DNA replication fork

Once DNA replication has been initiated, a large number of enzymes must function to allow DNA polymerase access to the template strand. The DNA is separated into single strands by DNA helicase and spontaneous re-annealing is prevented by single-stranded binding protein (SSB). The primosome is then formed in the developing replication bubble allowing the binding of DNA primase. This primase adds a few complementary RNA nucleotides to the template strand that acts as a primer for the main DNA replication complex DNA polymerase III in Figure 2. The extension of DNA complementary to the template strand means that the replication bubble can be extended quickly.  However it was less clear for many years how the other strand was replicated at the same time and yet still synthesized from 5? to 3?.

An elegant explanation of the extension of the replication bubble was that it was discontinuous again using DNA polymerase III extending from RNA primers. It was found that primase regularly adds short RNA fragments to the lagging strand allowing DNA polymerase III to extend from 5? to 3?.until it meets the next RNA primer where extension stops. The lagging strand is for a short time a DNA: RNA het- eroduplex, before DNA polymerase I functions in its repair mode and removes RNA with an exonuclease activity, replacing it with complementary DNA nucleotides. Any gaps in the newly synthesized lagging strand are then resolved by DNA ligase.  In this way both the template and leading and lagging strands of the replication bubble are quickly and accurately replicated.