Look at the two drawings of his autoradiographs on the next page.Ĭairns measured the length of the “silver” tracks, which usually consisted of three possible closed loops, or circles. He saw tracks of silver grains in the autoradiographs (the same kind of silver grains that create an image on film in old-fashioned photography). After Cairns developed the autoradiographs, he examined the results in the electron microscope. A sensitive film was placed over the membrane and time was allowed for the radiation to expose the film. The DNA released was allowed to settle and adhere to membranes. He then disrupted the cells gently to minimize damage to the DNA. coli cells for long periods on 3Hthymidine (3H-T) to make all of their cellular DNA radioactive. In 1963, John Cairns confirmed this fact by direct visualization of bacterial DNA. Recall the phenomenon of bacterial conjugation allowed a demonstration bacterial chromosomes were circular. Visualizing Replication and Replication Forks
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