I have read that DNA polymerase requires a primer to bind to the DNA, but I am confused as to why this is the case. When DNA undergoes replication in the cell, there are no primers in the nucleus. Why are they needed in PCR?
Many thanks :)
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$\begingroup$ Who said that there are no primers in the nucleus? Read about Primase. I advise that you pick up a good book on basic molecular biology and read the chapter on replication. Genes by Lewin is a decent book. $\endgroup$– WYSIWYGNov 24, 2015 at 16:57
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$\begingroup$ @WYSIWYG Thank you for your reply. I see now that there is RNA primase in the nucleus- this makes more sense! However i'm now confused as to why an RNA strand is synthesised? I understand that RNA polymerase works on only one strand and DNA polymerase works on two, however does it not make a difference to the finished product that the ends of the DNA strands are actually RNA? $\endgroup$– MeepNov 24, 2015 at 17:21
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$\begingroup$ There is a question about this here. DNA polymerase cannot synthesize without primers but RNA polymerase can. Primase is a kind of RNA polymerase. $\endgroup$– WYSIWYGNov 24, 2015 at 17:34
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Actually, when DNA replicate in cell, there is primer exists. DNA polymerase only can add deoxyribonucleotide (dNTP) to the 3’end of a growing DNA chain, instead of creating a new strand. Here is the picture for DNA replication in legging strand, but is same principal use in leading strand. Primer is synthesized by primase in replication.
It is also worth to mention that, looking at the last step, there is a primer there and must be removed, and DNA maybe shorter in each DNA replication time, and that is why DNA telomere mechanism is proposed. That may be a key point for aging.
So, in PCR, it also needs primers, for DNA polymerase only can only can add dNTP to the 3’end of a growing DNA chain.