Hand-tinted transmission electron micrograph showing the DNAs of two different, but related, bacterial viruses partially spliced together, producing a so-called heteroduplex DNA molecule. The two viral DNAs differ in length and content. The TEM shows both substitution loops (red), in which the two DNAs do not match, and two deletion loops (blue), where one of the DNAs has additional sequences not possessed by the other. Where the two DNAs do match, and as a result are spliced together, they are coloured yellow. The hetero- duplex is between the DNAs of two wild-type lambda bacteriophages.