why must PCR primers be present in great excess at the beginning of a PCR reaction?

Question

Please! Anyone who could shed some light on this question or where I should look for the answer. Your help will be greatly appreciated.
Thank you

Answer 1

PCR primers are used to bind to the desired sequence of DNA that you want to amplify. You are starting with a sample of unknown numbers of DNA pieces, so you want to make sure that you have plenty of primers to bind specifically to your sequence on each DNA strand.
Remember that it is the primer that allows for polymerization of the DNA strand since it provides the free hydroxyl group that polymerase enzyme neede to add new nucleotide. You start with a few copies of DNA and end with about a billion, so you need enough primers to make this happen.

For a visual, check out http://www.mun.ca/biology/scarr/Gr12-26.html

For a great interactive website that starts with a bacterial colony and ends with a DNA sequence (amplified using PCR and then sequenced using capillary electrophoresis), see http://www.hhmi.org/biointeractive/vlabs/ and enter Bacterial ID lab. The accompanying readings are very valuable.