orientation promotes genetic variation.

• ​Explain how meiosis leads to genetic variation in gametes.
• State the the number of chromosome combinations possible due to random orientation is 2^n.​

3.3.U8  Fusion of gametes from different parents promotes genetic variation.

• Outline the role of fertilization as a source of genetic variation.​

3.3.A1  Non-disjunction can cause Down syndrome and other chromosome abnormalities.  Studies showing age of parents influences chances of non-disjunction.

• Define non-disjunction.
• State the result of nondisjunction.
• Describe the cause and symptoms of Down syndrome.
• Explain the relationship between parental age and chances of non-disjunction.​​

3.3.A2  Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

• Describe the two procedures for obtaining fetal cells for production of a karyotype.

3.3.S1  Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.

• Outline the events of prophase, metaphase, anaphase and telophase in meiosis I and meiosis II.
• Draw diagrams of cells in prophase, metaphase, anaphase and telophase in meiosis I and meiosis II.​

3.3.NOS  Making careful observations- meiosis was discovered by microscope examination of dividing germ-line cells.

• Discuss difficulties in microscopic examination of dividing cells.
• Describe the discovery of meiosis.

In the News:

• Why Sex? Biologists Find New Explanations (2020-04-23)
• Molecular guardians monitor chromosomes during cell division (2018-10-04)
• How chromosomes 'cheat' for the chance to get into an egg (2017-11-02)
• ​​​​Scientists identify Down's Syndrome gene (2016-08-09)
• ​​From the somatic cell to the germ cell (2017-06-08)
• Cells simply avoid chromosome confusion (2014-09-15)