Mapping deletion mutations by genetic tools is one thing, seeing them displayed along the length of a DNA molecule is quite another. But how can one see what isn’t there? In 1968, Ronald Davis and Norman Davidson did just that—visualizing deletion mutations in the genome of coliphage l. They reasoned that if the double-stranded DNA chromosomes of wild type phage and of a deletion mutant were dissociated and allowed to anneal together, a deletion would show up in hybrid chromosomes as an (unannealed) single-stranded loop flanked by double-stranded (annealed) DNA regions. They set to work in the lab and successfully confirmed this idea.
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