1. Unwinding of the DNA double helix: Replication starts with the unwinding of the DNA double helix, which is tightly coiled to fit inside the cell. This unwinding creates two "replication bubbles," with the unwound DNA forming the replication forks at the center.
2. Binding of the helicase enzyme: The unwinding process is facilitated by an enzyme called helicase. Helicase binds to the DNA at the origins of replication and separates the two strands of the double helix, breaking the hydrogen bonds between complementary nucleotides.
3. Stabilization of the unwound DNA: As the DNA double helix unwinds, proteins called single-stranded DNA-binding proteins (SSBs) bind to the separated strands to prevent them from reannealing. These SSBs stabilize the unwound DNA and help maintain the replication fork.
4. Formation of the replication complex: At each replication fork, a replication complex forms. This complex includes multiple proteins and enzymes, including DNA polymerase (the enzyme that synthesizes new DNA strands), primase (an enzyme that synthesizes short RNA primers), and auxiliary factors involved in proofreading and maintaining the replication fork structure.
5. Synthesis of RNA primers: DNA polymerase, which can only extend existing DNA strands, requires a starting point for DNA synthesis. Primase synthesizes short RNA primers complementary to the template DNA strands. These primers provide a free 3' end for DNA polymerase to attach to and start DNA synthesis.
6. DNA synthesis by DNA polymerase: DNA polymerase binds to the RNA primers and starts synthesizing new DNA strands by adding nucleotides that are complementary to the template strand. The nucleotides are linked together by phosphodiester bonds, extending the growing DNA strands in the 5' to 3' direction.
7. Proofreading and correction: As DNA polymerase synthesizes new DNA strands, it also proofreads the newly added nucleotides to ensure accuracy. If a間違えたnucleotide is incorporated, the DNA polymerase can remove it and replace it with the correct nucleotide. This proofreading mechanism helps to maintain the fidelity of DNA replication.
The process of replication continues bidirectionally from each origin of replication, with the two replication forks moving in opposite directions until the entire genome is replicated. Once replication is complete, the RNA primers are removed, and the gaps they left behind are filled in by DNA polymerase. The ends of the newly synthesized DNA strands are then sealed by an enzyme called DNA ligase, completing the DNA replication process.
