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Genetics Lectures 19–23.doc

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McGill University
Biology (Sci)
BIOL 202
Tamara Western

Lecture 19Largescale chromosomal changes in plantsView of a chromosome3 parts to it p q and a centromere that connects the short and long arm PpetitQnext letterHow to visualize chromosomes1Take a mammalian blood sample Centrifuge the sample to separate the white blood cell nucleated from the remaining blood Put the white cells in culture Most normal mammalian cell will die eventually in culture2Stimulate the cells to divide in vitro by giving growth factor fetal bovine serum Stop them when they are dividing Stop the mitotic spindles with drugs Taxol and the cells will be stuck in mitosis 3Add hypotonic solution to expand the cells and then squash the cells on he slide fix them and stain 4Using a radioactive or fluorescent biotin probe one can see the location of the probes on the chromosomesPloidy the number of chromosomesFor an organism or a celleuploidcomplete normal sets of chromosomes aneuploid chromosome is over or underrepresented trisomy genetic imbalance polyploid extra set of chromosomes plants3n 4n 5ndiploidtwo sets of chromosomes human mammals n23tetraploids four sets of chromosomesRearrangements an abnormal chromosome deletion translocation duplicationSome species exist in monoploidy male bees wasps and antsMonosomicone less chromosome NOT one less set Like a diploid1 chromosome Trisomiccell will have an EXTRA chromosome Increase the total number of chromosomes by 1 Humans have 46 chromosomes to start with so can have 45 or 47 or 48 49 50Polyploidy Very common in plants but rarer in animalsAutopolyploids multiple chromosome set from within one species most triploidsAllopolyploids have chromosome sets from two or more speciesSterility in a triploid Meiosis pairing in a triploidThey have an extra chromosome that cannot pair with the other Meiosis will generate three things trivalent or bivalent and univalent Almost impossible for a triploid organism to produce gametes with an equal number of chromosomes For this reason TRIPLOID PLANTS ARE STERILEMeiosis uncertainty in TetraploidyTetraploidy is less intense but can still have univalent and trivalent segregation Trivalent and Univalent cells will be sterile What will drive the migration of the chromosomes in the daughter cells is the centromere Makes the chromosomes migrate If the segregation is equal lined as two bivalent or one quadravalent it will still be fertile with diploid gametesMost tetraploid plants will generate unfertile gametes but some like the bread wheat and Raphanobrassica are fertileAllopolyploidy The wonder of hybridizationRadish are 2n diploid18 chromosomes so the gametes will have 9 chromosomes eachFuse 9 and 9 from cabbage diploid 2n18 together and make 2n18 But it will be sterile It wont exit meiosis because it wont segregate the chromosomes properlySpontaneous doubling of the chromosomes 4n36 causes amphidiploidy These plants WERE fertile This is because although they have the same number of chromosomes and although they were close enough to make an individual from the two gametes all the chromosomes from the radish could not pair with the chromosomes from the cabbage The pairing was not happening so the chromosome would not segregate one from the other during meiosis A tetraploid is more of a double diploidthis can make a fertile amphidiploidOrigin of hexaploid wheat by sequential bybridization of different speciesWheat is a hexaploid plant It took many years do this AA and BB gametes from two diploid species that fuse They needed to be a double diploid AABB for it to be fertile a tetraploid individualPlants are quite tolerant to the duplication of chromosomesMany years later AABB individual was mixed with another plant and this became an AA BB DD plant ABD was sterile until the duplication the chromosomes through a sterile intermediate A B C D The wheat we have now is hexaploidIn cell culture the use of colchicine allows the generation of tetraploid cells
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