A primer dimer is formed and amplified in three steps. In the first step, two primers anneal at their respective 3 ends (step I in the figure). If this construct is stable enough, the DNA polymerase will bind and extend the primers according to the complementary sequence (step II in the figure). An important factor contributing to the stability of the construct in step I is a high GC-content at the 3 ends and length of the overlap. The third step occurs in the next cycle, when a single strand of the product of step II is used as a template to which fresh primers anneal leading to synthesis of more PD product.