DNA replication is a complex process that requires multiple molecules
to work together to enable replication to occur. "Research in the
past two decades has identified an ensemble of more than 20 proteins
involved in the process of replication initiation, illustrating the
complexity involved in coordinating initiation from hundreds or
thousands of origins (Bell and Dutta, 2002; Mendez and Stillman,
2003)."
"DNA replication and progression through S phase" by David Y Takeda1
and Anindya Dutta, Oncogene (2005) 24, 2827?2843
http://genome.bioch.virginia.edu/DNARepl/papers/Review.pdf. This
source provides the most comprehensive examination of current
knowledge regarding the processes involved in the initiation of DNA
replication I could locate.
I have also provided a variety of additional sources discussing
various aspects of initiating factors for DNA replication and cell
division in general. "Chapter 21: Regulating the Eukaryotic Cell
Cycle" by Lodish et al., Molecular Cell Biology, Fifth edition, W. H.
Friedman & Company (2004)
http://lifesci.rutgers.edu/~denhardt/Course/Lect2.dtd.cellcycle.pdf
provides a good overview of the eukaryotic cell cycle and its control
mechanisms, including those involved in DNA synthesis.
The following PowerPoint presentation discusses initiating and
regulating factors in DNA replication:
"Limiting Replication to Once per Cell Cycle
The Licensing Model
Factors recruited to origin prior to S-phase (G1), required for
replication to occur (licensing).
Licensing factor puts origin in replication competent state.
Origins are triggered to ?fire? at beginning of S-phase by ?S-phase
promoting factor? (SPF).
Replication initiates, replication forks leave behind a 'spent' origin."
"Limiting Replication to Once per Cell Cycle
Regulation of Eukaryotic Origins
Licensing Factor - MCM protein complex (helicase is loaded onto
origins in late M-G1)
Pre-replicative complex (PRC) - ?licensing? formed by recruitment of
MCM by CDC6 protein.
S-phase Promoting Factor (SPF) - protein kinases activated to initiate
S-phase (CDC28-cyclin, CDC7)."
"Biochemistry 410" (September 28, 2005)
http://www.biochem.ubc.ca/coursematerials/410IJSsept28pptnotes.ppt
"Eukaryotes typically possess large and complex genomes, which require
multiple origin points that initiate their replication. These sites
are called ORCs (origin recognition complexes). "
"A process known as licensing ensures that chromosomes are able to be
replicated only after they have successfully passed through mitosis
and find themselves in a new cell. And the mechanisms of licensing are
slowly being teased apart, uncovering a networks of control systems
with built in safety features.
To understand the control of the initiation of DNA replication, we
first need to consider some of the basic steps in the initiation of
eukaryl DNA replication. First, chromosomes contain multiple sets of
sequences that bind to a group of proteins to form the ORC. This
multi-protein/DNA complex remains intact throughout the cell cycle,
yet is completely insufficient to trigger DNA replication. To trigger
DNA replication, another hexameric protein complex, known as the MCM
(minichromosome maintenance) complex must be recruited to the ORC. It
is the MCM which effectively licenses the DNA for replication, as it
is composed of several helicases that unwind the DNA to facilitate
replication. So how do you get the MCM on the ORC? Two other proteins
work in concert to recruit MCM. The first is known as Cdc6. When the
cell is ready to replicate its DNA, Cdc6 is recruited by the ORC.
Another protein, Cdt1, is then loaded and it potentiates the activity
of Cdc6. Yet another protein, known as mcm10 may also be recruited to
facilitate the loading of MCM (since MCM does not seem to physically
contact Cdc6) [3]."
"The design problem that is solved by this level of complexity is
this: once you load the MCM onto a replication origin, you don't want
to reload another MCM until after synthesis has been completed and
mitosis has occurred. And one way this is ensured is through the use
of all three components: the ORC proteins, the Cdc6/Cdt1 complex, and
the MCM.
