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AP Biology 2006-2007 DNA The Genetic Material AP Biology Scientific History  The march to understanding that DNA is the genetic material  T.H. Morgan.

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Presentation on theme: "AP Biology 2006-2007 DNA The Genetic Material AP Biology Scientific History  The march to understanding that DNA is the genetic material  T.H. Morgan."— Presentation transcript:


2 AP Biology DNA The Genetic Material

3 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)  Hershey & Chase (1952)  Watson & Crick (1953)  Meselson & Stahl (1958)

4 AP Biology Genes are on chromosomes  T.H. Morgan  working with Drosophila (fruit flies)  genes are on chromosomes  but is it the protein or the DNA of the chromosomes that are the genes?  through 1940 proteins were thought to be genetic material… Why? 1908 | 1933 What ’ s so impressive about proteins?!

5 AP Biology The “Transforming Factor” 1928  Frederick Griffith  Streptococcus pneumoniae bacteria  was working to find cure for pneumonia  harmless live bacteria mixed with heat-killed infectious bacteria causes disease in mice  substance passed from dead bacteria to live bacteria = “Transforming Factor”

6 AP Biology The “Transforming Factor” Transformation? 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.

7 AP Biology DNA is the “Transforming Factor”  Avery, McCarty & MacLeod  purified both DNA & proteins from Streptococcus pneumoniae 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?

8 AP Biology Avery, McCarty & MacLeod Oswald Avery Canadian-born American physician & medical researcher Maclyn McCarty was an American geneticist Colin MacLeod Was a Canadian- American geneticist 1944

9 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?

10 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

11 AP Biology

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

13 AP Biology Hershey & Chase Martha Chase & Alfred Hershey  Martha C. Chase (1927 – August 8, 2003) A young laboratory assistant in the early 1950s when she and Alfred Hershey conducted one of the most famous experiments in 20th century biology. Devised by American bacteriophage expert Alfred Hershey at Cold Spring Harbor Laboratory New York, the famous experiment demonstrated the genetic properties of DNA over proteins. By marking bacteriophages with radioactive isotopes, Hershey and Chase were able to trace protein and DNA to determine which is the molecule of heredity 1952 | 1969 Hershey

14 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?

15 AP Biology Structure of DNA  Watson & Crick  developed double helix model of DNA  other scientists working on question:  Rosalind Franklin and Maurice Wilkins  Erwin Chargaff – Chargaff’s rules  Linus Pauling – alpha helical structure of protein 1953 | 1962 Franklin WilkinsPauling

16 AP Biology Watson and Crick 1953 article in Nature CrickWatson

17 AP Biology Rosalind Franklin ( ) The Secret of Photo 51

18 AP Biology





23  Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand  Competing models were the conservative model (the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new)

24 AP Biology Parent cell First replication Second replication (a) Conservative model (b) Semiconserva tive model (c) Dispersive model

25 AP Biology  Experiments by Matthew Meselson and Franklin Stahl supported the semiconservative model  They labeled the nucleotides of the old strands with a heavy isotope of nitrogen, while any new nucleotides were labeled with a lighter isotope

26 AP Biology Meselson and Stahl

27 AP Biology  The first replication produced a band of hybrid DNA, eliminating the conservative model  A second replication produced both light and hybrid DNA, eliminating the dispersive model and supporting the semiconservative model

28 AP Biology Fig a EXPERIMENT RESULTS Bacteria cultured in medium containing 15 N Bacteria transferred to medium containing 14 N DNA sample centrifuged after 20 min (after first application) DNA sample centrifuged after 20 min (after second replication) Less dense More dense

29 AP Biology Fig b CONCLUSION First replicationSecond replication Conservative model Semiconservative model Dispersive model

30 AP Biology DNA Replication: A Closer Look  The copying of DNA is remarkable in its speed and accuracy  More than a dozen enzymes and other proteins participate in DNA replication Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

31 AP Biology Central Dogma of Molecular Biology “The central dogma deals with the detailed residue-by-residue transfer of sequential information. It states that such information cannot be transferred back from protein to either protein or nucleic acid.” Francis Crick, 1958

32 AP Biology Conclusions  T.H. Morgan (1908) The chromosome theory of heredity  Frederick Griffith (1928)  Some component of heat-killed virulent bacteria can "transform" a non-virulent strain to become virulent  Avery, McCarty & MacLeod (1944)  DNA is the molecule that mediates heredity  Hershey & Chase (1952)  DNA is the molecule that mediates heredity, as shown in bacteriophage labeling experiments  Watson & Crick (1953)  DNA is in the shape of a double helix with antiparallel nucleotide chains and specific base pairing  Meselson and Stahl (1958)  Proved definitively the semi-conservative replication of DNA using radioactive isotopes of nitrogen

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