The Cell Cycle
The cell cycle is a series of stages that cells undergo to grow, replicate their DNA and divide. The cell cycle is divided into two main phases: interphase and the mitotic phase (M phase).
Mitosis ensures that each daughter cell receives an identical set of chromosomes, maintaining genetic consistency across cells. This process is important, not only for replacing damaged or dead cells, but also for enabling organisms to grow and develop from a single fertilised egg into a complex structure with millions of cells.
Interphase
Interphase is the period during which the cell prepares for division. It consists of three sub-phases:
- G1 Phase (First Gap/Growth): In this phase, the cell grows and carries out its normal functions. It also synthesises various enzymes and nutrients required for DNA replication. The cell checks for any DNA damage and ensures that it has all the necessary components for DNA synthesis.
- S Phase (Synthesis): During this phase, DNA replication occurs. Each chromosome is duplicated, resulting in two sister chromatids held together by a centromere. The cell ensures that each daughter cell will receive an identical set of chromosomes.
- G2 Phase (Second Gap/Growth): In this phase, the cell continues to grow and produce proteins necessary for mitosis. The cell checks for any DNA replication errors and begins to prepare for the physical process of cell division.
Mitotic Phase (M phase)
The mitotic phase is when the cell actually divides. It includes two key processes: mitosis and cytokinesis.
- Mitosis: Mitosis is the process of nuclear division, and it occurs in four stages:
- Prophase: The chromatin condenses into visible chromosomes, and the nuclear envelope begins to break down. The mitotic spindle, composed of microtubules, forms and attaches to the centromeres of the chromosomes.
- Metaphase: The chromosomes align along the metaphase plate or the equator, in the middle of the cell. The spindle fibres ensure that each sister chromatid is attached to opposite poles of the cell.
- Anaphase: The sister chromatids are pulled apart by the spindle fibres and move toward opposite poles of the cell. This ensures that each new cell will receive an identical set of chromosomes.
- Telophase: The chromosomes reach the poles, and the nuclear envelope re-forms around each set of chromosomes, creating two separate nuclei. The chromosomes begin to decondense back into chromatin.
- Cytokinesis: This is the final step, where the cytoplasm divides, resulting in two daughter cells. In animal cells, a cleavage furrow forms, pinching the cell in two. In plant cells, a cell plate forms, eventually leading to the formation of a new cell wall between the two daughter cells.
