Its History, Structure and Function DNA

Johann Friedrich Miescher Isolated DNA from leukocytes Called it “nuclein” WHO DISCOVERED DNA?

MEI 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%)

Frederick Griffith (1928) Worked with bacteria “Transformed” non- infectious bacteria into virulent bacteria Called the part that controlled virulence a “transforming principle” WAS DNA THE MOLECULE OF HEREDITY?

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)

In nuclear DNA: % A = % T % C = % G Led to idea of complementary base pairing CHARGAFF’S RULE

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