Genetic diversity arising from mutation or meiosis (3.4.3)
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Meiosis is a type of cell division that produces four genetically distinct gametes (sperm and egg cells) and generates genetic variation through independent assortment of chromosomes and crossing over of alleles between chromatids.
It reduces the chromosome number from diploid to haploid.
Meiosis involves two sequential divisions:
- Meiosis I – separates homologous chromosomes.
- Meiosis II – separates sister chromatids.
Genetic variation increases with random fertilisation of haploid gametes:
- Independent assortment (Meiosis I).
- Crossing over (Prophase I).
- Random fertilisation (after meiosis). The randomness of which particular gametes combine during fertilisation.
Gene mutations are changes in the base sequence of DNA within a gene. They can occur spontaneously during DNA replication.
- Base deletion occurs when a nucleotide is removed. This causes a frameshift and often results in an altered amino acid sequence.
- Base substitution occurs when one nucleotide is swapped for another. This mutation type does not cause a frameshift. A substitution may or may not change the amino acid specified by the codon. If not, this is called a silent mutation.
Mutagenic agents (e.g., radiation or chemicals) can increase the rate of mutation by damaging DNA or interfering with DNA replication.
Mutations in chromosome number can arise spontaneously through chromosome non-disjunction during meiosis.
Non-disjunction occurs when homologous chromosomes or sister chromatids fail to separate properly, producing gametes with an abnormal number of chromosomes (aneuploidy).