= Plasmid selection and confirmation
<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 $6000-1500=4500$ <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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