2 Origins of DNA Replication

Abstract

Initiation of DNA replication involves three sequential steps. First, one or more specific trans-acting proteins bind to specific cis-acting DNA sequences referred to as origins of replication (DePamphilis 1993a,b; Kornberg and Baker 1992). Second, DNA unwinding of the two complementary templates begins. This step is usually carried out by a DNA heli-case but can also be done by some DNA polymerases (e.g., adenovirus [Ad]). DNA unwinding is facilitated by single-strand-specific DNA-binding proteins such as replication protein A (RP-A), which coat the templates, and by topoisomerase I, which releases torsional stress generated by unwinding DNA. Third, DNA synthesis is initiated on one or both templates. In cellular chromosomes and DNA viruses that do not encode their own DNA polymerase (e.g., SV40, PyV, and PV), DNA polymerase-α:DNA primase complex synthesizes a short RNA-primed nascent DNA chain referred to as an Okazaki fragment. The first Okazaki fragment initiated on each template is extended continuously by DNA polymerase-δ and its accessory proteins to become the long nascent DNA strand on the forward arm of each of the two replication forks. The net result of these steps is bidirectional DNA replication employing bubble and fork structures such as those found in the chromosomes of prokaryotic and eukaryotic cells (Fig. 1). DNA replication is coupled to chromatin assembly, resulting in the random distribution of pre-fork histone octamers to both arms of the fork and rapid assembly of new histone octamers in the intervening regions of newly replicated DNA. Initiation of DNA replication can also occur in...