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Historical Events DNA Structure & Replication 1869 Bio 12 A. Allen C.P. Allen High School Friedrich Miescher identified DNA, which he called nuclein, from pus cells 1889 Richard Altman renamed nuclein nucleic acid 1931 Griffith discovered that genetic information could be passed from one bacterium to another; known as the transforming principle 1944 Avery showed that the transforming material was pure DNA not protein, lipid or carbohydrate. 1952 Hershey and Chase used bacteriophage (virus) and E. coli to show that only viral DNA entered the host 1953 Watson and Crick discovered the structure of DNA was a double helix 1 3 Walter Sutton-1902 Friedrich Miescher-1869 • observed that chromosomes obey Mendel's rules of inheritance • Suggested that “genes” are located on chromosomes. • Noticed that “genes” are inherited in the same fashion as chromosomes. • Swiss chemist • Isolated the nuclei from pus cells. • Discovered a chemical that didn’t act like protein. He named it “nuclein” • Many chemists continued his work • Found that nuclein was rich in phosphorus and contained no sulfur (unlike proteins) • Found that nuclein contained an acidic substance they termed “nucleic acid” • Found 2 types of nucleic acids: DNA & RNA Thomas Hunt Morgan-1910 4 • Stated that each “gene” had a locus on a particular chromosome. • Used Drosophila melanogaster (fruit fly) for his studies. 5 BIG questions remained… Hit the PAUSE button… • At this point, the idea of “genes” was an accepted notion. It was a very abstract idea: gene = trait that can be passed on. • DNA had been discovered. Chromosomes had been discovered. • Nobody had yet made the connection between genes & DNA. 6 • What IS the genetic material? What molecular substance is a “gene” made of? • Scientists agreed that—no matter what substance genes were made of—this substance must be: 1. able to store information 2. stable so that it can be copied and passed on 3. able to undergo rare changes called mutations in order for evolution to occur 7 Frederick Griffith-1931 Joachim Hammerling 1930's. • Giffith was trying to make a vaccine to prevent pneumonia infections in the "Spanish flu" influenza pandemic by using two strains of the Streptococcus pneumoniae bacterium. • The smooth strain (S strain) had a polysaccharide capsule and was virulent . • The rough strain (R strain) had a no polysaccharide capsule and was avirulent • Hammerling’s experiments with Acetabularia suggested that hereditary information was localized to nuclei. The capsule is a slimy layer on the cells' surface that allows the bacteria to resist the human immune system. The rough strain (R strain) did not cause pneumonia when injected into mice (it was avirulent), since it lacked a capsule. Somehow the type rough strain had been transformed into smooth strain, a process 9 Grififth christened the transforming principle • 8 • So Avery had proven that the genetic material was DNA, right? Oswald Avery & associates-1944 • Griffith’s experiment led Avery to design his own experiments to identify the “transforming agent.” • Avery’s evidence showed: 1. DNA from S strain bacteria caused R strain bacteria to be transformed. 2. Enzymes that degrade proteins and RNA did not prevent transformation. 3. Enzymes that digest DNA did prevent transformation. 4. The DNA segment that transformed the bacteria contained about 1600 nucleotides—enough for genetic variability to be possible. • Nope. Some folks still didn’t buy it. • The skeptics replied that maybe DNA was just used to activate proteinbased genes. • In the 1950s, bacteriophages were beginning to be used as scientific tools. • Scientists of the day were still asking: What does the virus inject into the bacteria? That’s where we’ll find the answer to our question: • Is protein or DNA the genetic material?? 10 11 Alfred Hershey/Martha Chase-1952 Hershey & Chase experiment Hershey & Chase’s conclusion? Note: DNA contains phosphorus but no sulphur Protein contains sulphur but no phosphorus 12 DNA—not protein—is the genetic material! DNA transmits all the genetic information needed to produce new viruses. 