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Meiosis- Sources of Genetic Variation — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 18 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

16.1.5 Meiosis: Sources of Genetic Variation — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how crossing over, independent assortment and random fertilisation generate genetic variation.Natural selection — genetically different offspring can be advantageous…

  • Mechanisms
  • Definition
  • Orientation
  • Separation
  • Number of combinations
  • Identical offspring

Part 1 of 6Why variation matters

Natural selection
  • Natural selection genetically different offspring can be advantageous.
Mechanisms
  • Mechanisms crossing over and independent assortment both produce different allele combinations in gametes.

Part 2 of 6Crossing over

Definition
  • Definition non-sister chromatids exchange alleles.
Pairing
  • Pairing in meiosis I homologous chromosomes pair closely.
Chiasmata
  • Chiasmata non-sister chromatids cross over and become entangled at these points.
Stress
  • Stress the entanglement strains the DNA, so a section may break and rejoin with the other chromatid.
Effect
  • Effect new combinations of alleles on the two chromosomes.
Frequency
  • Frequency usually at least one chiasma per bivalent, more likely away from the centromere.

Part 3 of 6Independent assortment

Definition
  • Definition production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs at metaphase I.
Orientation
  • Orientation each pair can face either way, independently of the others.
Separation
  • Separation homologues are then pulled to different poles.
Number of combinations
  • 2n where n is the haploid number; 223 = 8 388 608 for humans.

Part 4 of 6Random fertilisation

Fusion
  • Fusion any male gamete can fuse with any female gamete.
Result
  • Result each zygote has a unique combination of alleles.
Identical offspring
  • Identical offspring there is almost no chance of two individuals being genetically identical.
Siblings
  • Siblings explains why relatives can differ so much.

Part 5 of 6Mutation

Source
  • Source a random change in a gene during DNA replication can produce a new allele.
Effect
  • Effect increases genetic variation at gene level.

Part 6 of 6More from the specification

1
Meiotic Reduction Division
  • Meiosis is a specialized form of cell division in animal and plant germ cells where a diploid mother cell undergoes two successive nuclear division stages to produce four haploid gametes that contain half the chromosome number.
    • (Meiosis I and II)
2
Crossing Over During Prophase I
  • In prophase I, homologous chromosomes pair up to form bivalents along their length,
  • and non-sister chromatids exchange genetic segments at chiasmata,
  • producing recombinant chromatids.
3
Independent Assortment and Spindle Separation
  • In metaphase I, homologous pairs align randomly along the equatorial plate;
    • during anaphase I and telophase I, contracting spindle fibres separate whole chromosomes to opposite poles, generating substantial genetic variation.
4
Photomicrograph
  • A photograph taken through a light microscope.
5
Cytokinesis
  • Division of the cytoplasm.
6
Sexual reproduction
  • Reproduction involving fusion of two gamete nuclei, giving varied offspring.
7
Reduction division
  • Division that halves the chromosome number.
8
Spindle
  • Structure of microtubules that attaches to and moves chromosomes.
9
Diploid
  • Having two sets of chromosomes.
10
Prophase
  • First stage of mitosischromosomes condense and the nuclear envelope breaks down.
11
Telophase
  • Final stage in which chromosomes decondense and nuclear envelopes reform.
12
Anaphase
  • Stage in which centromeres divide and chromatids are pulled to opposite poles.
13
Nucleus
  • The part of the cell that controls its activities.
14
Equator
  • Middle of the spindle where chromosomes line up.
15
Cytoplasm
  • The jelly in which the cell's chemical reactions happen.

Key terms

TermMeaning
Crossing overExchange of alleles between non-sister chromatids.
ChiasmaPoint where non-sister chromatids cross over.
Independent assortmentRandom orientation of homologous pairs at metaphase I.
Genetic variationDifferences in allele combinations between individuals.
Random fertilisationChance fusion of any two gametes.
MutationRandom change in the DNA base sequence.

Avoid these mistakes

1
Saying crossing over exchanges chromatids of the same chromosome — it exchanges non-sister chromatids.
2
Saying independent assortment occurs in anaphase II only — it occurs at metaphase I.
3
Saying fertilisation is not a source of variation — random fusion creates unique zygotes.
4
Giving 2n as 2 × n — it means 2 to the power n.

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