Week 1 – 8/23-8/27Proof of Recombination Between Homologous ChromosomesMechanism for variation in linking is called crossing over – occurs at the start of meiosis through the process of synapsis(homologous chromosomes form pairs with their long axes parallel) in which tension resulting from the coilingof four chromatids about each other results in chromatids breaking at corresponding places and rejoining crossways or normally. This figure demonstrates the physical exchanges b/w homologous chromosomes. Homologous chromosomes have identical shapes, but occasionallythey are different(which is what is happening in the figure). The homologous c,w x progeny had to arise by crossing over b/w the C and w x loci. When such c, w xoffspring were cytologically examined, knob chromosomes were seen, showing that a knobless W x region had been physically replaced by a knobbed w x region. Recombination Frequency Measures the Distance between Linked Genes-likelihood of a recombination event b/w linked genes increases with distance-distances b/w genes are given as genetic map units: 10% recombination rate = 10 map units.-does not tell you anything about physical distance, only genetic distance(how often recombination occurs b/w two linked genes)-the 30 does not = 10+25, because of the existence of multiple crossovers, meaning there is a possibility of double crossovers occurring.-Genes on the far ends of a long chromosome will assort together at least 50% of the time because if an odd number of crossovers occurs between them, they will be separated – but if an even number occurs, they will end up together(called refactor crossing). -number of linkage groups = number of cytologically visible chromosomes. Each chromosome carries multiple genes, and genes on the same chromosome belong to the same linkage group. Gene Function can change via mutation-spontaneous mutation: necessarily rare mutations can occur in genes to give rise to altered forms. There is a strong advantage in there being a small but finite mutation rate; it provides a constant source of new variability, necessary for adaptation. -ex. the white-eyed fly in the bottle -induced mutation: human caused mutations-ex. First type was caused by muller & stadler who studied the effect of X-ray radiation on a fly’s morphologies and phenotypes-Gene size can be estimated by X-ray induced mutation rate, because the bigger a gene, the more likely its chromosome is to be hit by an X-ray. What is a gene made of?-The chemical composition of a gene is a chromosome, which is made up of nucleic acidsand protein. Since a gene must be self-replicating, and DNA is the self-replicating molecule, DNA must be the one to carry genetic info. Genes Control Enzyme Function• 1902, Archibald Garrod proposed that alkaptonuria (failure to metabolize tyrosine) is caused by a rare Mendelian factor: the wild-type gene is responsible for the presence of a particular enzyme, and in a homozygous mutant, the enzyme is congenitally absent. This general hypothesis of a gene-enzyme relationship wasextended by the following works. • Muriel Wheldale & Rose Scott-Moncrief studied plant anthocyanins synthesis, Fritz von Wettstein studied eye color in butterfly, Boris Ephrussi & George Beadle studied eye color in Drosophila: Came to the conclusion that a particular gene affected a particular step in the formation of the respective pigment whose absence changed the color of a particular structure.• 1941, George Beadle & Edward Tatum described the tryptophan biosynthetic pathway, and showed that each step was controlled by a different gene: The One Gene – One Enzyme Hypothesis-Hypothesizes that: • Genes work by controlling the synthesis of specific enzymes.• All biochemical processes in all living organisms are under genetic control.• All biochemical reactions in an organism are resolvable into separate steps.• Each step or reaction is under the control of a single gene.Mutation of a single gene results in the loss of function of the appropriate enzyme. (if youlose a gene, you lose a protein and the function of the enzyme)Griffith’s Transforming PrincipleDNA was thought to be a scaffold to support proteins(which represent the gene) in place. By the start of1928 though, there was evidence of it doing more than that. Dr. Griffith was studying how to prevent pneumonia(antibiotics hadn’t been invented) – found that streptococcus penuomoniae had two varieties: a virulent (smooth shaped colonies, kill people) strain that killed micewhen injected, and a non-virulent (rough colonies, don’t kill) strain that doesn’t do anything.. He heated the S strain to kill it, and injected mice - they were not harmed at all. However, Griffith found that by mixing live R strain and dead S strain, if you inject it into a mouse, it would die. Looking at its blood you would find S strain. R was converted to S. This was called the “Transforming Principle”. Even though bacteria cells are tiny, the change from R to S strain is a change in genetic information because R would propagate as R and vice versa, so R must have been converted to S.Transformation is the Transfer of Genetic Material Into Bacteria:- In the S strain, there is a gene called capS that makes the smooth coat that allows pneumonia bacteria to infect the host. DNA is released when heated, and the DNA gets incorporated into R-strain cell, and converts it into an S-cells. Transformation happens to MANY bacterial strains. Transforming material was Associated with DNA. Isolation of a chemically pure transforming agent- Heat-killed “S” strains werefractionated intotheir chemical constituents(tried to purify the protein, DNA, and RNA), and each fraction was tested for the ability to transform “R” strains into “S” strains. Took ten years to get clean fractions from the purification. Turned out that it was only the DNA fraction that could transform things. At the time, everyone was in love with protein though, so nobody believed them. They claimed that the DNA fraction was contaminated with protein. So they went further:-They used proteases, ribonucleases, and deoxyribonucleases. To prove their point, Averyand MacLeod and McCarty used these enzymes to attempt to eliminate the transformation. They destroyed protein, but it still worked. Destroyed RNA, still worked. Destroyed DNA, and transformation failed – further proof.Life Cycle of Lytic Bacteriophage-Phages are bacterial viruses, composed of