Plasmid selection and confirmation separate recovering candidates from establishing what they contain. In an original idealized comparison, stipulate a susceptible recipient and a selectable marker that permits growth under the chosen condition. The intended 6000-base pair construction and an empty carrier retaining that marker can both yield biological colonies. If removal of the 1500-base pair plasmid insert followed by DNA ligation joins its two boundaries without losing other material, the empty carrier is base pairs. Selection for their shared selectable marker cannot distinguish these candidates.
Use separate scientific controls for separate uncertainties. A recipient-only comparison tests background growth under selection. A known intact plasmid tests whether recipient preparation, genetic transformation and recovery can yield selected carriers. A carrier-only construction comparison tests recovery in the absence of added plasmid insert. Growth on supporting cell culture medium tests viability under that supporting condition. These are analytical comparisons for the constructed example, not a list claimed to have been performed in the 1973 plasmid construction experiment.
Recover candidate DNA, compare a complete restriction digest with the stipulated pattern, and inspect relevant joins by DNA sequencing. Each step narrows the claim: selection supports a growth phenotype; length comparison supports size and some structural features; DNA sequencing tests the letters sampled. Even a confirmed nucleotide sequence does not alone show the intended biological function. That requires an appropriate readout such as functional complementation, with its own scientific controls and recipient genetic background.
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