The cell cycle is the ordered sequence of events a cell undergoes to grow, duplicate its components, and divide into two daughter cells. The cycle is divided into two main parts: Interphase, a long preparatory period of growth and DNA replication, and the M phase, the relatively short period of physical cell division. Ensuring this process is executed with fidelity maintains genetic continuity across generations of cells.
G1 Phase: Initial Growth and Preparation
The G1 (Gap 1) phase is the first stage of Interphase, immediately following cell division. The cell increases significantly in size and actively synthesizes proteins, enzymes, and structural components. Organelles, such as mitochondria and ribosomes, are duplicated to support the needs of two future daughter cells. Accumulating sufficient energy reserves is a primary focus, preparing for the energetically expensive process of DNA replication that follows.
This stage is also where a cell decides whether to commit to division or exit the cycle. If conditions are unfavorable, or if the cell does not divide, it may enter a quiescent state known as G0. Passing the point of no return within G1 commits the cell to completing the rest of the cycle.
S Phase: DNA Replication
The S (Synthesis) phase is dedicated to accurately duplicating the cell’s genetic material. Specialized enzymes like DNA helicase unwind the double helix, separating the two strands to create a replication fork. DNA polymerase then moves along the exposed template strands, adding complementary nucleotides to form two new, identical DNA helices.
The complete duplication of the genome results in the formation of sister chromatids, which are two identical copies of a chromosome still attached to one another. Each chromosome now consists of two DNA molecules.
G2 Phase: Final Checks Before Division
The G2 (Gap 2) phase is the final subphase of Interphase, acting as a final period of preparation before physical division. The main focus shifts to synthesizing the specific proteins and components required for mitosis. For example, the cell manufactures and accumulates proteins that will form the mitotic spindle, such as tubulin.
During this stage, the cell performs a quality control check to ensure the genome was completely and accurately replicated in the preceding S phase. Any detected DNA damage is temporarily paused for repair mechanisms to fix the issue. Only after all checks are passed can the cell proceed into the division phase.
M Phase: Nuclear and Cell Division
The M phase encompasses the physical separation of the duplicated genetic material and the final splitting of the cell into two. This sequence is composed of two major overlapping events: Mitosis, the division of the nucleus, and Cytokinesis, the division of the cytoplasm. Mitosis is the process used by somatic cells to produce genetically identical daughter cells.
Prophase and Metaphase
Mitosis begins with Prophase, where the duplicated chromosomes condense and the mitotic spindle begins to form. The nuclear envelope then breaks down. In Metaphase, the chromosomes, each consisting of two sister chromatids, align precisely along the cell’s center plane, known as the metaphase plate.
Anaphase
This alignment is achieved by the spindle fibers, which attach to a protein structure on the chromosome called the kinetochore. Anaphase begins abruptly with the separation of the sister chromatids. They are pulled apart by the shortening spindle fibers toward opposite poles of the cell, and once separated, each chromatid is considered an individual chromosome.
Telophase and Cytokinesis
In Telophase, the separated chromosomes arrive at the poles and begin to decondense. A new nuclear envelope forms around each set of chromosomes, resulting in two distinct nuclei within the single parent cell. Concurrent with the latter stages of mitosis, Cytokinesis begins, physically dividing the cytoplasm and organelles. In animal cells, this occurs through the formation of a contractile ring that pinches the cell membrane inward until two separate daughter cells are formed.
Regulation: Cell Cycle Checkpoints
The cell cycle is controlled by surveillance mechanisms called checkpoints, which act as internal stops to prevent errors. These checkpoints pause the cycle until conditions are optimal and all preparatory tasks are complete.
The G1 checkpoint, often called the Restriction Point, determines if the cell size is adequate and if the DNA is undamaged before committing to replication. The G2/M checkpoint assesses the success of the S phase, ensuring that all DNA has been fully replicated and any damage has been repaired before the cell enters mitosis. The M or Spindle Checkpoint operates during metaphase, ensuring that every chromosome is correctly attached to the mitotic spindle fibers.
Driving the transitions between phases are two families of regulatory proteins: Cyclins and Cyclin-Dependent Kinases (CDKs). Cyclins are proteins whose concentration fluctuates cyclically. When Cyclins bind to and activate CDKs, the resulting complex phosphorylates target proteins to advance the cell to the next stage.