The cell cycle is the sequence of events that occurs in a cell, leading to its division and the duplication of its DNA to produce two daughter cells. The cell cycle consists of two major stages: interphase and mitosis. During interphase, the cell grows and replicates its DNA, while during mitosis, the replicated DNA is segregated, and the cell divides.
Interphase is divided into three phases: G1, S, and G2. During G1, the cell grows and synthesizes proteins required for DNA replication. At the end of G1, the cell enters the S phase, where the DNA is replicated. This phase is followed by G2, during which the cell prepares for cell division by synthesizing the proteins required for mitosis.
Mitosis consists of four stages: prophase, metaphase, anaphase, and telophase. During prophase, the chromatin condenses into discrete chromosomes, and the nuclear envelope disintegrates. During metaphase, the chromosomes align at the cell’s equator and attach to the spindle fibers. During anaphase, the spindle fibers pull the sister chromatids apart and move them toward opposite poles of the cell. Finally, during telophase, the chromosomes reach the poles of the cell, the nuclear envelope reassembles around each set of chromosomes, and the cell divides in a process called cytokinesis.
The cell cycle is tightly regulated to ensure each stage’s proper timing and completion. The cell cycle control is achieved by a group of proteins known as cyclins and cyclin-dependent kinases (CDKs). Cyclins are proteins that accumulate during specific cell cycle stages and bind to CDKs to activate them. Once activated, CDKs phosphorylate various target proteins that regulate the progression of the cell cycle.
There are several checkpoints during the cell cycle where it assesses whether it is ready to proceed to the next stage. The G1 checkpoint, the restriction point, ensures the cell has sufficient nutrients and energy to enter the S phase. The G2 checkpoint ensures that DNA replication has been completed and the cell is ready for mitosis. The spindle checkpoint ensures the chromosomes are properly aligned and attached to the spindle fibers before the cell proceeds to anaphase.
Mutations that affect the cell cycle regulation can lead to the uncontrolled cell division that characterizes cancer. Cancer cells often have defects in the checkpoints that normally prevent cells with damaged DNA from progressing through the cell cycle. As a result, cancer cells can accumulate mutations and divide uncontrollably.
In summary, the cell cycle is a highly regulated process that ensures the proper replication and segregation of DNA during cell division. The regulation of the cell cycle is achieved by cyclins and CDKs, and mutations that affect this regulation can lead to cancer.