Unit 2 – Genetics & Adaptation

Meiosis Chapter 12

Role of sexual reproduction Variation exists between members of a species - discontinuous or continuous Variation determined by inherited characteristics Sexual reproduction – fusion of 2 gamete (sex cell) nuclei Forms a zygote – genetically different from parents This maintains & increases genetic variation

Haploid & Diploid cells Every species has a certain chromosome complement Diploid cell (2n) – double set of chromosomes E.g. a zygote Haploid cell (n) – single set of chromosomes (half the DNA) E.g. a gamete Pairs of chromosomes match up gene for gene These are said to be homologous But different forms of each gene exist - alleles

Meiosis A diploid gamete mother cell (2n) forming four haploid gametes (n) Involves 2 nuclear divisions - firstly, chromosomes replicate – 2 chromatids - homologous chromosomes pair up gene by gene - at chiasmata chromatids join and genetic material can be swapped – crossing over - nuclear membrane disappears, spindle fibres form, homologous pairs align at equator - spindle fibres pull chromosomes to each pole Nuclear membrane re-forms, cytoplasm divides – 2 diploid cells Followed by 2 nd meiotic division – 4 gametes produced

Significance of meiosis Provides opportunity for new combinations of alleles of genes to arise Independent assortment Pairs of chromosomes line up randomly – independent assortment - more chromosomes = more possible combinations Crossing over / separation of linked genes At chiasmata linked chromosomes can exchange genetic information Results in new combinations of alleles – increased variation