1. Pre-replication Complex (Pre-RC) Formation:
- Before DNA replication can begin, pre-replication complexes (Pre-RCs) are formed at specific locations called origins of replication.
- Pre-RCs consist of several proteins, including the origin recognition complex (ORC), cell division cycle 6 (Cdc6), and mini-chromosome maintenance (MCM) helicase complex.
2. Activation of Cdc25 and CDKs:
- The start of DNA replication is regulated by the cell cycle checkpoints, which ensure that the conditions are favorable for replication.
- Checkpoints activate protein phosphatases such as Cdc25, which remove inhibitory phosphate groups from cyclin-dependent kinases (CDKs).
- CDK2 forms a complex with cyclin E, and CDK1 forms a complex with cyclin A. These CDK complexes are crucial for promoting the transition to the S phase and initiating DNA replication.
3. Cyclin-dependent Kinase Activity:
- Once activated, cyclin-dependent kinases phosphorylate various components of the pre-RC, leading to the recruitment of additional proteins and the remodeling of the pre-RC into a functional replication fork.
- Phosphorylation events by CDKs trigger the loading of MCM helicase onto the DNA and the unwinding of the double helix, allowing replication to commence.
4. DNA Helicase and Topoisomerase Activity:
- The MCM helicase complex unwinds the DNA duplex at the replication origin, creating a "replication bubble" where replication forks can extend bidirectionally.
- Topoisomerases help relieve the torsional stress caused by unwinding DNA by breaking and rejoining DNA strands, allowing for smooth DNA unwinding and replication fork progression.
5. Replication Factors and DNA Polymerases:
- Replication factors, such as single-strand DNA-binding proteins (SSBs) and replication protein A (RPA), help stabilize the unwound DNA and prevent premature reannealing.
- DNA polymerases, including DNA polymerase α, δ, and ε, are responsible for synthesizing new DNA strands. They add nucleotides to the growing DNA chains, using the parental DNA strands as templates.
6. Coordination of Replication Forks:
- Multiple replication forks are established along each chromosome during DNA replication.
- The firing of origins and the progression of replication forks are coordinated to ensure that the entire genome is replicated efficiently. This coordination involves regulatory factors and mechanisms to prevent collisions between replication forks.
Overall, the initiation of DNA replication in human cells is a complex and finely orchestrated process that relies on the interplay of numerous proteins and regulatory pathways. Proper coordination of these events ensures that DNA replication is accurate, efficient, and synchronized with other crucial cellular processes.
