Cells are the fundamental units of life, the smallest entities that carry out all essential biological functions, from metabolism to reproduction. Just as multicellular organisms follow a life cycle—birth, growth, reproduction, aging, death—each cell follows its own precise sequence of events, known as the cell cycle.
The cell cycle is divided into two broad segments: interphase and the M phase. Interphase occupies the majority of a cell’s lifespan and is further subdivided into the G1, S, and G2 stages. During G1, the cell enlarges and duplicates organelles, but chromosomes remain uncondensed. In the S phase, DNA replication occurs, duplicating all 46 chromosomes in human cells. G2 serves as a quality‑control checkpoint, ensuring that DNA is accurately replicated before the cell proceeds to division. Some cells enter a quiescent G0 state after mitosis, particularly in tissues with low turnover such as the liver.
At the end of G1, a protein‑mediated G1 checkpoint confirms that the cell is ready to commit to division. A second checkpoint at the G2 stage verifies that DNA replication has completed successfully. If a cell bypasses G2—directly proceeding from the S phase—this is typical for rapidly proliferating cells. These checkpoints are essential safeguards that prevent the propagation of damaged DNA.
Mitosis is the eukaryotic process that generates two genetically identical daughter cells from a single parent. It is an asexual division distinct from meiosis, which reduces chromosome number and introduces genetic diversity. In most animal cells, mitosis lasts roughly one hour, a brief moment in a cell’s lifetime. The term mitosis derives from the Greek for “thread,” reflecting the appearance of condensed chromosomes under a microscope. Mitosis itself involves only the nucleus; the overall cellular division is termed cytokinesis, while the nuclear division is known as karyokinesis.
Classically, mitosis comprises four stages: prophase, metaphase, anaphase, and telophase. Many textbooks include a fifth, prometaphase, which bridges prophase and metaphase. Below is a concise overview of each stage:
During prophase, chromatin condenses into discrete chromosomes. The centromere becomes a point of attachment for the kinetochore, while duplicated centrosomes migrate to opposite poles, initiating the mitotic spindle. In most higher eukaryotes, the nuclear envelope is enzymatically degraded, but in some organisms it remains intact and simply stretches.
Spindle microtubules reach out to kinetochores in a dynamic “search‑and‑capture” process. Once attached, tension is generated, aligning chromosomes along the metaphase plate.
All chromosomes line up precisely on the metaphase plate, ensuring that each daughter cell will receive an identical set of chromosomes. Microtubules may also extend to the cell cortex as astral microtubules, guiding spindle orientation.
Anaphase is split into two components: Anaphase A, where microtubules shorten and pull chromatids apart, and Anaphase B, where spindle poles move further apart, elongating the cell. A contractile actin ring forms beneath the plasma membrane, preparing for cytokinesis.
Chromosomes reach the opposite poles, decondense, and nuclear envelopes re‑form around each set. The spindle apparatus disassembles, and cytokinesis completes the process, yielding two identical daughter cells poised to enter G1 of their own cycles.
