The Cell Cycle
The cell cycle is the fundamental, regular sequence of events that enables a cell to duplicate its contents, grow, and divide into two new cells. It is essential for copying and passing on genetic information to the next generation, producing two genetically identical daughter cells, accurately replicating DNA, equally distributing replicated chromosomes, and coordinating cell growth with division to maintain proper size and contents.
Phases of the Cell Cycle The cell cycle consists of four distinct phases: Interphase (subdivided into G1, S, and G2 phases) and the M phase (Mitosis and Cytokinesis).
- Interphase: This is the long period between cell formation and cell division, during which the cell grows and prepares for division.
- G1 Phase (Gap 1): This is usually the longest and most variable phase of the cell cycle. During G1, the cell actively synthesises RNA and proteins, including those that control cell cycle progression, and grows in size to return to its pre-mitosis volume. It also duplicates cytoplasmic organelles.
- S Phase (Synthesis): This phase is characterised by DNA replication and histone synthesis. Centrosome duplication also begins here. The cell reaches a "restriction/commitment point" in the S phase, after which there is no turning back from division.
- G2 Phase (Gap 2): A relatively short phase where proteins required for mitosis accumulate, and the cell undergoes further growth and duplication of cytoplasmic organelles in preparation for division. Chromosomes also condense during this phase.
- M Phase (Mitosis and Cytokinesis): This is the period of actual cell division.
Cell Cycle Control System A complex network of regulatory proteins, known as the cell-cycle control system, ensures that the fundamental tasks of the cell cycle are completed accurately. This system involves an ordered series of biochemical switches that control main events like DNA replication and chromosome segregation. It responds to signals from both inside and outside the cell and is critical for regulating cell numbers.
- Checkpoints: The cell cycle has specific checkpoints that monitor critical events before allowing progression to the next phase.
- G1/S (Start or Restriction) Checkpoint: Located just before the S phase, this checkpoint assesses if cell nutrition, size, and environment are favourable, and if all DNA is intact. If conditions are met, DNA replication begins.
- G2/M Checkpoint: This ensures that DNA replication is completely finished and that the DNA is intact before the cell enters mitosis.