The professor needs a secondary transcript when making the recombinant plasmid because it is used as a marker. A molecular marker can be used to reveal the expression of another gene.
<h3>Plasmids and molecular markers</h3>
A plasmid is a circular fragment of a DNA molecule, which is not contained in the genome of an organism (single chromosome in bacteria).
A molecular marker is any sequence that is used to determine the presence/expression of another sequence in a given genetic material.
In this case, the marker can be used to reveal the presence of the oxytocin protein-coding gene within the plasmid.
Learn more about molecular markers here:
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