And keeps it in an extended state for dna polymerase

and keeps it in an extended state for DNA polymerase III
 The RNA primers are replaced with DNA by DNA polymerase I and the single
strand nicks left by polymerase I are sealed by DNA ligase
 As a replication fork moves along a parental double helix, two DNA strands
(leading and lagging strands) are replicated
 Replisome
o Complete replication apparatus moving along DNA molecule at a
replication fork
o Contains DNA polymerase III holoenzyme; one catalytic core
replicates the leading strand, the second catalytic core replicates the
lagging strand and the primosome unwinds the parental DNA
molecule and synthesized the RNA primers needed for the
discontinuous synthesis of the lagging strand
 In order for the two catalytic cores of the polymerase III holoenzyme to
synthesize both the nascent leading and lagging strands, the lagging strand is

thought to form a loop from the primosome to the second catalytic core of
DNA polymerase III
 In E. coli, the termination of replication occurs at variable sites within regions
called terA and terB, which block the movement of replication forks
advancing in the counterclockwise and clockwise directions
 DNA topoisomerases facilitate the separation of the nascent DNA molecules
 DNA is condensed into the nucleoid by the negative supercoiling introduced
by DNA gyrase
Rolling-circle replication
 Mechanism for replication circular DNA molecules
 One parental circular DNA strand remains intact and rolls while serving as a
template for the synthesis of a new complementary strand
 Replication is initiated when a sequence-specific endonuclease cleaves one
strand at the origin, producing 3’-OH and 5’-phosphate termini
 The 5’ terminus is displaced from the circle as the intact template strand
turns about its axis
 Covalent extension occurs at the 3’-OH of the cleaved strand
 Since circular template DNA may turn 36- many times, with the synthesis of
one complete DNA strand during each turn, rolling circle replication
generates single-stranded tails longer than the contour length of the circular
chromosome
 To produce double-stranded progeny molecules, the single-stranded tails are
used as templates for the discontinuous synthesis of complementary strands
prior to cleavage and circularization
Key Points
 DNA replication is complex, requiring participation of a large number of
proteins
 All DNA polymerases require a primer strand, which is extended, and a
template strand, which is copied
 Enzymes and DNA binding proteins involved in replication assemble into a
replisome at each replication fork and act in concert as the fork moves along
the parental DNA molecule
Eukaryotic chromosome replication
 RNA primers and okazaki fragments shorted in eukaryotes
 DNA synthesis takes place within a small portion of the cell cycle
 Contain multiple oriC
 Use two or more different polymerases to replicate the leading and lagging
strands at each replication fork
 Bidirectional replication just like in prokaryotes
 DNA replication restricted to S phase
o G1 phase, S phase, G2 phase and M phase