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3.3 Meiosis

IB Syllabus Statements

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.

  • An understanding of karyotypes has allowed diagnoses to be made for the purposes of genetic counselling.

  • 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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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.

Videos

Meiosis Part 1

Meiosis Extras

Meiosis Part 2

Non-disjunction and Down Syndrome

Hopefully Helpful Resource: 

PBS Mitosis vs. Meiosis Animation: http://www.pbs.org/wgbh/nova/miracle/divi_flash.html

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Topic 3.3 Meiosis SSaQ (Syllabus Statement as Questions)

  • Explain how making careful observations led to the discovery of meiosis. *NoS

  • In meiosis, what does one diploid nucleus divide to form?

  • What is the purpose of halving the chromosome number during meiosis?

  • In relation to meiosis, when is DNA replicated so that each chromosome consists of two sister chromatids?

  • When specifically do homologous chromosomes pair up during meiosis?

  • When does crossing over occur during meiosis?

  • What is meant by ‘condensation’ of chromosomes following pairing up of homologous chromosomes?

  • When do homologous chromosomes line up or orient themselves at the equator of the cell? Explain how this process is random.

  • Explain what happens during the first division of meiosis using the following terms: chromosome number, pair, homologous, chromosome, halve

  • Explain how crossing over and random orientation promote genetic variation.

  • Explain how the fusion of gametes from different parents promotes genetic variation.

  • Explain how non-disjunction can cause Down syndrome and other chromosome abnormalities.

  • Outline the relationship between the age of parents and the chances of non-disjunction. Suggest a causal relationship between these two factors and the possible outcomes.

  • Describe two methods that are used to obtain cells for karyotype analysis. What are the associated risks when using these methods?

  • Provide instructions for how you would guide someone to draw diagrams showing the stages of meiosis resulting in four haploid cells. (For this example: start with a cell with a diploid number of 4) (Note: You do not need to include chiasmata in the drawings or instructions.)

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