DNA primaseuses ribonucleoside triphosphates to synthesize short RNA primers . These primers are about 10 nucleotides long in eucaryotes, and they are made at intervals on the lagging strand, where they are elongated by the DNA polymerase to begin each Okazaki fragment.

WHY MIGHT AN ERASABLE RNA PRIMER BE PREFERRED TO A DNA PRIMER THAT NEED NOT BE ERASED?

The argument that a self-correcting polymerase cannot start chains de novo also implies its converse:

an enzyme that starts chains de novo cannot be efficient at self-correction. Thus any enzyme that

primes the synthesis of Okazaki fragments will of necessity make a relatively inaccurate copy (at least 1 error in 105). Even if the copies retained in the final product constituted as little as 5% of

the total genome (for example, 10 nucleotides per 200-nucleotide DNA fragment), the resulting

increase in overall mutation rate would be enormous. It therefore seems likely that the evolution

of RNA rather than DNA for priming entailed a powerful advantage, since the ribonucleotides in

the primer automatically mark these sequences as "bad copy" to be removed.