G1 phase
Overview
The G1 phase, or Gap 1 phase, is the first of four phases in the cell cycle that takes place in eukaryotic cell division. This phase follows the completion of cytokinesis and precedes the S phase. The G1 phase is a period of activity where cells grow and carry out normal functions.
Cell Cycle Context
The G1 phase is part of the cell cycle, which is a series of events that take place in a cell leading to its division and duplication. The cell cycle consists of four main phases: G1 phase, S phase, G2 phase, and M phase. The G1 phase is the first phase of the cell cycle and is followed by the S phase, where DNA replication occurs.
Function
The primary function of the G1 phase is to prepare the cell for DNA replication in the S phase. During the G1 phase, the cell increases in size, produces RNA and proteins, and prepares for the duplication of DNA. The G1 phase is also a checkpoint in the cell cycle; the cell checks the integrity of the DNA and whether the necessary nutrients and energy are available for the cell to proceed to the S phase.
Regulation
The G1 phase is tightly regulated by several mechanisms to ensure the integrity of the cell's DNA. The key regulator of the G1 phase is the G1/S checkpoint, also known as the restriction point. This checkpoint is controlled by the cyclin-dependent kinases (CDKs) and their regulatory proteins, the cyclins. The activity of the CDKs and cyclins is regulated by several mechanisms, including the retinoblastoma (Rb) protein, the p53 protein, and the p21 protein.
G1/S Checkpoint
The G1/S checkpoint, also known as the restriction point, is a critical control point in the G1 phase. This checkpoint ensures that the cell is ready to enter the S phase and begin DNA replication. If the cell does not pass this checkpoint, it may enter a resting state known as the G0 phase.
G0 Phase
The G0 phase is a resting state that cells may enter if they do not pass the G1/S checkpoint. Cells in the G0 phase are not actively preparing to divide. Some cells, such as nerve cells, stay in the G0 phase indefinitely. Other cells can be stimulated to re-enter the cell cycle and proceed to the S phase.
Clinical Significance
The regulation of the G1 phase and the G1/S checkpoint is often disrupted in cancer. Mutations in the genes that regulate the G1 phase, such as the Rb, p53, and p21 genes, can lead to uncontrolled cell division and the development of cancer. Therefore, understanding the mechanisms that regulate the G1 phase is critical for the development of new cancer therapies.