The cell cycle is a series of events within a cell that leads to its division and duplication. This process creates two daughter cells from a single parent cell. Its purpose includes the growth and development of multicellular organisms, the replacement of old or damaged cells, and the repair of injured tissues. The continuous and regulated progression through this cycle ensures the propagation of life and the maintenance of organismal integrity.
The G1 Phase: Growth and Preparation
The G1 phase, also known as “Gap 1” or “Growth 1,” is the initial period after cell division. During this stage, a cell increases in size, synthesizing proteins and organelles. These components, such as ribosomes and mitochondria, prepare the cell for DNA replication. This phase also involves the synthesis of enzymes and other molecules necessary for the upcoming DNA synthesis. A regulatory point, the G1 checkpoint, assesses its internal and external conditions. If conditions are unfavorable, or the cell is not ready to divide, it may exit the cycle and enter a quiescent state called G0.
The S Phase: DNA Replication
The S phase, or “Synthesis,” is defined by DNA replication. During this period, the cell duplicates its entire genome, ensuring that each new daughter cell receives a complete set of genetic instructions. Each chromosome is copied, resulting in two identical sister chromatids that remain joined at a region called the centromere. This replication involves unwinding the DNA double helix and synthesizing new complementary strands using enzymes like DNA polymerase. Accurate DNA duplication during the S phase is paramount for genetic stability; errors can lead to mutations or chromosomal abnormalities.
The G2 Phase: Final Checks and More Growth
Following DNA synthesis, the cell enters the G2 phase, also known as “Gap 2” or “Growth 2.” This period serves as a preparatory stage before cell division. Cells continue to grow and synthesize additional proteins, particularly those necessary for chromosome segregation and cell division, such as tubulin, which forms microtubules. A G2 checkpoint at the end of this phase inspects its replicated DNA. This checkpoint verifies that DNA replication has been completed accurately and that any DNA damage incurred has been effectively repaired. Only after passing this assessment can the cell proceed to division.
The M Phase: Cell Division
The M phase, representing “Mitosis” (or Meiosis in germline cells), is the shortest period of the cell cycle. This phase involves two primary processes: nuclear division and cytoplasmic division. Nuclear division, known as mitosis, is a series of events that ensures the separation of duplicated chromosomes. Mitosis itself is subdivided into several distinct stages: prophase, metaphase, anaphase, and telophase. During these stages, the nuclear envelope breaks down, chromosomes condense and align at the cell’s equator, sister chromatids separate and move to opposite poles, and new nuclear envelopes form around the segregated chromosomes. Following nuclear division, cytokinesis, the division of the cytoplasm, occurs. This process involves the formation of a cleavage furrow in animal cells or a cell plate in plant cells, resulting in two genetically identical daughter cells.