What would be the affect on pcr reaction, if primers are not added in the reaction?

In absence of the primer the reaction lacks a primer template junction, and thus a free 3′-hydroxyl group will cause the reaction to fail.

Explanation:

In the PCR reactions primers are used to bind to ssDNA template hybridize itself and define the segment or part of the sample that will be amplified. This hybridize template makes the primer template junction.

If primers not added this junction would not be created then the enzyme DNA polymerase cannot make new DNA due to there is no free 3′-hydroxyl group which is essential to bind with the first deoxynucleotide.

Thus, In absence of the primer the reaction lacks a primer template junction, and thus a free 3′-hydroxyl group will cause the reaction to fail.