BIOL 100 (Spring 2001) Lecture #4: DNA structure & functionpage 1Chapter for today: Chap. XIIIMajor points for the day:1. Early experiments showed DNA as the genetic material2. Structure of DNAMolecular Biology explores the nature of heredityDuring the last fifty years the new discipline of molecular biology has generated arevolution in the biological sciences• At the beginning of the 1940s we didn’t know what the chemical basis ofheredity was• We didn’t know how hereditary instructions were propagated to control theway cells behaved or developedMolecular biology is a discipline which attempts to trace the molecular causes of avariety of biological phenomena, though the problem of heredity has been centralto molecular biology since its beginningsAs a direct result of the work of molecular biologists over the last several decadeswe now understand the molecular basis of heredity to a large extentWe also understand the basics of how hereditary instructions are given to cellsIn addition, we have come to understand how those instructions can be controlledso that they are given only in the correct circumstancesDuring the next four lectures we will consider• How molecular biologists came to understand the structural basis ofheredity—that is, the structure of the DNA molecule• How the instructions in DNA are transfered to an unstable messenger moleculecalled RNA• How the RNA code is translated into proteins, the chemical products of thehereditary instructions• And finally, the ways in which scientists are using the tools of molecular biologyto alter genetic instructions in order to understand better how cells workDNA as the “genetic material”The book describes how in 1868 Friedrich Miescher, a Swiss physician, first isolated amolecule he called “nuclein”Miescher knew that the genetic material resided in nuclei, and was probably carriedon the bodies which microscopists had called chromosomesHowever, as you can imagine, seeing chromosomes does not explain how theyworkMiescher wanted to understand them chemicallyNuclein we would call deoxyribonucleic acid (literally, an acid from nuclei whichcontains the sugar deoxyribose)I’m sure that most of you take it for granted that DNA is the genetic materialI know that you have heard that DNA is the genetic material because it is inheritedfrom one generation to the nextBut what is the evidence that it is so?The early history of molecular biology is punctuated by a series of experiments sosimple, and yet so elegant, that they have become known by the authors namesBIOL 100 (Spring 2001) Lecture #4: DNA structure & functionpage 2which constitute almost a pantheon of biology—Avery, MacLeod and McCarty;Watson & Crick; Hershey–Chase; Meselson & Stahl; Jacob & MonodMost of these experiments date from the first decade of modern molecularbiology—the 1950s• Though they are over 40 years old, nearly every biology student learns aboutthese experiments• They form the intellectual basis for much of modern biology, or at least ofmodern geneticsAvery, MacLeod & McCarty provide evidence for DNA as the genetic materialMuch of the first half of the century microbiologists attempted to identify organismsresponsible for diseaseFred Griffith was an Army doctor who in 1928 wanted to make a vaccine againstStreptococcus pneumoniae, which caused a type of pneumoniaSince the time of Pasteur, about 50 years before, vaccines had been made using killedmicroorganisms which could be injected into patients to elicit the immune responseof live cells without risk of diseaseThough he failed in making the vaccine he stumbled on a demonstration of thetransmission of genetic instructions by a process we now call transformation• He found that the bacterium had two forms when grown on agar plates, asmooth (S) and a rough (R) form• The R bacteria were harmless, but the S bacteria were lethal when injected intomice• Heat–killed S cells were also harmless—the same effect seen by Pasteur• However, surprisingly when live R cells were mixed with killed S cells andinjected into mice the mice died, and the bacteria rescued from the mice had been“transformed” into S typeThis experiment strongly implied that genetic material had been transferred from thedead to the live cellIt was hard to be certain of this, or to know what the material was in this crudeexperimentSixteen years later in 1944 the team of Avery, MacLeod and McCarty revisited thisexperiment and attempted a more definitive experiment• A favorite hypothesis of the 1930s was that the genetic instructions inchromosomes was protein• Chromosomes, as we shall see, are largely made of proteins, and biologiststhought that DNA performed a structural role as a scaffold to hold theseinformational proteins• However, chemical analysis of DNA had begun to suggest that it might be amore interesting molecule than had been supposedAvery, MacLeod and McCarty repeated Griffith’s experiment using purified DNAmolecules and got the same “transformation” of R cells into S cells• Since it is often the case that favored hypotheses are given up only verygrudgingly, they did some critical control experiments• Knowing that many would claim that their DNA was contaminated with proteinwhich was the true active principle, they treated the “DNA” with two enzymes:– Protease: this degrades proteins to its monomers (amino acids)BIOL 100 (Spring 2001) Lecture #4: DNA structure & functionpage 3– DNAse: this degrades DNA to its monomers (nucleotides)• Pre–treatment with protease had no effect on transformation, whilepre–treatment with DNAse eliminated transformationThrough another example of bloody–mindedness in scientific history many refused toaccept these results, and suggested that it might apply only to bacteria!The Hershey–Chase experimentThe second classical demonstration of DNA as the genetic material was an outgrowth ofa group of molecular biologists who in the late 1940s and early 1950s started to useviruses that attack bacteria (bacteriophages—or literally “bacteria eaters”) to studythe nature of heredityAlfred Hershey was one of the founders of this group. One of his students, MarthaChase, performed an experiment which clearly distinguished