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4.4.5Additional lessons

Meiosis — IB Biology HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 44 of 101

📚 IB Biology HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6The Two Divisions

Meiosis is a reduction divisionit halves the chromosome number from diploid to haploid.

1
Before meiosis the DNA is replicated, so every chromosome consists of two sister chromatids.
2
Prophase I: homologous chromosomes pair up as bivalents and crossing over swaps segments between non-sister chromatids at chiasmata.
3
Metaphase I and anaphase I: bivalents line up at the equator, then whole homologous chromosomes are pulled to opposite poles.
4
Telophase I and cytokinesis give two haploid cells, each with chromosomes that still consist of two chromatids.
5
Meiosis II resembles mitosis: sister chromatids separate, producing four haploid cells in total.

Part 2 of 6Sources of Genetic Variation

Crossing over
  • Crossing over Exchange of segments between non-sister chromatids creates new combinations of alleles on each chromosome.
Independent assortment
  • Independent assortment Each bivalent lines up randomly, so maternal and paternal chromosomes are distributed to gametes in many combinations.
Number of combinations
  • Number of combinations With n pairs of chromosomes there are 2ⁿ possible combinations, which is 2²³ in humans, ignoring crossing over.
Random fertilisation
  • Random fertilisation Any sperm can fuse with any egg, multiplying the possible genotypes of offspring.
FeatureMitosisMeiosis
DivisionsOneTwo
Homologous pairingNoYes, in prophase I
Crossing overNoYes
Daughter cells2 diploid, identical4 haploid, different

Part 3 of 6Non-disjunction

Definition
  • Definition Chromosomes or chromatids fail to separate during anaphase, so some gametes receive an extra chromosome and others lack one.
Down syndrome
  • Down syndrome A gamete carrying two copies of chromosome 21 fuses with a normal gamete, giving trisomy 21 with 47 chromosomes.
Detection
  • Detection Karyotyping arranges chromosomes in homologous pairs by size and banding pattern, showing any extra or missing chromosomes.
Maternal age
  • Maternal age The risk of non-disjunction in egg cells rises with the age of the mother.

Part 4 of 6Haploid and Diploid

Haploid (n)
  • Haploid (n) A cell with one set of chromosomes, one from each homologous pair, as in human gametes with 23 chromosomes.
Diploid (2n)
  • Diploid (2n) A cell with two sets, one from each parent, as in human body cells with 46 chromosomes.
Why reduction matters
  • Why reduction matters Without halving, the chromosome number would double every generation after fertilisation.

Part 5 of 6Worked Example

Question Calculate the number of different chromosome combinations possible in the gametes of a human, ignoring crossing over.

  1. Humans have 23 pairs of homologous chromosomes, so n = 23.
  2. Each pair can orientate in 2 ways independently, so combinations = 2²³.
  3. 2²³ = 8 388 608.

Answer About 8.4 million different gametes, which is why siblings are genetically different even without crossing over.

Part 6 of 6Exam Checklist & Tips

4.4.5 – Key Points for the Exam

  • Bivalent A pair of homologous chromosomes held together during prophase I.
  • Crossing over The exchange of DNA between non-sister chromatids of homologous chromosomes.
  • Chiasma The point where chromatids cross and exchange material.
  • Non-disjunction Failure of chromosomes to separate correctly during meiosis.
  • Haploid Having a single set of chromosomes, n.

Common Mistakes

  • Saying homologous chromosomes separate in meiosis II;
    • this happens in meiosis I, and chromatids separate in meiosis II.
  • Describing crossing over between sister chromatids rather than non-sister chromatids.
  • Stating that meiosis produces identical cells.
  • Thinking gametes are formed by mitosis.

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