The cell cycle is an organized series of events that enables a cell to grow and divide, producing two new daughter cells. This process is divided into two major phases: interphase and the mitotic (M) phase. Interphase is the cell’s preparatory stage, where it grows, duplicates its DNA, and synthesizes necessary components for division. Within interphase, “G phases,” or Gap phases, ensure the cell is properly prepared for replication and division. These G phases involve the cell assessing its internal and external environments to ensure favorable conditions for progression.
The G1 Phase
The G1 (Gap 1 or Growth 1) phase marks the initial period of interphase, following cell division. During this time, the cell grows in size, actively synthesizing a variety of proteins and duplicating organelles such as mitochondria and ribosomes. This phase involves biochemical activity, as the cell accumulates building blocks for DNA replication and energy reserves. It carries out its normal metabolic functions while simultaneously preparing for DNA synthesis.
A key regulatory point within G1 is the G1 checkpoint, also known as the restriction point. Here, the cell evaluates its internal state, including size, energy reserves, and DNA integrity. It also assesses external conditions, such as growth factor availability, to determine if conditions are suitable for DNA synthesis and cell division. Passing this checkpoint commits the cell to completing the rest of the cell cycle. If conditions are unfavorable or DNA damage is detected, the cell can halt progression, allowing for repair or potentially entering a resting state.
The G0 Phase
The G0 (Quiescent) phase represents a state where cells have exited the active cell cycle. Cells in G0 are metabolically active, performing their specialized functions within the organism, but they are not undergoing DNA replication or cell division. This phase is considered an extended G1 phase, as cells typically enter G0 from G1.
Many differentiated cells, such as mature nerve and muscle cells, enter G0 and remain there permanently, performing their specialized roles without dividing. Other cells, like some liver cells and tissue stem cells, can enter G0 temporarily and re-enter the cell cycle in response to specific signals, such as tissue damage or growth factor availability. The G0 phase helps maintain tissue homeostasis and allows cells to conserve resources when division is not required. It also ensures specialized cells can maintain their functions without the constant demand of replication.
The G2 Phase
Following DNA replication in the S phase, the cell enters the G2 (Gap 2 or Growth 2) phase. This is another period of growth and preparation for cell division. During G2, the cell continues to grow, synthesizes additional proteins, and duplicates any remaining organelles necessary for mitosis. This phase ensures all components required for cell division are in place and properly organized.
A key event in G2 is the G2 checkpoint, often called the DNA damage checkpoint. Here, the cell checks for any DNA damage that occurred during S phase replication or incomplete DNA synthesis. If problems are detected, the cell cycle halts to allow DNA repair mechanisms to correct errors before proceeding to mitosis. This checkpoint helps maintain genomic integrity and prevents transmitting damaged DNA to daughter cells. Without a properly functioning G2 checkpoint, cells with compromised DNA could enter division, potentially leading to mutations or abnormal cell proliferation.