plant meiosis. animals vs. plants plant reproductionanimal reproduction life cycle alternation of...

Download Plant Meiosis. Animals vs. Plants Plant ReproductionAnimal Reproduction Life cycle Alternation of generations No alternation of generations GametesHaploid

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  • Plant Meiosis
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  • Animals vs. Plants Plant ReproductionAnimal Reproduction Life cycle Alternation of generations No alternation of generations GametesHaploid gametes SporesHaploid sporesNo spores Gametes made by Haploid gametophyte, by mitosis Diploid organism, by meiosis Spores made by Diploid sporophyte, by meiosis No spores
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  • Alternation of Generations Plants have a double life cycle with two distinct forms: Sporophyte: diploid, produce haploid cells by meiosis. Gametophyte: haploid, produce gametes by mitosis.
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  • Non-flowering plants Mosses, ferns, and related plants have motile, swimming sperm.
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  • Moss Life Cycle
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  • Fern Life Cycle
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  • Flower Higher Plant
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  • Angiosperm Life Cycle
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  • Gametogenesis: Male
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  • G ametogenesis: Female
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  • Double Fertilization
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  • Flower to Fruit
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  • Ovule to Seed
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  • Unique events in Meiosis Homologous ( matching ) chromosomes pair up before 1 st cell division Homologous chromosomes: -look alike -code for same traits -receive one from each parent
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  • During 1 st division, homologous chromosomes exchange genes during process called crossing over Unique events in Meiosis
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  • These homologous chromosomes separate during 2 nd division of meiosis so chromosomes in gametes are different from each other due to crossing over Crossing over increases genetic variation and is the reason why siblings look different Unique events in Meiosis
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  • Crossing Over Sometimes in meiosis, homologous chromosomes exchange parts in a process called crossing-over, or recombination.
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  • Process of Recombination Genes are on chromosomes. Meiosis is a mechanism for re-shuffling the chromosomes: each gamete gets a mixture of paternal and maternal chromosomes. However, chromosomes are long and contain many genes. To get individual genes re- shuffled, there needs to be a mechanism of recombining genes that are on the same chromosome. This mechanism is called crossing over.
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  • More Recombination Crossing over occurs in prophase of meiosis 1, when the homologous chromosomes synapse, which means to pair closely with each other. DNA strands from the two chromosomes are matched with each other. During synapsis, an enzyme, recombinase, attaches to each chromosome at several randomly chosen points. The recombinase breaks both DNA molecules at the same point, and re-attaches them to opposite partners. The result of crossing over can be seen in the microscope as prophase continues, as X-shaped structures linking the homologues. The genetic consequence of crossing over is that each chromosome that goes into a gamete is a combination of maternal and paternal chromosomes.
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  • Recombination Process
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  • Linkage. Linkage occurs when two genes are close to each other on the same chromosome. Two genes are syntenic, when they are on the same chromosome. Linked genes are syntenic, but syntenic genes are not always linked. Genes far apart on the same chromosome assort independently: they are not linked. Linkage is based on the frequency of crossing over between the two genes. Crossing over occurs in prophase of meiosis I, where homologous chromosomes break at identical locations and rejoin with each other. The failure of two genes to assort independently
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  • Discovery of Linkage In 1900, Mendels work was re-discovered, and scientists were testing his theories with as many different genes and organisms as possible. William Bateson and R.C. Punnett were working with several traits in sweet peas, notably a gene for purple (P) vs. red (p) flowers, and a gene for long pollen grains (L) vs. round pollen grains (l).
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  • Bateson and Punnetts Results PP LL x pp ll selfed F1: Pp Ll F2 results in table Very significant deviation from expected Mendelian ratio: chi- square = 97.4, with 3 d.f. Critical chi square value = 7.815. The null hypothesis for chi square test with 2 genes is that the genes assort independently. These genes do not assort independently. phenot ype obsexp ratio exp num P_ L_2849/16215 P_ ll213/1671 pp L_213/1671 pp ll551/1624
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  • Linkage Mapping Each gene is found at a fixed position on a particular chromosome. Making a map of their locations allows us to identify and study them better. In modern times, we can use the locations to clone the genes so we can better understand what they do and why they cause genetic diseases when mutated. The basis of linkage mapping is that since crossing over occurs at random locations, the closer two genes are to each other, the less likely it is that a crossover will occur between them. Thus, the percentage of gametes that had a crossover between two genes is a measure of how far apart those two genes are. As pointed out by T. H. Morgan and Alfred Sturtevant, who produced the first Drosophila gene map in 1913. Morgan was the founder of Drosophila genetics, and in his honor a recombination map unit is called a centiMorgan (cM). A map unit, or centiMorgan, is equal to crossing over between 2 genes in 1% of the gametes.
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  • Gene Mapping Gene mapping determines the order of genes and the relative distances between them in map units 1 map unit = 1 cM (centimorgan) In double heterozyote: Cis configuration = mutant alleles of both genes are on the same chromosome = ab/AB Trans configuration = mutant alleles are on different homologues of the same chromosome = Ab/aB
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  • Gene Mapping Gene mapping methods use recombination frequencies between alleles in order to determine the relative distances between them Recombination frequencies between genes are inversely proportional to their distance apart Distance measurement: 1 map unit = 1 percent recombination (true for short distances)
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  • Recombination
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  • 29 Gene Mapping Genes with recombination frequencies less than 50 percent are on the same chromosome = linked) Linkage group = all known genes on a chromosome Two genes that undergo independent assortment have recombination frequency of 50 percent and are located on nonhomologous chromosomes or far apart on the same chromosome = unlinked Recombination
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  • Recombination between linked genes occurs at the same frequency whether alleles are in cis or trans configuration Recombination frequency is specific for a particular pair of genes Recombination frequency increases with increasing distances between genes No matter how far apart two genes may be, the maximum frequency of recombination between any two genes is 50 percent. Recombination
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  • 31 Gene Mapping Recombination results from crossing-over between linked alleles. Recombination changes the allelic arrangement on homologous chromosomes Recombination
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  • Genetic Mapping The map distance (cM) between two genes equals one half the average number of crossovers in that region per meiotic cell The recombination frequency between two genes indicates how much recombination is actually observed in a particular experiment; it is a measure of recombination Over an interval so short that multiple crossovers are precluded (~ 10 percent recombination or less), the map distance equals the recombination frequency because all crossovers result in recombinant gametes. Genetic map = linkage map = chromosome map Recombination
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  • Gene Mapping: Crossing Over Two exchanges taking place between genes, and both involving the same pair of chromatids, result in a nonrecombinant chromosomes
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  • Gene Mapping: Crossing Over Crossovers which occur outside the region between two genes will not alter their arrangement The result of double crossovers between two genes is indistinguishable from independent assortment of the genes Crossovers involving three pairs of alleles specify gene order = linear sequence of genes
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  • 35 Gene Mapping: Crossing Over
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  • Genetic vs. Physical Distance Map distances based on recombination frequencies are not a direct measurement of physical distance along a chromosome Recombination hot spots overestimate physical length Low rates in heterochromatin and centromeres underestimate actual physical length
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  • Genetic vs. Physical Distance
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  • Discovery of Genetic Linkage Classical genetics analyzes the frequency of allele recombination in progeny of genetic crosses New associations of parental alleles are recombinants, produced by genetic recombination. Tests crosses determine which genes are linked, and a linkage map (genetic map) is constructed for each chromosome.
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  • MORGANs EXPERIMENTS Both the white eye gene (w) and a gene for miniature wings (m) are on the X chromosome. Morgan (1911) crossed a female white miniature (w


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