AP Biology DNA The Genetic Material

AP Biology 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 Chromosomes related to phenotype  T.H. Morgan  working with Drosophila  fruit flies  associated phenotype with specific chromosome  white-eyed male had specific X chromosome 1908 | 1933

AP Biology 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? 1908 | 1933 What ’ s so impressive about proteins?!

AP Biology The “Transforming Principle” 1928  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” Transformation = change in phenotype something in heat-killed bacteria could still transmit disease-causing properties live pathogenic strain of bacteria live non-pathogenic strain of bacteria mice diemice live heat-killed pathogenic bacteria mix heat-killed pathogenic & non-pathogenic bacteria mice livemice die A.B. C. D.

AP Biology 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 1944 What ’ s the conclusion? mice die

AP Biology Oswald AveryMaclyn McCartyColin MacLeod Avery, McCarty & MacLeod  Conclusion  first experimental evidence that DNA was the genetic material 1944 | ??!!

AP Biology Confirmation of DNA  Hershey & Chase  classic “blender” experiment  worked with bacteriophage  viruses that infect bacteria  grew phage viruses in 2 media, radioactively labeled with either  35 S in their proteins  32 P in their DNA  infected bacteria with labeled phages 1952 | 1969 Hershey Why use S ulfur vs. P hosphorus?

AP Biology Protein coat labeled with 35 S DNA labeled with 32 P bacteriophages infect bacterial cells T2 bacteriophages are labeled with radioactive isotopes S vs. P bacterial cells are agitated to remove viral protein coats 35 S radioactivity found in the medium 32 P radioactivity found in the bacterial cells Which radioactive marker is found inside the cell? Which molecule carries viral genetic info? Hershey & Chase

AP Biology

Blender experiment  Radioactive phage & bacteria in blender  35 S phage  radioactive proteins stayed in supernatant  therefore viral protein did NOT enter bacteria  32 P phage  radioactive DNA stayed in pellet  therefore viral DNA did enter bacteria  Confirmed DNA is “transforming factor” Taaa-Daaa!

AP Biology Hershey & Chase Alfred HersheyMartha Chase 1952 | 1969 Hershey

AP Biology 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% T = 29.4% G = 19.9% C = 19.8% 1947 That ’ s interesting! What do you notice? Rules A = T C = G

AP Biology Structure of DNA  Watson & Crick  developed double helix model of DNA  other leading scientists working on question:  Rosalind Franklin  Maurice Wilkins  Linus Pauling 1953 | 1962 Franklin WilkinsPauling

AP Biology Watson and Crick 1953 article in Nature CrickWatson

AP Biology Dr. Watson is not a nice man… He is "inherently gloomy about the prospect of Africa“….because… To him, the real cause of Africa's woes is that black people are inherently less intelligent than other races. "All our social policies are based on the fact that their intelligence is the same as ours — whereas all the testing says not really," –October 2007 He hoped that everyone was equal, he continued, but "people who have to deal with black employees find this not true".

AP Biology He didn’t stop there… (Amount of melanin in the skin) was why black people and Latinos had reputations as lovers — something, he said, of which the English could not be accused "Whenever you interview fat people, you feel bad because you know you're not going to hire them," In 1997, he even said a woman should be allowed to abort a fetus if tests had shown it would be a homosexual and the mother deemed this undesirable.

AP Biology WAIT…there are still more people to insult! Watson has repeatedly supported genetic screening and genetic engineering in public lectures and interviews, arguing that stupidity is a disease and the "really stupid" bottom 10% of people should be cured He has also suggested that beauty could be genetically engineered, saying "People say it would be terrible if we made all girls pretty. I think it would be great.”

AP Biology Rosalind Franklin ( )

AP Biology Hydrogen bonds form between complementary bases Complementary base pairs hold the two DNA strands together DNA is a double helix of two nucleotide strands The Watson-Crick Model of DNA Structure

AP Biology Hydrogen Bonds Nitrogen-containing bases protrude inward from sugar-phosphate backbone Hydrogen bonds hold certain nitrogenous base pairs together  A bonds with T, G bonds with C  Bonding bases called complementary base pairs  A and T bond with TWO Hydrogen bonds  G and C bond with THREE Hydro. bonds

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 material.”— Watson & Crick

AP Biology Models of DNA Replication  Alternative models  become experimental predictions conservativesemiconservative Can you design a nifty experiment to verify? dispersive 1 2 P

AP Biology Semiconservative replication  Meselson & Stahl  label “parent” nucleotides in DNA strands with heavy nitrogen = 15 N  label new nucleotides with lighter isotope = 14 N “The Most Beautiful Experiment in Biology” 1958 parentreplication Make predictions … 15 N parent strands 15 N/ 15 N

AP Biology Predictions 1st round of replication conservative 15 N/ 15 N 14 N/ 14 N semi- conservative 15 N/ 14 N dispersive 15 N/ 14 N conservative 15 N/ 15 N 14 N/ 14 N semi- conservative 15 N/ 14 N dispersive 15 N/ 14 N 2nd round of replication 14 N/ 14 N 15 N parent strands 15 N/ 15 N 1 2 P

AP Biology Franklin Stahl Matthew Meselson Franklin Stahl Meselson & Stahl

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)  semi-conservative replication

AP Biology proteinRNA The “Central Dogma” DNA transcriptiontranslation replication  Flow of genetic information in a cell

AP Biology Science …. Fun Party Time! Any Questions??