13 Now Put it all together! Case Closed! Well, not quite… • After almost 100 years, scientists had finally proven that DNA is the material that genes are made of. • But they only had a foggy idea of what DNA really was. • What was its structure? Scientist/Date Key Discovery. (if applicable, what previous works were used?) Miescher-1869 Found that DNA was rich in phosphorus and contained no sulfur (unlike proteins) Hammerling 1930s Genetic information is in the nucleus Griffith 1928 Genetic material could be passed from bacterium to another (transforming principle) Avery 1944 showed that the transforming material was pure DNA not protein. (Used Griffith’s work on the transforming principle) Hershey & Chase 1952 DNA—not protein—is the genetic material! (Used Meischer’s work –Phosphorous in DNA, not Sufur) 14 Since Friedrich Mieschner discovered “nuclein” in 1869, chemists had been working to figure out its chemical composition Chemists knew DNA was a polymer made of nucleotide monomers. Nucleotides are classified into 2 groups based on structure: 1. Purines contain these bases… – Adenine – Guanine They also knew that each nucleotide consisted of: 1.a phosphate group 2. a 5-carbon sugar 3.One of 4 nitrogencontaining bases 2. Pyrimidines = 5 carbon sugar 17 Purine Nucleotides contain these bases… – Cytosine – Thymine (DNA only) – Uracil (RNA only) 18 Pyrimidines Nucleotides Study tip: small name = BIG BASE STRUCTURE (2 RINGS) 19 Study tip: BIG NAME = small base structure (one ring) 20 Erwin Chargaff-1940s • • • Stop & Think Remember: each nucleotide has one of four possible bases, A, T, C & G Chargaff analyzed DNA from various species. Conclusions of his experiments came to be known as “Chargaff’s Rules” Suppose you were Chargaff who observed the ratios of A, T, C & G we just discussed. Use deductive reasoning to determine how the bases pair up. 1. What can you say about the relative amounts of A, T, C & G in different species 2. What can you say about the relative amounts of A, T, C & G in a given species 21 23 Rosalind Franklin-1953 Chargaff’s Rules • DNA from any cell of all organisms should have a 1:1 ratio of pyrimidine and purine bases. • The amount of guanine is equal to cytosine & the amount of adenine is equal to thymine. • An expert in X-ray crystallography. Used this technique to discover the physical shape of DNA! • This was the first indication that DNA was composed of a double helix structure which had a constant diameter of 2nm. • Died at age of 37 cancer 26 Watson & Crick’s model of DNA structure James Watson & Francis Crick-1953 • The rules of complementary base pairing (or nucleotide pairing) are: • Constructed a model for DNA structure • Awarded the Nobel Prize in 1962 • Rosalind Franklin’s work was essential to the Watson and Crick model. She died of cancer due to overexposure to x-rays. Her lab partner, Maurice Wilkins was also awarded the Nobel Prize along with Watson & Crick because of his contributions to Ms. Franklin’s work. • Purines are always paired with pyrimidines but only in certain combinations… • A with T: the purine adenine (A) always pairs with the pyrimidine thymine (T) • C with G: the purine guanine (G) always pairs with the pyrimidine cytosine (C) 27 28 Watson & Crick’s model of DNA structure Stop & Think So why can`t… • The 2 strands of DNA are held together by the hydrogen bonds between the complimentary bases. A (purine) pair with C (pyrimidine) or G (purine) pair with T (pyrimidine)? 29 Stop & Think 30 What`s this? Why can`t … A (purine) pair with G (purine) or C (pyrimidine) pair with T (pyrimidine)? 32 35 DNA Packaging DNA Structure • DNA – Double Helix • Histones – Proteins • Nucleosome – DNA coils around histones. • Coils • Supercoils • Chromosome 37 38 Strands of DNA are Antiparallel Linked Nucleotides in DNA • • • • Nucleotides are linked by phosphodiester bonds. Hydroxyl groups from the 3rd carbon and a phosphate group (off 5th carbon) of two nucleotides react to form water. A phosphodiester bond5 is a type of condensation reaction • That wraps up one mystery! The two strands in a DNA molecule run antiparallel to each other (the two strands have opposite orientations; the 5' end of one strand aligns with the 3' end of the other strand. 3’ and 5’ pertain to the 3rd and 5th carbons in the deoxyribose molecules. Now we know “what”… but how? • What is the genetic material? –DNA! • How is it passed on to the next generation? How is it copied? But in science, new information almost always raises new questions… 41 42