Please look here for details on how Transactional replcaition works

http://msdn.microsoft.com/en-in/library/ms151706%28v=sql.105%29.aspx

I think you ahv confused it with snapshot replcaition which needs to occur they first time in order to have a synced copy of the databases to begin with. The agents in transactional replcaition can be scheduled or running continously.