The Search for the Genetic Material "If I have seen further, it is by standing on the shoulders of giants." – Isaac newton

What was known so far Chromosomes are made up of DNA and protein Protein is the more likely candidate made of 20 different building blocks specificity of function Not much was known about DNA simpler contained deoxyribose, plenty of phosphate and four bases

An Early Study on DNA Friedrich Miescher (1871) isolated an acidic substance from the nuclei of white blood cells “nuclein”  nucleic acid

What is the Genetic Material made of ? Frederick Griffith (1928) Streptococcus pneumoniae type S  pathogenic type R  non-pathogenic studies led to the identification of DNA as the genetic material

Griffith’s Experiment A B C D Autopsy revealed presence of S cells

Questions What had caused the non-pathogenic R cells to become pathogenic in the presence of heat killed S cells? Since heat denatures most proteins, is it still likely that the genetic material is made up of protein? TRANSFORMATION

What is the “transforming factor”? Oswald Avery, Maclyn McCarty & Colin MacLeod (1944) attempt to identify Griffith’s “transforming agent” purified chemicals from heat-killed S cells  tested with live R cells for transformation only DNA worked  “transforming principle” was DNA

AVERY, MCCARTY & MACLEOD’S EXPERIMENT

Additional evidence for DNA Alfred Hershey & Martha Chase (1952) E. coli infected with T2 phage labeled T2 with radioactive isotopes: 35S for protein 32P for DNA after infection, agitation and centrifugation, the supernatant and pellet were analyzed  radioactivity detected in bacterial cells in the tubes labeled with 32P

HERSHEY-CHASE EXPERIMENT

 DNA injected into the host cell!

Very little was known about DNA Elements present: C, H, N, O and P Contained the sugar deoxyribose, plenty of phosphate and four bases (A, C, T, G) Precise structure not known

More Studies on DNA Erwin Chargaff (1950) The composition of DNA varied from one species to another, in particular in the relative amounts of the bases A, C, T, G. DNA in several species contained equal amounts of A and T and equal amounts of G and C

More Studies on DNA Maurice Wilkins & Rosalind Franklin (1952) X-ray crystallography revealed the structure of monomers that repeated at regular intervals DNA molecule probably had the corkscrew shape of a helix Sugar phosphate backbone on the outside of the helix

DNA structure Unraveled James Watson & Francis Crick (1953) Built a molecular model of DNA Deduced the dimensions of the DNA molecule: diameter = 2 nm N bases stacked 0.34 nm apart DNA a double helix sugar and phosphate backbone (on the outside) pairs of N bases stacked on the inside

Watson & Crick’s DNA Model

DNA model had equal amounts of A & T and equal amounts of G & C observed that A fitted together with T while G paired naturally with C each base pair held together by hydrogen bonding Explains Chargaff’s observation  The Principle of Complementarity: Each base can pair with only one other, called its complement.

“It has not escaped our notice that the pairing… immediately suggests a possible copying mechanism for the genetic material.” (Watson & Crick, 1953) In fact, it is the key to the gene’s main functions: replication and protein synthesis.

Watson and Crick, together with Wilkins, jointly received the Nobel Prize for the elucidation of DNA structure.

Because the Nobel Prize can not be awarded posthumously, Rosalind Franklin, who died of cancer in 1958, could not be honored.

How Replication works Each strand of the double helix contains the information necessary to make its complementary strand. How it happens: When the DNA is ready to multiply, its two strands pull apart. Along each one, a new strand forms. Resulting in two copies of the original.

Meselson-Stahl experiment

Activity: DNA Puzzle Solve the puzzle by assembling the different pieces to make up the molecular model of DNA. Each piece represents the components of a nucleotide: sugar, phosphate, nitrogenous base (four distinct pieces for each base). Draw your finished puzzle and label the different pieces. How does the puzzle illustrate the specificity of base pairing? Why is this particular feature necessary for replication? Illustrate the semi-conservative model of replication using the sequence derived from your puzzle. How did Meselson and Stahl’s findings prove that this was indeed the mechanism for DNA replication?