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Identifying DNA as the Hereditary Material. Initially. People thought that proteins were the hereditary material. 1920’s Frederick Griffith . A microbiologist who studied the pathology of Streptococcus pneumoniae Used two strains of the bacterium
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Initially... People thought that proteins were the hereditary material...
1920’s Frederick Griffith • A microbiologist who studied the pathology of Streptococcus pneumoniae • Used two strains of the bacterium • S-Strain – highly pathogenic but can be made non-pathogenic by heating it
1920’s Frederick Griffith • Used two strains of the bacterium • R-Strain – non-pathogenic What does ‘pathogenic’ mean?
1920’s Frederick Griffith • Discovered that mice died after being injected with a mixture of heat-killed S-strain and living R-strain bacteria
1920’s Frederick Griffith • He called this the transforming principle – because something from the heat-killed pathogenic bacteria must have transformed the living non-pathogenic bacteria to make them deadly
1944 Avery, MacLeod & McCarty • Oswald Avery, Colin MacLeod (both Canadian!) and Maclyn McCarty • Continued with Griffith’s work • Wanted to identify what IS the transforming principle
1944 Avery, MacLeod & McCarty • Prepared identical extracts of the heat-killed S-strain and added different enzymes to each extract • One enzyme destroyed RNA, one destroyed DNA and one destroyed proteins
1944 Avery, MacLeod & McCarty • Each extract/enzyme mixture was mixed with live R-strain cells RESULT? • The only extract that did not cause transformation was the one that was treated with the DNA-destroying enzyme
1944 Avery, MacLeod & McCarty RESULT? • Therefore the transforming principle is DNA
1952 Alfred Hershey & Martha Chase • Designed one of the most famous experiments in the history of genetics • Used bacteriophages (viruses that infect bacteria)
1952 Alfred Hershey & Martha Chase • Have an inner nucleic acid core and an outer protein coat, called a capsid Capsid DNA
1952 Alfred Hershey & Martha Chase • Their experiment was designed to find out which part of the virus – the DNA or the protein in the capsid – enters bacterial cells and directs the production of more viruses
Used radioactive isotopes to trace each type of molecule • Radioactive sulfur (35-S) – since proteins contain sulfur and DNA does not • Radioactive phosphorus (32-P) – since DNA has phosphorus and proteins do not
Result? Bacteria exposed to P-32 showed radioactivity, while those exposed to S-35 did not Provided conclusive evidence that viral DNA was transferred to bacterial cells and that viral DNA held the info needed for the viruses to reproduce
1869 Friedrich Miescher • Isolated the nuclei of WBC that he obtained from pus • Extracted a weakly acid substance containing nitrogen & phosphorus
1869 Friedrich Miescher • Called it nuclein (since it was found in the nucleus) • Renamed nucleic acid once the chemical composition was discovered
Early 1900’s Phoebus Levene • Isolated two types of nucleic acids (RNA and DNA) • After many years of experiments, Levene proposed that DNA and RNA are made up of individual units that he called nucleotides
Nucleotides have 3 main parts: • nitrogen-containing base • 5-carbon sugar • Phosphate group
4 types of nitrogenous bases – A, C, T and G Double ringed purines (A & G) Single ring pyrimidines (C & T)
1949 Erwin Chargaff • Studied the chemistry of nucleic acids • Came to two significant conclusions: • There is variation in the composition of nucleotides among different species
1949 Erwin Chargaff Came to two significant conclusions: • Nucleotides are present in characteristic proportions Pg 209
Chargaff’s Rule • In DNA the % composition of adenine is equal to thymine • % composition of guanine is equal to cytosine
1951 Linus Pauling • Discovered that many proteins have helix-shaped structures
1953 Rosalind Franklin & Maurice Wilkins • used x-ray diffraction analysis of DNA to determine its structure • Franklin was able to conclude that DNA has a defined helical structure
1953 Rosalind Franklin & Maurice Wilkins • Franklin determined DNA has two repeating patterns at intervals of 0.34 nm and 3.4 nm
1953 Rosalind Franklin & Maurice Wilkins • Franklin observed how DNA reacted with water and concluded that nitrogenous bases were located on the inside of the helical structure, and the sugar-phosphate backbone was located on the outside
1953 James Watson & Francis Crick • Used the results, conclusions and conjectures of their peers and produced a model for DNA • In 1962 they were awarded a Nobel Prize (along with Wilkins)
