A denaturation map of the λ phage DNA molecule determined by electron microscopy

The helix—random coil transition of DNA from λ bacteriophage has been studied by electron microscopy. When a λ DNA solution is partially denatured by heating in the presence of formaldehyde, denatured regions are found on the DNA molecules. In many denatured regions one can see two subunits; presumably they are individual DNA strands. Denaturation therefore does not appear to be an all-or-none process. When the position of each denatured region is measured with respect to one end of the molecule, it is found that the sites occur with high frequency at three zones on a DNA denaturation map. The midpoints of these zones correspond to 0·5 2 , 0·7 3 and 0·9 8 of the total map distance. Possibly these regions correspond to adenine- and thymine-rich sequences in the λ DNA molecule.

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