The Search for the Genetic Material Scientific Inquiry Early in the 20th century the identification of the molecules of inheritance loomed as a major challenge to biologists When T H Morgan s group showed that genes are located on chromosomes the two components of chromosomes DNA and protein became candidates for the genetic material 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 Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Evidence That DNA Can Transform Bacteria The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 Griffith worked with two strains of a bacterium one pathogenic and one harmless When he mixed heat killed remains of the pathogenic strain with living cells of the harmless strain some living cells became pathogenic He called this phenomenon transformation now defined as a change in genotype and phenotype due to assimilation of foreign DNA Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 16 2 Mixture of heat killed Living S cells Living R cells Heat killed S cells and control control S cells control living R cells EXPERIMENT RESULTS Mouse dies Mouse healthy Mouse healthy Mouse dies Living S cells Evidence That Viral DNA Can Program Cells More evidence for DNA as the genetic material came from studies of viruses that infect bacteria Such viruses called bacteriophages or phages are widely used in molecular genetics research Phage head Tail fiber DNA Bacterial cell In 1952 Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2 To determine the source of genetic material in the phage they designed an experiment showing that only one of the two components of T2 DNA or protein enters an E coli cell during infection They concluded that the injected DNA of the phage provides the genetic information Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 16 4 3 EXPERIMENT Phage Empty protein Radioactive shell protein Radioactivity phage protein in liquid Bacterial cell Batch 1 radioactive sulfur 35S DNA Phage DNA Centrifuge Pellet bacterial cells and contents Radioactive DNA Batch 2 radioactive phosphorus 32P Centrifuge Pellet Radioactivity phage DNA in pellet Additional Evidence That DNA Is the Genetic Material It was known that DNA is a polymer of nucleotides each consisting of a nitrogenous base a sugar and a phosphate group In 1950 Erwin Chargaff reported that DNA composition varies from one species to the next This evidence of diversity made DNA a more credible candidate for the genetic material Chargaff s rules state that in any species there is an equal number of A and T bases and an equal number of G and C bases Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Building a Structural Model of DNA After most biologists became convinced that DNA was the genetic material the challenge was to determine how its structure accounts for its role Maurice Wilkins and Rosalind Franklin were using a technique called X ray crystallography to study molecular structure Franklin produced a picture of the DNA molecule using this technique Franklin s X ray crystallographic images of DNA enabled Watson to deduce that DNA was helical and was made up of two strands forming a double helix Watson and Crick built models of a double helix to conform to the X rays and chemistry of DNA Franklin had concluded that there were two antiparallel sugarphosphate backbones with the nitrogenous bases paired in the molecule s interior Purine purine too wide Pyrimidine pyrimidine too narrow Purine pyrimidine width consistent with X ray data 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 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 Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Adenine A Guanine G Thymine T Cytosine C The Basic Principle Base Pairing to a Template Strand Since the two strands of DNA are complementary each strand acts as a template for building a new strand in replication In DNA replication the parent molecule unwinds and two new daughter strands are built based on base pairing rules 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 Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 16 9 3 A T A T A T A T C G C G C G C G T A T A T A T A A T A T A T A T G C G C G C G C a Parent molecule b Separation of strands c Daughter DNA molecules each consisting of one parental strand and one new strand DNA Replication The copying of DNA is remarkable in its speed and accuracy More than a dozen enzymes and other proteins participate in DNA replication Replication begins at special sites called origins of replication where the two DNA strands are separated opening up a replication bubble A eukaryotic chromosome may have hundreds or even thousands of origins of replication Replication proceeds in both directions from each origin until the entire molecule is copied Copyright 2008 Pearson Education Inc publishing as Pearson Benjamin Cummings Fig 16 12a Origin of replication Parental template strand Daughter new strand Doublestranded DNA molecule Replication fork Replication bubble 0 5 m Two daughter DNA molecules a Origins of replication in E coli Fig 16 12b Origin of replication Double stranded DNA molecule Parental template strand Daughter new strand 0 25 m Bubble Replication fork Two daughter DNA molecules b Origins of replication in eukaryotes Enzymes that play a role in DNA synthesis At the end of each replication bubble is a replication fork a Yshaped region where new DNA strands are elongating Helicases are enzymes that untwist the double helix at the replication forks Single strand binding protein binds to and stabilizes singlestranded DNA until it can be used as a template