Question

Discuss how important primers are in a biochemical context. How do primers work?

Answer 1

Primers are short stretches of single stranded oligonucleotides that can be RNA primers (made up of ribonucleotides and used by living organisms) or DNA primers (made up of deoxyribonucleotides and used in laboratories). In biochemistry, primers are used during the process of DNA replication. The DNA dependent DNA polymerase cannot initiate the de novo synthesis of DNA. It needs a primer in order to initiate replication. The primers are synthesized by enzymes called primase which synthesize primers on the leading and the lagging strands.

How primers work:

The primers provide the 3'-OH end that is then further elongated by DNA polymerase. The DNA polymerase recieves the incoming nulceotide (dNTPs) according to base pairing complimentarity with the template strand and adds then to the 3'-OH of the primer, thereby forming a phosphodiester bond between them. The leading strand is elongated along the direction of replication fork and hence only one primer is sufficient. The lagging strand is synthesized in the opposite direction discontinuously and hence each fresh primers are required for each fragment (Okazaki fragment).

Another use of primers is found for the enzyme Reverse transcriptase, which is an RNA dependent DNA polymerase. It requires the presence of primers in order to synthesize DNA (using RNA as template).

In vitro biochemical experiments often involve the use of synthetically made primers in Polymerase chain reactions (PCR) in order to amplify DNA.

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