the mechanism for microorganism replicating their genome without primer in vivo - (Mar/08/2013 )

I want to ask in vitro, we add buffer, enzyme, template, primer to prepare PCR mixture. When in vivo, there is no primer, how does organism replicate their genome?

good overview: http://en.wikipedia.org/wiki/DNA_replication
(also with eucaryotic and procaryotic DNA replication)