A Comparison of Mitosis and MeiosisA Comparison of Mitosis and MeiosisA Comparison of Mitosis and MeiosisSlide 4Independent Assortment of ChromosomesCrossing OverRandom Fertilization and EvolutionMendelian GeneticsMendel’s Experimental, Quantitative ApproachMendel’s Experimental, Quantitative ApproachMendel’s Experimental, Quantitative ApproachMendel’s ModelMendel’s ModelMendel’s ModelMendel’s ModelSegregation modelUseful Genetic VocabularyThe Molecular Basis of InheritanceEvidence That Viral DNA Can Program CellsEvidence That Viral DNA Can Program CellsEvidence That Viral DNA Can Program CellsDNA replication and repairDNA replicationDNA replicationDNA ReplicationDNA replicationDNA replicationSynthesizing a New DNA StrandAntiparallel ElongationAntiparallel ElongationProofreading and Repairing DNASlide 32Slide 33Slide 34Slide 35A Comparison of Mitosis and Meiosis•Mitosis conserves the number of chromosome sets.producing cells that are genetically identical to the parent cell•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•The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosisA Comparison of Mitosis and MeiosisA Comparison of Mitosis and Meiosis•Three events are unique to meiosis, and all three occur in meiosis l:Synapsis and crossing over in prophase I.Homologous chromosomes physically connect and exchange genetic informationAt the metaphase plate, there are paired homologous chromosomes (tetrads).instead of individual replicated chromosomesAt anaphase I, it is homologous chromosomesinstead of sister chromatids, that separateGenetic variation produced in sexual life cycles contributes to evolution•Mutations (changes in an organism’s DNA) are the original source of genetic diversity•Mutations create different versions of genes called allelesReshuffling of alleles during sexual reproduction produces genetic variation•Origins of Genetic Variation Among OffspringThe behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generationThree mechanisms contribute to genetic variation:Independent assortment of chromosomesCrossing overRandom fertilizationIndependent Assortment of Chromosomes•Homologous pairs of chromosomes orient randomly at metaphase I of meiosis•In independent assortment:Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs•# of combinations possible when chromosomes assort independently into gametes.•2n, where n is the haploid number•For humans (n = 23).•more than 8 million (223) possible combinations of chromosomesCrossing Over•Produces recombinant chromosomes.Combines genes inherited from each parentBegins very early in prophase I, as homologous chromosomes pair up gene by gene•In crossing over, homologous portions of two nonsister chromatids trade placesCrossing over contributes to genetic variation by combining DNA from two parents into a single chromosomeRandom Fertilization and Evolution•Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)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•The Evolutionary Significance of Genetic Variation Within PopulationsNatural selection results in the accumulation of genetic variations favored by the environmentSexual reproduction contributes to the genetic variation in a population, which originates from mutationsMendelian Genetics•What genetic principles account for the passing of traits from parents to offspring?•The “blending” hypothesis.Genetic material from the two parents blends together•The “particulate” hypothesis.Parents pass on discrete heritable units (genes)•Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments•Advantages of pea plants for genetic study:There are many varieties with distinct heritable features, or characters character variants (such as purple or white flowers) are called traitsMating of plants can be controlledEach pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from anotherMendel’s Experimental, Quantitative Approach1. Purple stamens were removed2. Added pollen (sperm carrying) from white flower to eggs of purple flower3. A pollinated carpel matures from this “cross”4. Planted these seeds5. Color of offspring flowers.Mendel’s Experimental, Quantitative Approach•In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization•The true-breeding parents are the P generation•The hybrid offspring of the P generation are called the F1 generation•When F1 individuals self-pollinate, the F2 generation is producedMendel’s Experimental, Quantitative Approach•Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids•Mendel called the purple flower color a dominant trait and the white flower color a recessive trait•Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits•What Mendel called a “heritable factor” is what we now call a geneMendel’s Model•Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspringFour related concepts make up this model•The first concept is that alternative versions of genes account for variations in inherited charactersThese alternative versions of a gene are now called allelesEach gene resides at a specific locus on a specific chromosomeMendel’s Model•The second concept is that for each character an organism inherits two alleles, one from each parentMendel made this deduction without knowing about the role of chromosomes•The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation–Alternatively, the two alleles at a locus may differ, as in the F1 hybridsMendel’s Model•The third concept is