On your first attempt to run a PCR, you realize you forgot to add one of the two primers. The graduate student you work with suggests you throw away your reaction and start over. Why

Explanation:

During  PCR, we use two primers one is forward primer and the other one is reverse primer they match the  sequence of one one of the two complementary strands of the target DNA, they flank the target region (that the region which we has to be copied). if we add only one primer it copies only one strand of the DNA in multiple copies. Usually we call this as Asymmetric PCR.

Generally Primers are synthetic short stretch of oligonucleotides that are complementary to the target DNA. They act as a foundation for the amplification process of  DNA to form multiple copies

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.