The term replisome is used to refer to the multiprotein replication complex. Numerous replication systems from E.coli, bacteriophage, and viral systems have been described. The first eukaryotic replication system to be purified was for the small circular DNA genome of the virus SV40. Table 24.4 lists the functions of the proteins in three reconstituted DNA replication systems.

Figure 24.36 shows a generalized replication scheme for single strand phage DNA. The main points are as follows:

1. The single plus (+) strand genome enters cells. It serves as the template for the synthesis of the complementary minus (-) strand.

2. The duplex, called RFI, has superhelical turns introduced into it.

3. A site-specific initiation protein nicks the DNA at a specific sequence in the (+) strand and attaches to its 5' end.

4. Replication proceeds via extension of the 3' end. The 5' end is displaced in a rolling circle mechanism and single strand DNA binding proteins attach to the displaced strand.

5. 5' to 3 extension continues.

6. Completion of one full circle of replication causes a protein to nick and release the original plus strand and generate a new duplex (called RFII) containing the original minus strand.