Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

260 Chapter 5: DNA Replication, Repair, and Recombination
G 1
DNA
Cdt1
Mcm helicase
Cdc6
origin
prereplicative complex
ORC (origin recognition complex)
+
PHOSPHORYLATION
OF Mcm AND ORC
HELICASES ACTIVATED; ORC DISPLACED
RECRUITMENT OF DNA POLYMERASE
AND OTHER REPLICATION PROTEINS;
ORC REBINDS; DNA SYNTHESIS BEGINS
Figure 5–31 DNA replication initiation in
eukaryotes. This mechanism ensures that
each origin of replication is activated only
once per cell cycle. An origin of replication
can be used only if a prereplicative
complex forms there in G 1 phase. At the
beginning of S phase, specialized kinases
phosphorylate Mcm and ORC, activating
the former and inactivating the latter. A new
prereplicative complex cannot form at the
origin until the cell progresses to the next
G 1 phase, when the bound ORC has been
dephosphorylated. Note that the eukaryotic
Mcm helicase moves along the leadingstrand
template, whereas the bacterial
helicase moves along the lagging-strand
template (see Figure 5–25). As the forks
begin to move, ORC is displaced, and new
ORCs rapidly bind to the newly replicated
origins.
S
P
P
P
P
COMPLETION OF
DNA REPLICATION
P
G 2
P
The protein kinases that trigger DNA replication simultaneously prevent
assembly of new prereplicative complexes until the next M phase resets the entire
cycle (for details, see pp. 974–975). They do this, in part, by phosphorylating ORC,
rendering it unable to accept new helicases. This strategy provides a single window
of opportunity for prereplicative complexes to form (G 1 phase, when kinase
activity is low) and a second window for them to be activated and subsequently
disassembled (S phase, when kinase activity is high). Because these two phases of
the cell cycle are mutually exclusive and occur in a prescribed order, each origin
MBoC6 n5.600/5.31
of replication can fire once and only once during each cell cycle.
Features of the Human Genome That Specify Origins of
Replication Remain to Be Discovered
Compared with the situation in budding yeast, the determinants of replication
origins in other eukaryotes have been difficult to discover. It has been possible to
identify specific human DNA sequences, each several thousand nucleotide pairs
in length, that are sufficient to serve as replication origins. These origins continue
to function when moved to a different chromosomal region by recombinant DNA
methods, as long as they are placed in a region where the chromatin is relatively
uncondensed. However, comparisons of such DNA sequences have not revealed
specific DNA sequences that mark origins of replication.