How do we know that DNA is the genetic material?

Griffith, 1928

Avery et al., 1944

4 Bacteriophage = Virus that attacks bacteria and replicates by invading a living cell and using the cell’s molecular machinery. The T2 phage Bacteriophages are composed of DNA & protein

Hershey and Chase, 1952

What information was available to Watson and Crick as they worked to determine the structure of DNA? 1.Genetic material is composed of nucleotides

What information was available to Watson and Crick as they worked to determine the structure of DNA? 2. These nucleotides have variable binding properties. Image from from

3. The relative abundances of these nucleotides determine the melting (denaturation) temperature for a given genome. (G+C)/(G+C+A+T) Melting temperature of genomic DNA A relative of Mycobacterium tuberculosis

What information was available to Watson and Crick as they worked to determine the structure of DNA? 4. Chargaff’s “Rules” regarding the ratios of the nucleotides,… Table from Manchester 2008 Recall from your reading of Watson and Crick, 1952: It has been found experimentally that the ratio of the amounts of adenine to thymine, and the ratio of guanine to cytosine, are always very close to unity for deoxyribose nucleic acid.

Chargaff’s “Rules” Table from Manchester 2008 “pur”=purines including adenine and guanine in DNA, as well as caffeine, and theobromine (the vasodillator and heart stimulant found in chocolate) “pyr”=pyrimidines including thymine, cytosine, and uracil in DNA. Other forms are useful as anesthetics, and in treating HIV.

5. … and a highly informative photograph by Rosalind Franklin and Raymond Gosling Photo 51

This structure has two helical chains each coiled round the same axis. We have made the usual chemical assumptions, namely that each chain consists of phosphate diester groups… with 3’-5’ linkages…owing to the dyad, the sequences of the atoms in the two chains run in opposite directions.

It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.