It turns out that all three complexes can be independently
phosphorylated by a protein kinase known as Clb-Cdc28. This kinase has
a dual role. Once the entire initiation complex has formed, it is
"turned on" by Clb-Cdc28 (which was just previously activated itself)
through phosphorylation. Yet the phosphorlyated complexes have not
only been altered to initiate replication, but can no longer
reinitiate replication."
"INITIATING DNA REPLICATION IN EUKARYOTES: Another Story About Smart
Cells" by Mike Gene, ID Think http://www.idthink.net/biot/dna/
"But what proteins are needed to trigger initiation of DNA
replication, and how do they intersect the other proteins and
processes involved? Two sets of factors are required. One factor,
called the origin recognition complex (ORC), recognizes origins of
replications, and signals to the DNA replication apparatus where to
carry out its function.... Another factor 'licenses' DNA replication,
that is, it gives the go ahead for initiation of DNA replication.
Information from other organisms, particularly yeast and Xenopus, has
allowed the identification of a group of genes that code for called
minichromosome maintenance (MCM) proteins that serve to license DNA
replication. MCM proteins regulate initiation of DNA synthesis in a
cell cycle dependent fashion."
"Biological Overview" By Thomas B. Brody (March 7, 1998)
http://flybase.bio.indiana.edu/allied-data/lk/interactive-fly/polycomb/discpab1.htm
"The master controller is the cyclin-dependent kinase.
The three major cell cycle events - DNA replication, mitosis and cell
division, are controlled by cyclin-dependent kinases (Cdks), which are
active only when bound to a cyclin partner.
Yeast cells have only one essential Cdk subunit, referred to as Cdk1,
and they use different B-type cyclins in combination with the same
Cdk1 subunit to initiate S phase and M phase. The situation in
mammalian cells is more complex, as they have other important Cdks
(Cdk2, 4 and 6) in addition to Cdk1. In budding yeast, Cdk1 binds with
Clb5 (an S-phase cyclin) to form a Cdk1/Clb5 complex (also called
S-phase promoting factor or SPF for short) which can phosphorylate
proteins at the origin of replication and trigger the onset of DNA
replication. By binding with Clb2 (an M-phase cyclin) and forming a
Cdk1/Clb2 complex (referred to as M-phase promoting factor, MPF), Cdk
phosphorylates histones and initiates mitosis. MPF can substitute for
SPF in initiating S phase, but the reverse is not possible."
"Generic Model of Eukaryotic Cell Cycle Control: Biological Details"
(June 15, 2005) http://jigcell.biol.vt.edu/generic_model/GenericBio.html
"Initiation of DNA replication requires the ordered assembly of
macromolecular complexes at replication origins (Diffley and Labib,
2002). The first step in the assembly of the initiation complex
involves the binding of MCM proteins to ORC-associated chromatin
during the G1 phase of the cell cycle to form pre-Replicative Complexs
(pre-RCs). Binding of DNA polymerases and other accessory proteins to
pre-RCs follows this step and is required to support both primer
synthesis and strand elongation. Activation of DNA synthesis at the G1
to S phase transition requires both Cdk and Cdc7 kinase activities.
These kinases are believed to induce a conformational change within
the pre-RC that allows loading of DNA polymerases and final assembly
of the replication complex (Sclafani et al., 2002)."
"DNA REPLICATION CHECKPOINTS IN FISSION YEAST" by Gennaro D?Urso,
University of Miami School of Medicine http://www.med.miami.edu/mnbws/DUrso-.html
"We have shown that activation of the mammalian DNA replication
machinery is catalysed by cyclin A-cdk2, and that cyclin E-cdk2 plays
an essential role in the assembly of DNA replication complexes in
cells that re-enter the cell cycle from the quiescent state. More
recently we identified a new protein, Ciz1 and demonstrated that it is
involved in mammalian DNA replication. "
"Dr. Dawn Coverley " University of York
http://www.york.ac.uk/depts/biol/staff/dc.htm
Search terms: S-phase promoting factor; Cyclin A-CDK2; replication
licensing factor S-phase promoting factor; MCM proteins; DNA
replication signal |
