DNA: The Stuff of Inheritance

Finding the Genetic Material zIn the first half of the twentieth century, after biologists began to appreciate Mendel’s and Morgan’s work with genetics, the race began to find the part of the cell that was the genetic material. zEukaryotic chromosomes are made of only two materials: DNA and protein. Thus, one of these must be the genetic material. zThe greatest biologists of the time were split. Roughly half of them thought that DNA was the genetic material, and the other half thought that is was protein. zA series of important experiments led to the identification of the genetic material. zScientists whose experiments contributed to our knowledge of the structure and function of the genetic material included: 1928 Frederick Griffith 1944 Oswald Avery 1947 Erwin Chargaff 1952 Alfred Hershey and Margaret Chase 1952 Maurice Wilkins and Rosalind Franklin 1953 James Watson and Francis Crick

Frederick Griffith zIn 1928, Frederick Griffith was trying to produce a vaccine against a type of bacterial pneumonia. zGriffith isolated two strains of pneumonia bacteria from mice. He could grow both strains in the lab, but only one caused pneumonia. zThe strains looked slightly different from each other. The colonies of disease-causing bacteria had a smooth coat and the harmless bacteria had a rough coat. zGriffith did not succeed in producing a vaccine for bacterial pneumonia, but his results were very important to scientists anyway. zGriffith’s experiment showed that the genetic material could be transferred from one organism to another when he demonstrated bacterial transformation.

Griffith’s Experiment and Bacterial Transformation Control (no growth)Harmless bacteria (rough coated) Mouse lives Disease-causing bacteria (smooth colonies) Mouse dies of pneumonia Heat-killed disease- causing bacteria (smooth colonies) Mouse lives Harmless bacteria (rough colonies) Heat-killed disease- causing bacteria (smooth colonies) Mouse dies of pneumonia Live disease-causing bacteria (smooth colonies!) The heat-killed bacteria passed their disease-causing trait to the harmless bacteria = bacterial transformation!

In the tube with disabled DNA, no transformation occurred! Oswald Avery zIn 1944, Oswald Avery and a group of scientists repeated Griffith’s experiment, with some changes. zThey took and extract from the heat-killed bacteria and filled 5 tubes. They treated each tube to destroy one type of molecule: either protein, carbohydrate, lipid, RNA, or DNA. zThen they inoculated harmless bacteria with the tubes. Transformation still occurred in all tubes except the one that contained destroyed DNA. zTheir results demonstrated that DNA appeared to be the material responsible for bacterial transformation. No Protein No Carbo- No Lipid No RNA No DNA hydrate

Erwin Chargaff zAdditional evidence pointed to the idea that DNA was the genetic material. zBiochemists knew that before mitosis, during the S-phase of the cell cycle, a dividing eukaryotic cell exactly doubles its DNA content, and during mitosis this DNA is equally distributed between the two daughter cells. z Additional analysis showed that diploid cells contained exactly twice as much DNA as haploid cells contained. zScientists knew that DNA was a polymer of four different nucleotides, each containing a different nitrogen base. zChargaff analyzed the DNA from a number of different organisms and found that the composition of DNA differs from species to species, thus showing molecular diversity. zHe also found that in the DNA of a species, adenine and thymine always appear in equal amounts and cytosine and guanine also appear in equal amounts. These two discoveries were known as Chargaff’s Rules, or base-pairing rules. A = T, C = G

Hershey and Chase zIn 1952, Hershey and Chase showed that the genetic material was definitely DNA and not protein, using radioactively-labeled bacteriophage viruses. T2 Phages labeled with radioactive 32 P, which marks DNA T2 Phages labeled with radioactive 35 S, which marks protein Agitate in a blender to separate viruses from bacterial host Radioactive 32 P found inside the bacteria! Thus the genetic material transferred from the virus must be the marked DNA. Radioactive 35 S found still in the viruses! Thus, the marked protein was NOT transferred to the bacteria, and therefore cannot be the genetic material. Viruses infect bacterial host

Watson and Crick zAfter the results of the Hershey-Chase experiment were published, the race for the genetic material focused on the structure of DNA. zIn London, Maurice Wilkins and Rosalind Franklin worked on X-ray diffraction studies of the crystalline forms of DNA. zIn Cambridge, England James Watson and Francis Crick were also trying to determine the structure of DNA. zWatson saw Franklin’s picture, and he and Crick correctly deciphered DNA’s structure. zThey made a model with two strands having the phosphates and sugars alternating on the outside and the nitrogen bases pairing on the inside. zTheir structure explained how DNA could copy itself and could code for the production of proteins responsible for genetic traits. zWatson, Crick, and Wilkins won the Nobel Prize in Franklin died in The Nobel Prize is not awarded posthumously. Rosalind Franklin’s X-ray diffraction picture of the structure of DNA showed that it was a double helix.

The Structure of DNA (Part 1) zThe two side chains of DNA are built of alternating molecules of the sugar deoxyribose and phosphate groups. They run in opposite directions. zThe interior “rungs” of the DNA ladder result from the pairing of the nitrogen bases. zA purine base always pairs with a pyrimidine base. The bases are held together by hydrogen bonds, which are relatively weak and easy to break. zAdenine and Guanine are purines, consisting of two carbon-nitrogen rings. zThymine and Cytosine are pyrimidines, made of only one carbon-nitrogen ring.

The Structure of DNA (Part 2) zBecause of the placement of hydrogen atoms in the four nitrogen bases, Adenine will only pair with Thymine, zGuanine will only pair with Cytosine. zAlthough the base-pairing rules determine the combinations of bases that form the “rungs of the ladder,” they do not determine the sequence of nucleotides along each DNA strand. zThis means that the linear sequence of DNA’s bases can be varied in an infinite number of ways, enough to code for the genes of every living thing. AdenineThymine N N N NNH H (sugar) N N CH 3 O H (sugar) GuanineCytosine N N O N N (sugar) N NO H H H H N N H