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

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