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DNA Its History, Structure and Function WHO DISCOVERED DNA? • Johann Friedrich Miescher • Isolated DNA from leukocytes • Called it “nuclein” MIESCHER’S WORK • Late 1800s • Disproved that the nucleus is mostly protein • Isolated the components of DNA: mostly carbon, hydrogen, oxygen, nitrogen, and phosphorus • No known proteins had that much phosphorus (3%) WAS DNA THE MOLECULE OF HEREDITY? • Frederick Griffith (1928) • Worked with bacteria • “Transformed” noninfectious bacteria into virulent bacteria • Called the part that controlled virulence a “transforming principle” GRIFFITH’S EXPERIMENT AVERY AND MACLEOD (1944) THEIR EXPERIMENT CONCLUSIONS • A chemical is responsible for the virulence of Pneumococcus bacteria • The chemical was not a protein, a lipid, a carbohydrate, or RNA • The chemical was DNA HERSHEY AND CHASE (1952) THEIR EXPERIMENT CONCLUSIONS • The active component of the bacteriophage that transmits the infective characteristic is the DNA. • There is a clear correlation between DNA and genetic information. • Source: Access Excellence WHAT IS DNA’S STRUCTURE? • Erwin Chargaff (1952) • Maurice Wilkins and Rosalind Franklin (1953) • J.D. Watson and Francis Crick (1953) CHARGAFF’S RULE • In nuclear DNA: • %A=%T • %C=%G • Led to idea of complementary base pairing WILKINS, FRANKLIN, AND PHOTO 51 PHOTO 51 WATSON AND CRICK WHAT IS THE STRUCTURE OF DNA? • Double helix • Two “backbones”: alternating deoxyribose and phosphate groups • Twisted “ladder”: the “rungs” are the paired nitrogenous bases • Antiparallel • Strands run in opposite directions WHAT IS THE STRUCTURE OF DNA? • Complementary base pairing • A pairs with T, C with G • Hydrogen bonding between bases • Two bonds between A & T; three between C & G • Easily broken, easily reformed THE DOUBLE HELIX NUCLEOTIDE STRUCTURE FIVE NUCLEOTIDES STEPS IN REPLICATION • Helicase unwinds the double helix, forming replication forks • DNA polymerase III binds free nucleotides to the exposed complementary bases • Replication occurs in the 5‘ to 3‘ direction (lead strand and lag strand); bases can only be added to the 3’ end of the growing DNA molecule • Other enzymes correct errors, remove primers, seal “nicks” in the backbone • The resulting daughter DNA molecules each consist of an original strand and a new strand PROKARYOTE ORIGIN OF REPLICATION EUKARYOTE ORIGIN OF REPLICATION