1. G1 Phase (First Gap Phase):
* Cell Growth: The cell increases in size and synthesizes new proteins and organelles.
* Checkpoints: The cell undergoes a crucial checkpoint to assess if it's ready to proceed with DNA replication. It checks for:
* Sufficient nutrients and energy reserves: This ensures the cell has the resources to support DNA replication and subsequent cell division.
* Absence of DNA damage: Any damaged DNA needs to be repaired before replication to avoid errors in the new cells.
* Presence of growth factors: These signals tell the cell to proceed with the cell cycle.
2. S Phase (Synthesis Phase):
* DNA Replication: The cell replicates its entire genome, ensuring each daughter cell receives an identical copy. This process involves:
* Unwinding the DNA: The double helix of DNA is unwound by enzymes like helicase.
* Separation of DNA strands: The two strands are separated, serving as templates for new strands.
* Building new DNA strands: DNA polymerase enzymes synthesize new complementary strands of DNA.
* Proofreading: DNA polymerase ensures accuracy by proofreading the newly synthesized DNA.
3. G2 Phase (Second Gap Phase):
* Further Growth and Protein Synthesis: The cell continues to grow and produces additional proteins required for mitosis, such as those involved in chromosome segregation and spindle formation.
* Checkpoints: The cell undergoes another checkpoint to assess if the DNA replication is complete and error-free:
* Correct DNA replication: Ensures that the newly synthesized DNA is free from errors and complete.
* Presence of necessary proteins: Checks if the cell has the proteins needed for proper chromosome condensation and spindle formation.
In summary:
* Growth and preparation: The cell needs to grow and accumulate necessary resources and proteins.
* DNA replication: The cell must duplicate its DNA to provide each daughter cell with a complete genome.
* Quality control: The cell must ensure the duplicated DNA is error-free and that it has the necessary proteins and resources to proceed with mitosis.
Only once these crucial events are successfully completed can a cell enter the mitotic phase, resulting in two genetically identical daughter cells.
