MEIOSIS, DOWNS AND POLYPLOIDY ESSAY DESCRIBE THE MECHANISM BY WHICH MEIOSIS CONTRIBUTES TO GENETIC VARIATION AND DESCRIBE HOW ABNORMAL MEIOSIS LEADS TO DOWNS SYNDROME AND POLYPLOIDY. ALSO, DESCRIBE THE ADVANTAGES OF POLYPLOIDY IN AGRICULTURE? o Find the main headings from the information given o Then make clear points to gain the full 17 marks o The other 3 marks will be given by the examiner only if the essay is logical and focused o It will be marked on an ‘explanation’ of Crossing Over, Random arrangement of chromosomes, Downs Syndrome, Polyploidy and the Advantages of Polyploidy in Agriculture Meiosis and Genetic Variation Crossing Over o Homologous chromosomes pair up o Each chromosome has 2 chromatids, o The chromatids cross over. The points at which crossing over takes place are referred to as a chiasmata o Genetic material is exchanged between non sister chromatids o After the process of crossing over chromosomes have genes from its homologous partner o This means that each gamete formed will have a mixture of genes from the paternal and maternal parents o This brings about variation in the gametes formed and also to offspring Max 5 Random Arrangement of Chromosomes at the Equator o Each pair of homologous chromosomes line up at the equator of the spindle independently of what the other pairs are doing- this is called independent assortment o This means the gametes formed will have a different number of maternal and paternal chromosomes Max 3 Abnormal Meiosis: Downs Syndrome (page 33 Dr Isaac) o In Meiosis I, the chromosome pair 21 does not separate or In Meiosis II the chromatids of chromosome 21 do not separate or the centromere does not divide – this is referred to as NON-DISJUNCTION o One gamete will have an extra copy of chromosome 21 o If this gamete fuses with a normal gamete, the resulting zygote will have 3 copies of chromosomes 21 instead of 2 o The zygote therefore ahs 47 chromosomes leading to Downs Syndrome Max 4 Polyploidy o During Meiosis I, there is a lack of separation of ALL homologous chromosomes this is called NON DISJUNCTION o One gamete will inherit the diploid set of chromosomes o When a diploid gamete is fertilized by a normal haploid gamete o The zygote will have 3 sets of chromosomes (triploid) o In a similar way, tetraploid and other polyploid offspring could be formed Max 3 Advantages of Polyploid in Agriculture o Polyploidy forms seedless varieties of fruit such was watermelons, bananas, grapes, some apples o Polyploidy cells are bigger and they produce larger flowers and storage organs o Infertile plants become fertile eg wheat o Plants may be healthier or resistant to disease. Max 2 LIFE SCIENCE ACADEMICS - 2014
Posted on: Wed, 19 Nov 2014 17:15:00 +0000
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