The cell cycle is a precisely orchestrated sequence of events within a cell. This cyclical process leads to the division of one parent cell into two new daughter cells. Its overarching purpose encompasses the growth of organisms, the repair of damaged tissues, and the reproduction of new cells. It ensures accurate duplication and distribution of genetic material.
Interphase: Preparing for Division
Before a cell divides, it undergoes a preparatory stage known as Interphase. This phase is characterized by significant cell growth and the replication of its genetic material. Interphase is further subdivided into three distinct stages: G1 phase, S phase, and G2 phase.
G1 Phase
The G1 phase marks a period of active growth where the cell increases in size and synthesizes various proteins and organelles. During this time, the cell accumulates the necessary building blocks and energy reserves required for subsequent DNA replication. Cells are metabolically active in G1, carrying out their normal functions while preparing for division.
S Phase
Following G1, the cell enters the S phase, where the key event of DNA replication takes place. Each chromosome is duplicated, resulting in two identical sister chromatids joined together. This semi-conservative replication ensures that each new daughter cell will receive a complete copy of the genetic information.
G2 Phase
The final stage of interphase is the G2 phase, a period of continued growth and preparation for cell division. The cell synthesizes additional proteins and organelles essential for mitosis. During G2, the cell also checks for any DNA damage, making repairs before proceeding to the M phase.
M Phase: The Division Process
The M phase encompasses the process of cell division, consisting of two events: mitosis, which is the division of the nucleus, and cytokinesis, the division of the cytoplasm. This coordinated process ensures that the duplicated genetic material and cellular contents are evenly distributed into two daughter cells.
Prophase
Mitosis begins with Prophase, where the loosely organized chromatin within the nucleus condenses into compact chromosomes. The mitotic spindle, a structure made of microtubules, starts to form, and the nuclear envelope surrounding the genetic material begins to break down.
Metaphase
In Metaphase, the condensed chromosomes align themselves along the cell’s equatorial plate, often referred to as the metaphase plate. This alignment is facilitated by the attachment of spindle fibers to the chromosomes. The arrangement at the metaphase plate is crucial for ensuring that each daughter cell receives an identical set of chromosomes.
Anaphase
Anaphase is characterized by the abrupt separation of the sister chromatids. The newly separated chromosomes are pulled by the shortening spindle microtubules toward opposite poles of the cell. Simultaneously, the cell elongates as the poles move further apart, preparing for the final division.
Telophase
Telophase follows anaphase as the separated chromosomes arrive at opposite poles of the cell. The chromosomes begin to decondense. New nuclear envelopes form around each set of chromosomes, creating two distinct nuclei, and the mitotic spindle disassembles.
Cytokinesis
Cytokinesis completes the M phase by dividing the cytoplasm. In animal cells, a cleavage furrow forms and pinches the cell into two. In plant cells, a new cell wall forms in the middle to separate the daughter cells. This results in two genetically identical daughter cells.
Regulating the Cell Cycle
The precise control of the cell cycle is essential to prevent errors that can lead to diseases, such as cancer. Cells employ checkpoints, which act as critical decision points throughout the cycle. These checkpoints assess the cell’s internal and external conditions before allowing progression to the next phase.
There are three main checkpoints: the G1 checkpoint, the G2 checkpoint, and the M (spindle) checkpoint. The G1 checkpoint, often considered the primary decision point, evaluates cell size, nutrient availability, growth factors, and DNA integrity. If conditions are favorable, the cell commits to division; otherwise, it may enter a resting state or undergo repair. The G2 checkpoint ensures that DNA replication has been completed and that the cell is ready for mitosis. It checks for any DNA damage. The M checkpoint, occurring during metaphase, verifies that all chromosomes are aligned at the metaphase plate and attached to the mitotic spindle before sister chromatid separation begins.
The regulation of these checkpoints involves molecules, cyclins and cyclin-dependent kinases (CDKs). Cyclins are proteins whose levels fluctuate throughout the cell cycle, while CDKs are enzymes that become active when bound to cyclins. These cyclin-CDK complexes phosphorylate other proteins, signaling the cell to advance through phases.