Final answer:
DNA is added into the PT21A plasmid, which is an extrachromosomal circular DNA molecule used as a vector in molecular biology for gene manipulation.
Step-by-step explanation:
The student is asking about the contents added to the PT21A plasmid. Based on the information provided, the PT21A plasmid would have DNA added into it. A plasmid is an extrachromosomal, covalently closed, circular DNA molecule, which is a common vector used in molecular biology for cloning and gene manipulation. It is particularly prevalent in prokaryotes. The process of inserting DNA into a plasmid involves the use of enzymes to add a poly-A tail or similar modifications to the DNA to facilitate its insertion into the vector. The poly-A tail is a modification added to the 3' end of pre-mRNAs—which refers to messenger RNA and not the plasmid DNA directly—to protect mRNA from degradation and assist in its export from the nucleus.
When considering the historical context of DNA experiments, such as those conducted by Hershey and Chase, it helps clarify why DNA is the material used for genetic manipulation. Hershey and Chase's experiment demonstrated that genetic material responsible for viral replication is DNA, not protein. Therefore, for genetic engineering and producing recombinant DNA, scientists would introduce foreign DNA into the plasmid and not RNA or protein.