Priming the engine of DNA synthesis
Synthesis of new DNA is carried out by DNA polymerases that copy DNA templates. Curiously, these enzymes cannot start DNA strands; they can only extend preexisting ‘‘primers’’ that have a 30-OH group available to attack the a-phosphate of incoming complementary dNTPs. It is the job of the primases to produce the initiating (RNA) primers for every DNA strand. Primases are unusual DNA-dependent RNA polymerases that generate short segments of RNA:DNA heteroduplex, and in bacteria, the primase activity is attributed to the DnaG protein.