A complete guide to how cells grow, replicate their DNA, and divide — the foundation of all life.
The cell cycle is the ordered series of events a cell undergoes from the moment it is formed by division of a parent cell to the point at which it itself divides into two daughter cells. First observed through microscopy in the nineteenth century, the cell cycle is now understood as a tightly regulated sequence of molecular events that ensures accurate duplication and distribution of genetic material.
Every living organism — from a single bacterium to a trillion-cell human body — depends on the cell cycle. Without it, growth would be impossible, damaged tissues could not be repaired, and life could not reproduce.
“The cell cycle is the essential mechanism by which all living things reproduce. In unicellular organisms, the cell cycle is the mechanism of reproduction. In multicellular organisms, it is responsible for growth, development, and the replacement of worn-out cells.”
— Bruce Alberts et al., Molecular Biology of the Cell, 7th Edition (2022)
The Cell Cycle at a Glance
G1
Cell growth
S
DNA replication
G2
Preparation
Mitosis
Nuclear division
Cytokinesis
Cell splits
Back to G1
Cell division is not random — it is driven by three fundamental biological needs:
Multicellular organisms develop from a single fertilised cell (the zygote) into complex bodies containing trillions of cells. This increase in cell number — not cell size — is how organisms grow. A human adult, for example, is made of roughly 37 trillion cells, all descended from one original zygote.
Cells have limited lifespans. Red blood cells survive roughly 120 days; skin cells are replaced every two to three weeks. When tissue is damaged — a cut, a bruise, a broken bone — cells around the wound divide to regenerate the lost tissue. Without this capacity, organisms could not heal.
For unicellular organisms such as bacteria or yeast, cell division is reproduction — each division produces an entirely new organism. Even in multicellular organisms, specialised germ cells undergo a modified division (meiosis) to produce gametes for sexual reproduction.
Quick Check
Which of the following is NOT a reason cells undergo division?
The cell cycle is divided into distinct phases, each with specific molecular tasks that must be completed before the next phase can begin.
Gap 1. The cell grows in size, synthesises proteins and organelles, and carries out its normal functions. A key checkpoint here determines whether the cell will proceed to DNA replication.
Synthesis. The cell replicates its entire genome so that each daughter cell will receive a complete copy. After S phase, every chromosome consists of two identical sister chromatids.
Gap 2. The cell continues growing and produces proteins essential for mitosis, such as tubulin for the mitotic spindle. A second checkpoint verifies that DNA replication is complete and error-free.
Mitosis. The replicated chromosomes are separated and distributed equally to two daughter nuclei through a carefully orchestrated series of steps: prophase, metaphase, anaphase, and telophase.
Cytokinesis. The cytoplasm physically divides, splitting the single cell into two genetically identical daughter cells. In animal cells this occurs via a cleavage furrow; in plant cells, a cell plate forms.
Fill in the Blank
The cell cycle consists of two major periods:________(where the cell grows and replicates DNA) and the mitotic phase (where the cell divides).
A typical mammalian cell in culture completes one full cycle in approximately 24 hours, though this varies widely between cell types. Embryonic cells can divide in as little as 30 minutes, while some adult cells rarely divide at all. Roughly 95% of the total cycle time is spent in interphase; the dramatic events of mitosis account for only about one hour.
| Phase | Duration | % of Cycle |
|---|---|---|
| G1 | 11 hours | ~46% |
| S | 8 hours | ~33% |
| G2 | 4 hours | ~17% |
| M | 1 hour | ~4% |
Quick Check
In a typical mammalian cell, which phase of the cell cycle takes the longest?
This study guide walks through each stage of the cell cycle in depth. Each chapter includes interactive quizzes and diagrams to reinforce your understanding.