For living organisms to grow, repair damaged tissues, and reproduce, their cells must divide. This fundamental process, mitosis, allows a single parent cell to produce two genetically identical daughter cells. Before this intricate division, crucial preparatory steps must unfold to ensure the cell is ready to distribute its genetic material.
The Cell’s Essential Cycle
Cells continuously progress through the cell cycle, an organized sequence of growth, DNA replication, and cell division. The preparatory period before mitosis, known as interphase, is a stage of significant activity. Interphase is subdivided into distinct stages, each with specific activities that equip the cell for division.
The First Growth Phase
The G1 phase is the initial stage of interphase, where the cell experiences significant growth, increasing its size and synthesizing proteins and organelles. It performs normal functions with high metabolic activity, accumulating building blocks and energy for DNA replication. A critical decision point, the G1 checkpoint, occurs towards the end of this phase. Here, the cell assesses its internal state, including size and energy, and evaluates external conditions for division. DNA damage is also checked; if conditions are not met, the cell may halt progression or enter a quiescent state.
The DNA Duplication Phase
Following passage through the G1 checkpoint, the cell enters the S phase, or “synthesis” phase, dedicated to duplicating its entire DNA content. Accurate replication ensures each daughter cell receives a complete and identical set of genetic instructions. During this process, each chromosome is copied, resulting in two identical sister chromatids attached at the centromere.
DNA replication is semi-conservative; each new DNA molecule consists of one original and one newly synthesized strand. Enzymes like DNA helicase unwind the double helix, while DNA polymerase synthesizes new strands. The cell also synthesizes histone proteins, important for packaging duplicated DNA into compact nucleosomes, which organize genetic material within the nucleus.
The Second Growth and Checkpoint Phase
After DNA replication, the cell enters the G2 phase. During this period, the cell continues to grow and synthesizes additional proteins and organelles required for mitosis. This includes components like microtubules, important for forming the spindle apparatus that will separate the chromosomes.
A final quality control step, the G2 checkpoint, occurs at the end of this phase. The cell checks the duplicated DNA for errors or damage from the S phase and confirms complete replication. This checkpoint acts as a safeguard, preventing the cell from entering mitosis with compromised or incomplete genetic material. If issues are detected, the cell cycle halts, allowing time for DNA repair before division.