The cell cycle is the series of events that occur in a cell leading to its division into two daughter cells. It is divided into two main phases: interphase and mitotic phase. Interphase is further divided into three sub-phases: G1 (gap 1), S (synthesis), and G2 (gap 2), while the mitotic phase is divided into five sub-phases: prophase, prometaphase, metaphase, anaphase, and telophase.
- Interphase: Interphase is the longest phase of the cell cycle and accounts for about 90% of the total time. During this phase, the cell grows, replicates its DNA, and prepares for cell division. The three sub-phases of interphase are:
- G1 phase: In this phase, the cell grows and carries out its normal functions. It also synthesizes RNA and proteins necessary for DNA replication and cell division.
- S phase: During this phase, the cell replicates its DNA, forming two identical copies of each chromosome. At the end of this phase, each chromosome consists of two identical sister chromatids held together by a centromere.
- G2 phase: In this phase, the cell synthesizes the proteins necessary for cell division, such as the microtubules that will pull the chromosomes apart during cell division.
- Mitotic phase: The mitotic phase is the phase of the cell cycle during which the cell divides into two daughter cells. It is divided into five sub-phases:
- Prophase: In this phase, the chromatin condenses into visible chromosomes, and the nuclear envelope breaks down. The centrosomes move to opposite poles of the cell and begin to form the spindle fibers that will attach to the chromosomes.
- Prometaphase: During this phase, the spindle fibers attach to the kinetochores of the chromosomes, which are protein structures on the centromeres. The chromosomes begin to move toward the center of the cell.
- Metaphase: In this phase, the chromosomes align at the equator of the cell, known as the metaphase plate. The spindle fibers are fully formed and attach to each sister chromatid.
- Anaphase: During this phase, the sister chromatids are pulled apart by the spindle fibers and move toward opposite poles of the cell. The cell elongates as the poles move apart.
- Telophase: In this phase, the chromosomes arrive at the poles of the cell, and the nuclear envelope reforms around each set of chromosomes. The spindle fibers break down, and the cell undergoes cytokinesis, dividing into two daughter cells.
Overall, the cell cycle is tightly regulated by a complex network of proteins and signaling pathways, which ensure that each phase is completed before the cell progresses to the next phase. Any defects or mutations in these regulatory mechanisms can lead to cell cycle abnormalities and diseases, such as cancer.