The cell cycle is the sequence of events a eukaryotic cell follows from its formation until it divides into two new daughter cells. Its purpose is to ensure the precise duplication and distribution of genetic material for growth, tissue repair, and reproduction. This entire process is divided into two main periods: Interphase and the Mitotic (M) phase. Interphase is the lengthy preparatory period when the cell grows and copies its DNA, while the M phase is the relatively short period of physical cell division.
The Preparatory Phase (Interphase)
Interphase is the longest part of the cell cycle, accounting for approximately 90% of the total time for a typical human cell. Although once incorrectly labeled the “resting phase,” the cell is highly active, performing its normal functions while preparing for division. Preparation involves a tightly controlled sequence of three sub-phases: G1, S, and G2, which collectively ensure the cell is ready to divide.
The cycle begins with the Gap 1 (\(\text{G}_1\)) phase, where the newly formed cell focuses on intensive growth. During \(\text{G}_1\), the cell synthesizes proteins, accumulates energy reserves, and duplicates most of its organelles, effectively increasing its cytoplasmic volume. This is the phase where the cell commits to division, passing a restriction point that determines whether it will proceed or enter a non-dividing state called \(\text{G}_0\).
Following sufficient growth, the cell enters the Synthesis (S) phase, defined by DNA replication. The cell’s entire genome is duplicated, resulting in two identical copies of each chromosome, known as sister chromatids, which remain attached. This replication ensures that each future daughter cell receives a complete and identical set of genetic instructions.
The Gap 2 (\(\text{G}_2\)) phase is where the cell makes its final preparations for physical division. The cell continues to grow and synthesizes proteins, particularly those necessary for chromosome manipulation and the formation of the mitotic spindle. Once the cell completes \(\text{G}_2\), it possesses double the original amount of DNA and is ready to transition into the division phase.
The Division Phase (M Phase)
The M phase, or Mitotic phase, is the shortest and most visible part of the cell cycle, during which the cell’s contents are partitioned into two new cells. This phase is composed of two major overlapping events: mitosis, which is the division of the nucleus, and cytokinesis, which is the division of the cytoplasm. The entire M phase typically lasts only about one hour in a rapidly dividing human cell.
Mitosis itself is a continuous process divided into four stages—prophase, metaphase, anaphase, and telophase—whose primary purpose is to segregate the duplicated chromosomes. Specialized structures called the mitotic spindle, made of microtubules, attach to the sister chromatids and pull them apart to opposite ends of the cell. The result of mitosis is the formation of two separate nuclei, each containing an identical, full set of chromosomes.
Cytokinesis completes the M phase by splitting the cell into two daughter cells. In animal cells, a contractile ring of actin and myosin filaments forms beneath the cell membrane and pinches the cell inward, creating a cleavage furrow. Plant cells, which have a rigid cell wall, form a structure called a cell plate down the center to separate the two new cells. This separation yields two genetically identical cells that can then begin their own cycle in \(\text{G}_1\).
Ensuring Accuracy and Control
The orderly progression through the cell cycle is maintained by internal control mechanisms known as checkpoints. These checkpoints are surveillance points where the cell assesses its internal state and environment before committing to the next major phase. They act as gatekeepers to ensure the integrity of the genome is preserved.
The G1 checkpoint, often called the restriction point, is the primary decision point. Here, the cell checks for adequate size, available nutrients, and undamaged DNA before initiating the S phase and committing to replication. The G2 checkpoint occurs before the M phase, verifying that DNA replication was complete and any damaged DNA has been repaired.
The M checkpoint, or spindle checkpoint, operates during metaphase. This checkpoint ensures that all duplicated chromosomes are correctly aligned and attached to the mitotic spindle before the sister chromatids are pulled apart in anaphase. Activation of regulatory proteins, particularly cyclins and cyclin-dependent kinases (CDKs), determines whether the cell proceeds past these checkpoints. If a cell fails a checkpoint and cannot repair the damage, it may trigger programmed cell death, preventing the transmission of flawed genetic material.