6.6.4.3 Initiation of DNA Replication. Most of our current knowledge of how DNA replication is
initiated comes from prokaryotes. Bacterial chromosomes have individual origins of replication. InE. coli
this is called oriC. OriCis about 250 nucleotide pairs long and has the structure shown in Figure 6.28. Various
proteins interact with the origin of replicationto initiate copying of the DNA. DnaA protein molecules
bind first to a 9 bp motif, then to each other to form a complex. This leads to unwinding of the helix in the
A/T-rich 13 bp motifs. Thereafter, a DNA helicase, the DnaB protein, in concert with the DnaC protein, binds
to form a pre-priming complex.
6.6.4.4 DNA Elongation. During the elongation process (Figure 6.29), the DnaB helicase protein
migrates along the duplex, attached to the ‘lagging strand’, breaking base pairs as it goes.^50 Every turn of the
double helix that is removed in this way generates an extra turn ‘upstream’ of the fork, which is relieved by
the enzyme DNA topoisomerase (Section 2.3.5). The newly generated DNA single strands are protected by
single strand binding proteins(SSBs) from DNA damage or unwanted binding to other nucleic acids or pro-
teins. These are later displaced as DNA polymerase moves in to make the new complementary lagging strand.
The leading strand can be copied without any discontinuity (Figure 6.30) but the lagging strand requires
a new primer every thousand or so nucleotides. This is synthesised by the primase and then the DNA poly-
merase (DNA polymerase III in E. coli) takes over, extending the primer for about 1000 nucleotides before
234 Chapter 6
Figure 6.29 Elongation of DNA synthesis involves DnaB helicase and DNA topoisomerase
Figure 6.28 Structure of the E. coli origin of DNA replication OriC