Question

Why does PCR fail if primers are not added?

Answer 1

Emma, there is a two-part answer to your question. The first reason is that DNA polymerase has to sit on a small region of double stranded DNA before it can travel down the single (template) strand of DNA and start adding new complementary bases to it.

The second and more important reason is that primers guide the DNA polymerase to amplify a very specific region of DNA (like a special gene you want to produce lots of DNA copies of). This is because primers are short single stranded segments of DNA which is complementary to the genetic code of a region of interest. (Say you were interested in examining a region of DNA which you knew was different between suspect A and suspect B in a forensics science case. To do so, you would first amplify up that region of interest by doing PCR - and then identify the changes in some other way like gel electrophoresis. To do so, you would first work out the gene sequence of interest, and design primers which can bind before and after that region of interest, so that the DNA polymerase knows what is important to needs to be amplified). There's a lot of DNA around, and usually people want to amplify specific genes that are of interest to them.