Replication - primer - please help!

[woi]

I've been so frustreted these past few days because i can't find the answer for this question - why are primers consisted of ribonucleotides and not of

deoxyribonucleotides? I know that the DNA polymerase can not start the replication process because they have the proofreading activity, so DNA polymerase alfa start the replication process and adds a couple of hundred ribonucleotides and after that DNA polymerase delta adds on that a couple of thousand deoxyribonucleotides and so on...But why does DNA polymerase alfa add ribonucleotides instead of deoxy? it would be much easier, no enzymes would be needed after to elimniate the primers.

Please help me with this! I know that the answer must be very simple but my mind is deffinetly stuck at this point...

[CharonY]

There are a number of reasons why primases are needed. The simplest one is that Primases (which are DNA-dependent RNA-polymerases) are able to directly synthesize onto RNA onto ssDNA, whereas DNA-dependent DNA polymerases, involved in the actual elongation, lack this ability. However, this synthesis step is somewhat error prone and by having another polymerase run over that bit again decreases the risk of replication errors.

If the primases would add desoxyribonucleotides there would be no easy way to recognize these bits (which is otherwise done be RNase H in case of DNA-RNA hybrids).