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60 ch14dn ahistory2008

The document summarizes the key scientific discoveries that established DNA as the genetic material: 1) Experiments in the early 1900s by Morgan, Griffith, and Avery et al. provided early evidence that DNA carries hereditary information and can alter phenotypes. 2) Further experiments in the 1940s-1950s by Chargaff, Hershey and Chase, and Watson and Crick revealed that DNA has a consistent base ratio, is the transforming principle, and forms a double helix structure. 3) The 1958 experiment by Meselson and Stahl, considered "the most beautiful experiment in biology," verified that DNA replication is semi-conservative through labeling DNA with different nitrogen isotopes.

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DNA The Genetic Material AP Biology 2006-2007
Scientific History  The march to understanding that DNA is the genetic material  T.H. Morgan (1908)  Frederick Griffith (1928)  Avery, McCarty & MacLeod (1944)  Erwin Chargaff (1947)  Hershey & Chase (1952)  Watson & Crick (1953)  Meselson & Stahl (1958) AP Biology
1908 | 1933 Chromosomes related to phenotype  T.H. Morgan  working with Drosophila  fruit flies  associated phenotype with specific chromosome  white-eyed male had specific X chromosome AP Biology
1908 | 1933 Genes are on chromosomes  Morgan’s conclusions  genes are on chromosomes  but is it the protein or the DNA of the chromosomes that are the genes?  initially proteins were thought to be genetic material… Why? What’s so impressive about proteins?! AP Biology
1928 The “Transforming Principle”  Frederick Griffith  Streptococcus pneumonia bacteria  was working to find cure for pneumonia  harmless live bacteria (“rough”) mixed with heat-killed pathogenic bacteria (“smooth”) causes fatal disease in mice  a substance passed from dead bacteria to live bacteria to change their phenotype  “Transforming Principle” AP Biology
The “Transforming Principle” mix heat-killed pathogenic & live pathogenic live non-pathogenic heat-killed non-pathogenic strain of bacteria strain of bacteria pathogenic bacteria bacteria A. B. C. D. mice die mice live mice live mice die Transformation = change in phenotype something in heat-killed bacteria could still transmit disease-causing properties AP Biology
1944 DNA is the “Transforming Principle”  Avery, McCarty & MacLeod  purified both DNA & proteins separately from Streptococcus pneumonia bacteria  which will transform non-pathogenic bacteria?  injected protein into bacteria  no effect  injected DNA into bacteria  transformed harmless bacteria into virulent bacteria mice die What’s the AP Biology conclusion?
1944 | ??!! Avery, McCarty & MacLeod  Conclusion  first experimental evidence that DNA was the genetic material Oswald Avery AP Biology Maclyn McCarty Colin MacLeod
1952 | 1969 Confirmation of DNA Hershey  Hershey & Chase  classic “blender” experiment  worked with bacteriophage  viruses that infect bacteria  grew phage viruses in 2 media, Why use radioactively labeled with either Sulfur  35S in their proteins vs. Phosphorus?  32P in their DNA  infected bacteria with labeled phages AP Biology
Protein coat labeled DNA labeled with 32P with 35S Hershey T2 bacteriophages are labeled with & Chase radioactive isotopes S vs. P bacteriophages infect bacterial cells bacterial cells are agitated Which to remove viral protein coats radioactive marker is found inside the cell? Which molecule carries viral 35 S radioactivity 32 P radioactivity found genetic info? AP Biology found in the medium in the bacterial cells
AP Biology
Blender experiment  Radioactive phage & bacteria in blender  35S phage  radioactive proteins stayed in supernatant  therefore viral protein did NOT enter bacteria  32P phage  radioactive DNA stayed in pellet  therefore viral DNA did enter bacteria  Confirmed DNA is “transforming factor” Taaa-Daaa! AP Biology
1952 | 1969 Hershey & Chase Hershey AP Biology Martha Chase Alfred Hershey
1947 Chargaff  DNA composition: “Chargaff’s rules”  varies from species to species  all 4 bases not in equal quantity  bases present in characteristic ratio  humans: A = 30.9% Rules A = T T = 29.4% C = G G = 19.9% C = 19.8% That’s interesting! What do you notice? AP Biology
1953 | 1962 Structure of DNA  Watson & Crick  developed double helix model of DNA  other leading scientists working on question:  Rosalind Franklin  Maurice Wilkins  Linus Pauling AP Biology Franklin Wilkins Pauling
1953 article in Nature Watson and Crick Watson AP Biology Crick
Rosalind Franklin (1920-1958) AP Biology
But how is DNA copied?  Replication of DNA  base pairing suggests that it will allow each side to serve as a template for a new strand “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic AP Biology material.” — Watson & Crick
Can you design a nifty experiment to verify? Models of DNA Replication  Alternative models  become experimental predictions conservative semiconservative dispersive P 1 2 AP Biology
1958 Semiconservative replication  Meselson & Stahl  label “parent” nucleotides in DNA strands with heavy nitrogen = 15N  label new nucleotides with lighter isotope = 14N “The Most Beautiful Experiment in Biology” parent replication Make predictions… 15 N/15N 15 N parent strands AP Biology
Predictions 14 N/14N 1st round of 15 N/14N N/14N 15 15 N/15N replication semi- conservative dispersive conservative 2nd round of replication P 14 N/14N 14 N/14N 15 N/14N 15 N/14N 15 N/15N 1 15 N/15N semi- 2 N parent 15 conservative dispersive AP Biology conservative strands
Meselson & Stahl Matthew Meselson Franklin Stahl Franklin Stahl Matthew Meselson AP Biology
Scientific History  March to understanding that DNA is the genetic material  T.H. Morgan (1908)  genes are on chromosomes  Frederick Griffith (1928)  a transforming factor can change phenotype  Avery, McCarty & MacLeod (1944)  transforming factor is DNA  Erwin Chargaff (1947)  Chargaff rules: A = T, C = G  Hershey & Chase (1952)  confirmation that DNA is genetic material  Watson & Crick (1953)  determined double helix structure of DNA  Meselson & Stahl (1958) AP Biology  semi-conservative replication
The “Central Dogma”  Flow of genetic information in a cell transcription translation DNA RNA protein replication AP Biology
Science …. Fun Party Time! Any Questions?? AP Biology 2006-2007
Ghosts of Lectures Past (storage) AP Biology 2006-2007
1958 Semiconservative replication  Meselson & Stahl  label “parent” nucleotides in DNA strands with heavy nitrogen = 15N  label new nucleotides with lighter isotope = 14N “The Most Beautiful Experiment in Biology” parent replication 15 N/15N 15 N parent strands AP Biology
1958 Semiconservative replication  Make predictions…  15 N strands replicated in 14N medium  1st round of replication? where should the bands be?  2nd round? AP Biology