DNA and The Genome Structure of DNA: Experimental Evidence What are we learning? To understand the experiments which led to the discovery of DNA and its structure. Textbook Chapter 1: Pages 5-9

Proteins or DNA? During the early part of the twentieth century, genes were known to be part of chromosomes and chromosomes to be composed of protein and DNA. However, scientists did not know which molecule made up the genetic material.

Griffith’s experiment In 1928, Griffith carried out the bacterial transformation experiment using 2 strains of pneumonia in mice. One strain was virulent (harmful) and one harmless. Fredrick Griffith

Griffith concluded that some chemical component had passed from dead cells to live cells. The live cells had become transformed.

Avery’s experiment Avery repeated Griffith’s experiment. He and his team isolated each constituent of the disease-causing strain of the bacterium. Over 14 years they tested each substance for its ability to transform dead cells. They identified DNA fragments as the only substance that could do this. Oswald Avery

Chargaff’s experiment In the late 40s, Chargaff analysed the base composition of DNA extracted from different species. He found that the quantities of the four bases were not all equal, but they always occurred in a characteristic ratio, regardless of the species. The findings called Chargaff’s rules were: The number of adenine bases = the number of thymine bases The number of cytosine bases = the number of guanine bases Erwin Chargaff

Wilkins and Franklin Wilkins and Franklin used X-ray crystallography to create 3D images of DNA molecules from different species. When x-rays are passed through a crystal of DNA a pattern is created, determined by the arrangement of atoms. Wilkins and Franklin discovered that the images made from the DNA of different species were identical.

Watson and Crick Watson and crick deduced from Wilkins and Franklin’s findings that DNA must be a long, thin molecule with a constant diameter in the shape of a helix. Using Chargaff’s rules, they deduced the base pair rule. They set about building a wire model and in 1953 were first to establish the 3D double helix structure.