Rebecca Davenport BIOL 1406 11 14 16 Ch 13 blueprint 1 Meiosis i Meiosis reduces the number of chromosomes by half 2n to n ii Form of sexual reproduction iii Egg Sperm undergoes Fertilization to form a Zygote Which undergoes mitosis to form multicellular Germ cells circle back to gametes undergoing meiosis iv Only diploid cells can undergo meiosis v Preceded by the replication of chromosomes vi Meiosis takes place in two consecutive cell divisions called meiosis I and meiosis II vii Two rounds of Meiosis results in four haploid daughter cells viii Genetically unique daughter cells ix 1 Two phases of Meiosis a Meiosis 1 separates homologous mom set and dad set chromosomes i Prophase I 1 Each chromosome pairs with its homolog and crossing over occurs sections of each chromatin swap 2 X shaped regions called chiasmata are sites of crossover ii Metaphase I 1 Pairs of homologs line up at the metaphase plate with one chromosome facing each pole 2 Microtubules are attached to the kinetochore iii Anaphase I 1 Pairs of homologous chromosomes separate and move to opposite poles 2 Sister chromatids remain attached at the centromere and move as one unit toward the pole iv Telophase I and Cytokinesis 1 Nuclear envelope reforms spindle dissipates 2 Each half of the cell has a haploid set of chromosomes each chromosome still consists of two sister chromatids 3 Cytokinesis usually occurs simultaneously forming two haploid daughter cells v b Meiosis 2 separates sister chromatids Mom has sister chromatids and dad sister chromatids when one of each is paired they are called non sister chromatids i Prophase II 1 In prophase II a spindle apparatus forms 2 In late prophase II chromosomes each still composed of two chromatids move toward the metaphase plate ii Metaphase II 1 Sister chromatids arranged at the metaphase plate 2 Sister chromatids of each chromosome are no longer genetically identical due to crossing over Meiosis I 3 Kinetochores of sister chromatids attach to microtubules extending from opposite poles iii Anaphase II 1 Sister chromatids separate 2 Sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles iv Telophase II and Cytokinesis 1 Chromosomes arrive at opposite poles 2 Nuclei form and the chromosomes begin decondensing 3 At the end of meiosis there are four daughter cells each with a haploid set of unreplicated chromosomes 4 Each daughter cell is genetically distinct from the others and from the parent cell c x Crossing over and Synapsis during Prophase I 1 Sister chromatids held together by cohesins 2 Nonsister chromatids are broken at corresponding positions 3 A zipper like structure the synaptonemal complex holds the homologs together 4 DNA breaks are repaired joining DNA from one nonsister chromatid to the corresponding segment of another 5 Crossover Chiasmata 6 b Inheritance of genes i Heredity is the transmission of traits from one generation to the next ii Genetics is the scientific study of heredity and variation iii Genes are the units of heredity and are made up of segments of DNA iv Genes are passed to the next generation via reproductive cells called gametes sperm and eggs v A gene s specific position along a chromosome is called the locus c Comparison of Asexual and Sexual reproduction i Asexual reproduction a single individual passes all of its genes to its offspring without the fusion of gametes produces a clone ii Sexual reproduction two parents give rise to offspring that have unique combinations of genes inherited from the two parents d Sets of chromosomes in human cells i A karyotype is an ordered display of the pairs of chromosomes from a cell ii The two chromosomes in each pair are called homologous chromosomes iii Number of chromosomes in one set is n 1 Haploid n 2 Diploid 2n iv Duplicating chromosomes 1 e Origins of genetic variation among offspring i The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation ii Three mechanisms contribute to genetic variation1 Independent assortment of chromosomes a Homologous pairs of chromosomes orient randomly at metaphase 1 of meiosis b In independent assortment each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs c 2 Crossing over a Crossing over produces recombinant chromosomes which combine DNA inherited from each parent b Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome c In humans an average of one to three crossover events occurs per chromosome f d 3 Random fertilization a Random fertilization adds to genetic variation because any sperm can fuse with any ovum unfertilized egg b The fusion of two gametes each with 8 4 million possible chromosome combinations from independent assortment produces a zygote with any of about 70 trillion diploid combinations Fertilization and zygotes i Fertilization is the union of gametes the sperm and the egg ii The fertilized egg is called a zygote and has one set of chromosomes from each parent iii The zygote produces somatic cells by mitosis and develops into an adult iv Meiosis of germ cells results in one set of chromosomes in each gamete v n Gametes n n MEIOSIS 2n FERTILIZATION Zygote 2n Mitosis Diploid multicellular organism Haploid n Diploid 2n 2 Comparison of Mitosis and Meiosis a Mitosis conserves the number of chromosome sets producing cells that are genetically identical to the parent cell b Meiosis reduces the number of chromosomes sets from two diploid to one haploid producing cells that differ genetically from each other and from the parent cell c d
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