Are you talking about eukaryotic cells?if you are,the replication begins at multiple points at the same time,because in that way,it speeds up the process.
For instance,the average human chromosome contains 150,000,000 nucleotide pairs which are copied at about 50 base pairs per second.The process would take a month (rather than the hour it actually does) but for the fact that there are many places on the eukaryotic chromosome where replication can begin.That makes it can complete the replication in about 1 hour.
