What is the role of a primer in DNA replication? What would happen if you forgot to add a primer in a tube containing the reaction mix for a DNA sequencing reaction?

The role of a primer is to provide to the DNA polymerase a 5' end, from where start to synthesize the new straid. DNA polymerase can not work on 3' end direction, so if you forget to add the primers on your PCR mix reaction, when you procede to reveal the result, you will see nothing. The rection would never take places, becouse polymerase could not act.