Topic 7 The Discovery of DNA & Its Roles October 7-14, 2005 Biology 1001

Introduction to DNA DNA is deoxyribonucleic acid, a polymer of the four nucleotide monomers adenine, guanine, cytosine, & thymine  Arranged as a double helix DNA is the molecule of heredity  DNA is precisely replicated during cell division  Each cell in the organism has a copy and a copy is transmitted from parent to offspring DNA encodes the amino acid sequences of all the proteins in an organism  All biochemical, anatomical, physiological, and behavioural characteristics of organisms are at least partially determined by DNA

Science as a process  A series of classic experiments elucidated the molecular basis of inheritance Griffith (1928) – The genetic material transforms bacteria Avery & MacLeod (1944) – The transforming agent is DNA Hershey & Chase (1952) – DNA is the hereditary material of viruses Watson & Crick (1953) – Modeled the 3D structure of DNA Meselson & Stahl (1958) – DNA replication is semi-conservative

7.1 The Genetic Material Is DNA A. Griffith’s (1928) Experiment  Worked with two strains of Streptococcus pneumoniae, a pathogenic one that causes pneumonia (S) and a nonpathogenic one (R)  R bacteria mixed with heat-killed S bacteria became pathogenic  The pathogenicity was inherited by the offspring of these R bacteria  Griffith reasoned that the R bacteria were able to take up a chemical from the environment Figure 16.2!

7.1 The Genetic Material Is DNA Griffith called this process transformation, but he did not know what the transforming chemical was Avery and colleagues spent 14 years testing various chemicals from the S bacterial remains to see which would transform nonpathogenic bacteria into pathogenic ones (R into S)  Only DNA worked Their discovery was met with considerable skepticism because it was widely held that proteins were a better candidate for the genetic material The question than became whether this finding was generalizable to other organisms, or specific to bacteria

7.1 The Genetic Material Is DNA B. The “Blender” Experiment of Hershey and Chase (1952) Studied the T2 bacteriophage that infects E. coli  T2 is a virus comprised of DNA & protein  Infected E. coli produce new viruses; viral DNA OR protein is responsible  Separately radiolabelled each component; DNA with 32 P and protein with 35 S  Allowed infection to proceed, then blended & centrifuged the bacteria  32 P was found in the pellet, 35 S in the supernatant  DNA is the hereditary material in viruses

7.2 The Discovery of the Model of DNA What was known to Watson & Crick  DNA is the hereditary material  DNA is a polymer of nucleotides Nucleotides contain a pentose sugar (deoxyribose), a phosphate group, and a nitrogenous base (A,C,G, or T) A & G are purines, C & T are pyrimidines The sugars and phosphates form the backbone of the polymer and give it directionality The proportion of each base varies from species to species – first evidence of diversity of DNA But in each species, the proportion of adenine equals that of thymine, and the proportion of guanine equals that of cytosine  These are Chargaff’s rules A=T and G=C

7.2 The Discovery of the Model of DNA The challenge was to devise a 3D structure that would account for DNA’s role in inheritance  Watson had also seen Rosalind Franklin’s X-ray diffraction data suggesting that the molecule was helical and wide enough to accommodate two strands of DNA – a double helix  Franklin reasoned that the sugar phosphate backbone faced outward, allowing the hydrophobic bases to occupy the interior of the molecule  What remained to be determined was the specific pairing of bases holding the two strands together

7.2 The Discovery of the Model of DNA In 1953, Watson and Crick reported their model in a one page paper to Nature entitled Molecular Structure of Nucleic Acids –Its key feature was the arrangement of bases between the two strand strands of the helix –Watson and Crick proposed that adenine paired with thymine, and guanine with cytosine, with hydrogen bonds between bases holding together the two strands of the helix

Figure 16.7 The Structure of DNA

Review - The 3D Structure of DNA Features of the model –Two strands of DNA arranged in a double-helix, antiparallel with the bases inward and the sugar phosphate backbone outward –The two strands are held together by hydrogen bonding between base pairs, A with T and C with G –The helix is “right-handed” with a 3.4 nm spacing between adjacent turns of the helix, and a 0.34 nm spacing between adjacent base pairs –The model fits the X-ray diffraction data and Chargaff’s rules