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Lecture 3

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Queen's University
BIOL 205

Chapter 7: DNA Structure and Replication DNA: THE GENETIC MATERIAL • To fulfill its role, the genetic material must meet several criteria – 1. Information: It must contain the information necessary to make an entire organism – 2. Transmission: It must be passed from parent to offspring – 3. Replication: It must be copied • In order to be passed from parent to offspring – 4. Variation: It must be capable of changes • To account for the known phenotypic variation in each species DNA: THE GENETIC MATERIAL • The data of many geneticists, including Mendel, were consistent with these four properties – However, the chemical nature of the genetic material cannot be identified solely by genetic crosses • Indeed, the identification of DNA as the genetic material involved a series of outstanding experimental approaches – These will be examined next Frederick Griffith 1928: Experiments with Streptococcus pneumoniae • Griffith studied a bacterium Streptococcus pneumoniae –causes pneumonia in humans- normally lethal in mice • S. pneumoniae comes in two strains – Type S  Smooth • Secrete a polysaccharide capsule – Protects bacterium from the immune system of animals- LETHAL Produce smooth colonies on solid media – Type R  Rough • Unable to secrete a capsule • Produce colonies with a rough appearance • NOT LETHAL Transforming R cells into S cells Figure 7-2 • Griffith concluded that something from the dead type S was transforming type R into type S – He called this process transformation • The substance that allowed this to happen was termed the transformation principle – Griffith did not know what it was • The nature of the transforming principle was determined using experimental approaches that incorporated various biochemical techniques Avery, MacLeod and McCarty, 1944 Halifax Port Hastings Nova Scotia • Avery, MacLeod and McCarty realized that Griffith’s observations could be used to identify the genetic material – At that time, it was known that DNA, RNA, proteins and carbohydrates are major constituents of living cells • They prepared cell extracts from type S cells containing each of these macromolecules DNA is the transforming agent Page 268 The Nobel laureate Arne Tiselius said that Avery was the most deserving scientist not to receive the Nobel Prize for his work. Hershey and Chase Experiment with Bacteriophage T2 • In 1952, Alfred Hershey (Nobel prize -1969) and Martha Chase provided further evidence that DNA is the genetic material • They studied the Inside the bacteriophage T2 capsid – It is relatively simple since its composed of only two Made up macromolecules of protein • DNA and protein The phage genetic material is DNA Figure 7-4 Ultracentrifuge >100,000 X g 10-12 Xg Centrifuges Pellet Supernatant Pellet In salt solution (Cesium chloride) Can separate molecules by their Density (size, dimensions) • The Hershey and Chase experiment can be summarized as such: – Used radioisotopes to distinguish DNA from proteins • 32P labels DNA specifically • 35S labels protein specifically – Radioactively-labeled phages were used to infect nonradioactive Escherichia coli cells – After allowing sufficient time for infection to proceed, the residual phage particles were sheared off the cells • => Phage ghosts and E. coli cells were separated – Radioactivity was monitored using a scintillation counter The first model of DNA - 1953 James Watson Francis Crick Maurice Wilkens Rosalind Franklin’s critical experimental result BBC news October 12, 2010 Newly-found letters of the scientists who discovered the structure of DNA highlight tensions around this major scientific breakthrough "To think that Rosie had all the 3D data for nine months and wouldn't fit a helix to it, and there was I taking her word for it that the data was anti-helical," wrote Dr Wilkins to Dr Crick. "Christ." Just before Dr Franklin was to leave King's College, Dr Wilkins wrote to the Cambridge scientists that "the smoke of witchcraft will soon be getting out of our eyes". "…cheer up, and take it from us that even if we kicked you in the pants, it was between friends," the pair wrote in response. "We hope our little burglary will at least produce a united front in your group!" DNA STRUCTURE • DNA is a large macromolecule with several levels of complexity – 1. Nucleotides form the repeating units • Nucleotides composed of phosphate group, deoxyribose sugar and any one of four bases A, G, C, T – 2. Nucleotides are linked to form a strand – 3. Two strands can interact to form a double helix – 4. The double helix folds, bends and interacts with proteins resulting in 3-D structures in the form of chromosomes Structures of the four DNA nucleotides Phosphomonoester bond Figure 7-5 Glycosidic bond Pyramids are never found in the CiTy Major difference between DNA and RNA Chargaff’s rules of base composition: 1.[T+C]=[A+G] 2.[T]=[A]; [C]=[G] 3.[A+T]does not always equal [G+C] Chargaff’s rule helped solve the structure of DNA The structure of DNA Figure 7-8 5’ 3’ Flat planar Structure Stack on top
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