Expert: Dana Krempels, Ph.D. - 6/24/2010

Question
My question is: Why an RNA primer is needed to begin the synthesis of Okazaki fragments in the lagging strand of the DNA and not a DNA primer? I know that the probability of RNA primase to make a mistake is very low compared to DNA primase, but why is that?

Thanks in advance!

Answer
Dear Anastas,

This is one of those "we don't really know" evolutionary questions.  It does seem like a waste of energy to make an RNA primer, only to remove it and replace it with DNA.  But this is simply how the primase works in our cells (and those of prokaryotes).

In  some cases, we try to ascribe a selective advantage to something like this.  But it's hard to imagine how this could be adaptive.  Still, it works.  And since all cells do this, it obviously was either the only way enzymes evolved to manufacture new DNA strands (and remember, it is very likely that RNA was the *original* genetic material, so this could simply be a holdover from those simpler times), or that the energy spent is  not enough to confer a selective disadvantage on cells using this mechanism.

Wish I had a magic answer for you!  But this is one of those hypothetical areas where we may never know.

Take care,

Dana