The work of Rosalyn Franklin using x-ray crystallography established the shape

as an helix

Watson and Crick gathered information from various

sources and built a model.The model established the

method by which DNA replication could occur.

A Science Odyssey:

You Try It: DNA Workshop Activity

Method of DNA replication suggested by Watson and Crick

•DNA helix unwinds•DNA helicase breaks the H bonds between complementary base pairs and double helix separates•DNA polymerase moves along the DNA pairing complementary nucleotides to the exposed bases•DNA ligase joins the sugar-phosphate backbone to form a new strand of DNA

Scan in diagram froggy page 104

Use of cards to show process

In 1958 Meselsohn and Stahl

conducted a series of experiments

that gave strong support to the theory

of semi-conservative replication.

•They grew E.coli bacteria for many generations in a medium containing 15N, a heavy isotope of nitrogen.

•The bacteria grew, incorporating the 15N into their DNA

•This made their DNA heavier than normal

•The bacteria grown in 15N were then transferred to a 14N medium and left for periods of time that corresponded to the generation time of E.coli

Bacteria grown in15N (heavy) medium

Transfer some bacteria to

14N (light) medium

sample0 mins

sample20 mins

sample40 mins

15N/ 15N

15N/ 14N

14N/ 14N

semi-conservative replication

Results• In the first generation, all DNA was mid-

way between that of light and heavy DNA, so it contained equal amounts of each.

• In the second generation there were two sorts of DNA, one was light and the other contained equal amounts of 15N and 14N (i.e. it was like the DNA in the first generation bacteria).

• Throughout the investigation the DNA from the control culture produced only light bands, indicating it contained only 14N