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Discovery of DNA: Questions • On a paper to turn in, and using your notes: • Describe the experiment and the thinking that first indicated that DNA and not protein was the material carrying hereditary information. How do we know what DNA does? Who figured this out? How? - Something from killed, lethal bacteria could - transform living, non- lethal bacteria to lethal - Phage DNA, which contains phosphorus, ended up in bacteria - X-rays of DNA showed the backbone is actually on the outside - In all living things, amounts of A=T and C = G Students will be able to.... AKA I can..... • describe the chemical structure of DNA, and explain how we know the structure. • explain how the coiling of DNA into chromosomes takes place and why this is important. • explain 4 differences between DNA and RNA, and why each is significant. • explain the transcription of RNA from DNA. • describe three types of RNA and explain the role of each in protein synthesis. • explain how mRNA is translated into protein. • trace the synthesis of protein from the transcription of DNA into RNA through the production of a finished protein. What Does DNA Do? Early researchers knew DNA made up chromosomes Doubted that it was hereditary material. How could only four bases code for all sorts of proteins? Researchers, including Linus Pauling, thought protein also found in chromosomes was probably the hereditary factor. The Search for the Genetic Material: Scientific Inquiry • The key factor in determining the genetic material was choosing appropriate experimental organisms • The role of DNA in heredity was first discovered by studying bacteria and the viruses that infect them Friedrich (Fritz) Miescher • • • • 1869 extracted a substance from white blood cells Called it nuclein. What do you think he was actually extracting? Frederick Griffith 1928 experimented on pneumonia bacteria in mice. Used 2 strains of a bacterium one pathogenic one harmless Discovery: something in heat-killed virulent bacteria could be transferred to live, harmless bacteria and make them virulent. Griffith’s Experiments • Mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic (deadly). • Called this phenomenon transformation • genotype and phenotype changed due to assimilation of foreign DNA Fig. 16-3 Phage head Tail sheath Tail fiber Bacterial cell 100 nm DNA Hershey and Chase • 1952 • performed experiments showing that DNA is the genetic material of a phage known as T2 • T2 Phage has 2 components: protein and DNA – One of the components was the genetic material • designed an experiment showing that either DNA or protein enters an E. coli cell during infection Hershey and Chase Protein contains sulfur. DNA contains phosphorous. How can this knowledge help to identify protein or DNA as the carrier of genetic information? Hershey – Chase Experiment Phage Bacterial cell Batch 1: radioactive sulfur (35S) Radioactive protein (35S) DNA Radioactive DNA (32P) Batch 2: radioactive phosphorus (32P) Phage Radioactive protein (35S) Empty protein shell Bacterial cell Batch 1: radioactive sulfur (35S) DNA Radioactive DNA (32P) Batch 2: radioactive phosphorus (32P) Phage DNA Phage Radioactive protein (35S) Empty protein shell Radioactivity (phage protein 35S) in liquid Bacterial cell Batch 1: radioactive sulfur (35S) DNA Phage DNA Centrifuge Pellet (bacterial cells and contents) (35S) Radioactive DNA (32P) Batch 2: radioactive phosphorus (32P) Centrifuge Radioactivity Pellet (phage DNA) in pellet (32P) Hershey and Chase • Concluded that DNA entered the E. coli and caused the changes, so is the genetic material. What we knew so far: • There are 4 nucleic acids • Purines (1 CN ring) – Adenine – Guanine • Pyrimadines – Thymine – Cytosine DNA structure DNA is made up of four bases. RNA also has four bases, but has uracil instead of thymine. Sugar–phosphate backbone 5 end Nitrogenous bases Thymine (T) Adenine (A) Cytosine (C) DNA nucleotide Phosphate Sugar (deoxyribose) 3 end Guanine (G) Erwin Chargaff and his data, 1950 What pattern(s) do you notice? Erwin Chargaff Conclusion: The amount of A = T and amount of C = G in each species, although the amounts differ from species to species. Linus Pauling • His model did not quite work. Why not? Rosalind Franklin Photo 57 Rosalind Franklin Franklin’s X-ray diffraction photograph of DNA Franklin concluded: - two antiparallel sugar-phosphate backbones - nitrogenous bases paired in the molecule’s interior Maurice WIlkins • King’s College, London • Was to collaborate with Franklin, but this didn’t work out. Why not? Watson and Crick Cavendish Laboratory, Cambridge Wilkins consulted with Watson and Crick. Without Franklin’s knowledge, he handed them Franklin’s data. Watson immediately recognized the significance of Photo 57. He and Crick went to work on a model of DNA. • Franklin’s X-ray crystallographic images of DNA were used by Watson to deduce that DNA was a helix • The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases • The width suggested that the DNA molecule was made up of two strands, forming a double helix At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data Fig. 16-UN1 • Watson and Crick reasoned that the pairing was more specific, dictated by the base structures • They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C) • The Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C Instead, pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray Fig. 16-8 Adenine (A) Thymine (T) Guanine (G) Cytosine (C) SO: Many proteins work together in DNA replication and repair • The relationship between structure and function is manifest in the double helix • Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material The First DNA Model DNA structure Across the DNA double-ladder, A always pairs with T, C always pairs with G because of the number of hydrogen bonds the bases form. DNA structure The DNA ladder forms a spiral, or helical, structure, with the two sides held together with hydrogen bonds. 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm (a) Key features (b) Partial of DNA chemical structure structure 5 end (c) Spacefilling model Fig. 16-7a 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm (a) Key features of DNA structure (b) Partial chemical structure 5 end Fig. 16-7b (c) Space-filling model DNA Replication Before cells divide, they must double their DNA so that each cell gets identical copies of the DNA strands. DNA replication helps assure that the bases are copied correctly. Enzymes carry out the process. Overview of DNA replication. • Hydrogen bonds break to “unzip” the DNA strand. • Enzymes guide free nucleotides to the exposed single strands and match the nucleotides. How enzymes carry out the replication process: • DNA Helicase unzips the DNA. DNA Polymerase synthesizes the new strands, using the old strands as templates. DNA Replication Build a DNA Model interactive feature (web) DNA Replication animation (web) Summary DNA is a nucleic acid made up of nucleotides. The order of the nucleotides is important, and is maintained by matching of bases across the DNA ladder (A-T, C-G), and by enzymes that patrol the DNA DNA replication occurs before cell division, and is an orderly, enzyme-driven process.