1. Origin Recognition Complex (ORC) Binding:
- ORC is a protein complex that recognizes and binds to specific DNA sequences called autonomously replicating sequences (ARS) in yeast or DNA replication origins in other eukaryotes.
- ORC can bind to DNA even before DNA replication begins.
2. Recruitment of Additional Proteins:
- Once ORC is bound to the DNA, it recruits additional proteins, including Cdc6 and Cdt1, to form a pre-replication complex (pre-RC).
3. Licensing Factors:
- Licensing factors, such as Cdc6 and Cdt1, are essential for the initiation of DNA replication.
- Cdc6 prevents the re-initiation of DNA replication at origins that have already fired.
4. DNA Helicase and Replication Fork Assembly:
- At the appropriate time in the cell cycle, the pre-RC is activated by additional factors, including the helicase MCM (minichromosome maintenance) complex.
- MCM helicase unwinds the DNA double helix, creating a replication bubble around the origin, and two replication forks are assembled, each moving in opposite directions.
5. Elongation and Termination:
- DNA polymerases extend the DNA strands at the replication forks, synthesizing new DNA using the unwound parental DNA as a template.
- DNA replication continues until the entire genome is replicated.
- Replication termination occurs when the replication forks reach specific termination sequences or when they collide with each other.
It's important to note that the detailed mechanisms of replication origin establishment and firing can vary between different species and cell types. The presence of multiple replication origins ensures efficient and coordinated DNA replication of the large eukaryotic genomes.
