Licensing factor
Encyclopedia
A Licensing Factor is a protein or complex of proteins that allows an origin of replication
to begin DNA replication
at that site. They are thought to primarily occur in Eukaryotic cells, since Prokaryotes use simpler systems to initiate replication.
of the cell in question. The origins are required to fire only once per cell cycle
, an observation that lead to the postulated existence of licensing factors by biologists in the first place. If the origins were not carefully regulated then DNA replication could be restarted at that origin giving rise to multiple copies of a section of DNA. This could be very damaging to cells and could have detrimental effects on the organism as a whole and so is to be avoided.
The control that licensing factors exert over the cycle represents a flexible system, necessary so that different cell types in an organism can control the timing of DNA replication to their own cell cycle.
has finished the cell cycle starts again, entering G1 phase of the cycle. At this point protein synthesis of various products required for the rest of the cycle begins. Two of the proteins synthesised are called Cdc6 and Cdt1 and are only synthesised in G1 phase. These two together bind to the Origin Recognition Complex
(ORC), which is already bound at the origin and in fact never leaves these sites throughout the cycle. Now we have a so-called Pre-Replication Complex, which then allows a heterohexameric protein complex of proteins MCM2 to 7 to bind. This entire hexamer acts as a helicase unwinding the double stranded DNA. At this point Cdc6 leaves the complex and is inactivated, by being degraded in yeast but by being exported from the nucleus in metazoans, triggered by CDK
-dependent phosphorylation. The next steps included the loading of a variety of other proteins like MCM10, a CDK, DDK and Cdc45, the latter directly required for loading the DNA Polymerase
. During this period Cdt1 is released from the complex and the cell leaves G1 phase and enters S phase when replication starts.
From the above sequence we can see that Cdc6 and Cdt1 fulfil the role of licensing factors. They are only produced in G1 phase in addition to which binding of all the proteins in this process excludes binding of additional copies. In this way their mode of action is limited to starting replication once since once they have been ejected from the complex by other proteins, the cell enters S phase when they are not re-produced or re-activated. Taking this into account it becomes clear that they can indeed act as licensing factors but only together. It has been suggested that the whole Pre-Replication Complex be called the licensing factor since the whole is required for assembling additional proteins to initiate replication.
Origin of replication
The origin of replication is a particular sequence in a genome at which replication is initiated. This can either be DNA replication in living organisms such as prokaryotes and eukaryotes, or RNA replication in RNA viruses, such as double-stranded RNA viruses...
to begin DNA replication
DNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
at that site. They are thought to primarily occur in Eukaryotic cells, since Prokaryotes use simpler systems to initiate replication.
Function of licensing factors
Origins of replication represent start sites for DNA replication and so their "firing" must be regulated to maintain the correct KaryotypeKaryotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
of the cell in question. The origins are required to fire only once per cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
, an observation that lead to the postulated existence of licensing factors by biologists in the first place. If the origins were not carefully regulated then DNA replication could be restarted at that origin giving rise to multiple copies of a section of DNA. This could be very damaging to cells and could have detrimental effects on the organism as a whole and so is to be avoided.
The control that licensing factors exert over the cycle represents a flexible system, necessary so that different cell types in an organism can control the timing of DNA replication to their own cell cycle.
Location of licensing factors
The factors themselves are found in different places in different organisms. For example in metazoan organisms, they are commonly synthesised in the cytoplasm of the cell to be imported into the nucleus when required. The situation is different in yeast where the factors present are degraded and resynthesised throughout the cell cycle but are found in the nucleus for most of their existence.Example of licensing factor action in Saccharomyces cerevisiae, brewers' yeast
Immediately after mitosisMitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
has finished the cell cycle starts again, entering G1 phase of the cycle. At this point protein synthesis of various products required for the rest of the cycle begins. Two of the proteins synthesised are called Cdc6 and Cdt1 and are only synthesised in G1 phase. These two together bind to the Origin Recognition Complex
Origin Recognition Complex
ORC or Origin Recognition Complex is a multi-subunit DNA binding complex that binds in all eukaryotes in an ATP-dependent manner to origins of replication....
(ORC), which is already bound at the origin and in fact never leaves these sites throughout the cycle. Now we have a so-called Pre-Replication Complex, which then allows a heterohexameric protein complex of proteins MCM2 to 7 to bind. This entire hexamer acts as a helicase unwinding the double stranded DNA. At this point Cdc6 leaves the complex and is inactivated, by being degraded in yeast but by being exported from the nucleus in metazoans, triggered by CDK
Cyclin-dependent kinase
thumb|350px|Schematic of the cell cycle. outer ring: I=[[Interphase]], M=[[Mitosis]]; inner ring: M=Mitosis; G1=[[G1 phase|Gap phase 1]]; S=[[S phase|Synthesis]]; G2=[[G2 phase|Gap phase 2]]...
-dependent phosphorylation. The next steps included the loading of a variety of other proteins like MCM10, a CDK, DDK and Cdc45, the latter directly required for loading the DNA Polymerase
DNA polymerase
A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....
. During this period Cdt1 is released from the complex and the cell leaves G1 phase and enters S phase when replication starts.
From the above sequence we can see that Cdc6 and Cdt1 fulfil the role of licensing factors. They are only produced in G1 phase in addition to which binding of all the proteins in this process excludes binding of additional copies. In this way their mode of action is limited to starting replication once since once they have been ejected from the complex by other proteins, the cell enters S phase when they are not re-produced or re-activated. Taking this into account it becomes clear that they can indeed act as licensing factors but only together. It has been suggested that the whole Pre-Replication Complex be called the licensing factor since the whole is required for assembling additional proteins to initiate replication.