Chapter 12 Notes - Meiosis
Chapter 12 Notes - Meiosis Biol 2311
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This 4 page Class Notes was uploaded by Rachael Couch on Wednesday October 14, 2015. The Class Notes belongs to Biol 2311 at University of Texas at Dallas taught by John Burr in Fall 2014. Since its upload, it has received 13 views.
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Date Created: 10/14/15
Chapter 12 Meiosis Meiosis results in the formation of sperm and eggs Gametes haploid cells with only one copy of each chromosome produced by quotgerm linequot cells eggsperm o Haploid N Somatic cells body cells diploid two homologous copies of each chromosome 0 Diploid 2N Meiosis Overview 1 Maternal and paternal chromosomes duplicate DNA synthesis so that each homolog now consists of two joined chromatids 2N4N o In humans 2N 46 chromosomes 23 maternal 23 paternal After S phase there are 92 chromosomes in a human cell 4N 2 ln meiosis 1 one 4N cell goes to two 2N cells 3 ln meiosis 2 each 2N cell becomes two 1N haploid cell So overall process is 1X4NII4X1N 4 A full complement of chromosomes is restored when a sperm and egg come together in fertilization to from a new 2N animal N N 2N lnterphase 2N4N and Meiosis 1 4N2N Cell enters interphase and duplicates in S phase just like mitosis ln mitosis the pairs of chromatids line up on the metaphase plate and separate from each other during anaphase ln prophase of meiosis 1 each of the homologous chromosome forms a with its sister homolog by binding along the full length 0 This process of pairing during prophase is called o Bivalents are held together by a structure of proteins called the synaptonemal complex 0 ln mitosis the kinetochores of the two chromatids attached to microtubules face opposite directions and are pulled away but in meiosis the two kinetochores of the paternal chromatids are fused to form a single functional structure and both face the same direction The two maternal chromatids do the same and both face the other directions This fusing of two bivalents which consists of 4 chromatids total 2 paternal and 2 maternal is called a 0 Because of this when anaphase of meiosis 1 begins sister chromatids stay together and the and move to the opposite poles Meiosis 2 o In meiosis 2 the chromatid pairs in the daughter cells line up in the metaphase plate and this time anaphase separates the chromatids form each other The daughter cells born out of this event are now haploid o Meiosis 2 is similar to mitosis Crossing Over in Prophase 1 0 When the homologs pair up with each other a chromatid from one homolog will cross over to pair with one of the chromatids of the other homolog This crossing over forms a plural chiasmata 0 At these chiasmata certain enzymes cut the DNA in the adjacent maternal and paternal chromatids and then ligate the maternal DNA molecule with the paternal one This is called in short the cutting and joining of chromatid DNA molecules o It is not uncommon for crossing over to occur more than once along the length of a pair of nonsister chromatids in a bivalent on average 3 times 0 These recombinant chromatid pairs then migrate to the opposite poles in anaphase Meiosis and Genetics Meiosis creates gametes that have a mix of chromosome from each of the two parents o The tetrads are lined up randomly during metaphase 0 Different arrangements cause different gametes o A diploid organism can produce 2n different combinations of maternal and paternal chromosomes nhaploid number 0 Each chromosome in the gamete has new combinations of genes not found on either of the parents chromosomes Life Cycles 0 In animals and most plants the diploid phase is long and complex and the haploid state is simple and short In many algae however the fertilized egg is the only diploid cell and it promptly undergoes meiosis The haploid cells go on to form a multicellular adult In both ferns and mosses the haploid state is multicellular o In ferns the diploid stage is more prominent o In mosses the haploid stage in more prominent Advantages of Meiosis Bacteria and many singlecelled eukaryotic organisms reproduce asexually by cell division mitosis Hydra multicellular animal can reproduce by budding Sea anemones and marine worms can split into 2 half organisms that then regenerates the other half 1978 John Maynard Smith pointed out that the existence of sexual reproduction presents a paradox Asexually reproducing organisms can produce twice as many grand offspring as individuals that reproduce sexually because any sex can make babies wheras in sexual reproduction only females can Darwinian natural selection production of more offspring over time is the basic measure of success would expect that natural selection would have eliminated sexual reproduction then if there was no other selective advantage of meiosis The true selective advantage of meiosis is the large that it creates o If a pathogen or parasite were to invade a genetically diverse population there will likely be at least a few individuals with genetic resistance These resistant individuals then live to reproduce and create a large number of offspring that are resistant to that pathogenparasite o In the same situation asexually reproducing populations will be all genetically the same and are at risk of being completely killed off by the pathogenparasite 0 Because of this sexually reproduced offspring are favored when parasitism rates are high Mistakes in Meiosis Most common error an entire tetrad in Meiosis 1 or pair of chromatids in Meiosis 2 fails to separate from each other and both go to the same pole This is called o Nondisjunction produces gametes that are either missing a chromosome or that have an extra copy of one of the chromosomes o If a normal sperm fertilizes an egg with two copies of a chromosome the result is a zygote with 3 copies of that chromosome called 0 Usually it is lethal for a zygote to be missing one chromosome or have an extra copy but not always 0 Trisomy 21 Down s syndrome 3 copies of chromosome 21 Frequency of Down s syndrome increases with mother s age Might be the result of the maintenance of her eggs at such an early stage immature eggs arrest in early prophase 1 of meiosis for such a long time
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