Topic Nuclear Division Essay - Main Points!

How do meiosis and fertilization give rise to genetic variation?

Main points
Key idea:	Crossing over and independent assortment of bivalents during meiosis result in genetic variation
· Genetic variation due to : (i) Crossing over of homologous chromosomes during prophase I (ii) Indep arrangement of pairs of homologous chromosomes (bivalents) during metaphase I + subsequent separation during anaphase I
Key idea:	Details of crossing over (prophase I) + result
· Non-sis chromatids of a pair of homologous chromosome may form chiasma (or chiasmata) ; result - breakage + exchange of an equivalent segment of non-sis chromatids · After rejoinig, result - exchange of genetic material b/w homologous chrom · Produce recombinant gametes w new combinations of alleles
Key idea:	Details of indep assortment (metaphase I, anaphase I) + result
· Indep arrangement + separation of homologous chrom during metaphase I and anaphase I, respectively results - a large number of diff allele combinations · Reason : arrangement of bivalents at metaphase plate is random + independent of each other - alleles at specific gene loci in a bivalent are separated independently of other alleles at other bivalents · Eventually, each gamete is haploid as it receives one of each pair of alleles + genetically diff fr other gametes · Ref to no. of possible allele combinations = 2n (where n = haploid no.)
Key idea:	Fertilization – another source of genetic variation
· In sexually reproducing organisms, fusion of male + female gametes fr 2 diff ind is random, result – genetic variation
Comments: (i) Visualize the answer with the aid of a labelled diagram. (ii) Verbalize the answer + write down the main points w/o referring to the answer (use abbreviations) (iii) Read again within the next 24h & once more within 72h (do not spend more than 10min) (iv) Linking to evolution - Meiosis and fertilization are sources of genetic variation in a population (or species) which is required for natural selection to exert its effect. Individuals best suited or adapted to the existing environmental conditions are more liikely to survive to reproductive age and produce offspring compared to less adapted individuals. Hence, genetic variation ensures the population change and adapt to the environment when the conditions changes.