By Kevin Beck – Updated Aug 30, 2022

Cells are the fundamental units of all living organisms, each harboring specialized structures that perform essential functions, much like organs in a body. Just as humans progress through distinct life stages—infancy, childhood, adolescence, adulthood, and old age—cells follow a defined life cycle with smoothly transitioning phases.

Prokaryotes (Bacteria and Archaea) are single‑cell organisms that lack a structured cell cycle; they simply grow and divide by binary fission. In contrast, eukaryotic organisms—animals, fungi, and plants—undergo a regulated series of cell cycle phases.

Core Phases of the Cell Cycle

At the heart of cellular life is reproduction: each cell duplicates itself so that the organism can grow, repair tissue, and ultimately produce offspring. The cell cycle is traditionally divided into two principal stages: interphase, during which the cell prepares for division, and mitosis (the M phase), in which the genetic material is partitioned into two daughter nuclei.

Overview of the Cell Cycle

During interphase, a cell enlarges, synthesizes proteins and organelles, and replicates its DNA. Once the genome is duplicated, the cell performs quality‑control checks before entering mitosis, where the duplicated chromosomes are segregated and the cell splits into two genetically identical daughter cells. The cycle then restarts.

While the duration of interphase varies widely among cell types and physiological conditions, mitosis is generally brief in comparison.

Interphase: G1, S, and G2

Interphase consists of three sub‑phases:

• G1 (Gap 1) – The cell grows, synthesizes proteins, and produces ATP, preparing the machinery for DNA synthesis.
• S (Synthesis) – DNA replication occurs; chromosomes unwind, exposing DNA to replication enzymes, resulting in sister chromatids joined at the centromere.
• G2 (Gap 2) – The cell continues to grow, assembles components for mitosis, and conducts a final integrity check of the replicated DNA.

After G2, the cell transitions into the M phase.

Chromosomes and Chromatin

Chromosomes are compacted strands of chromatin, consisting of DNA wrapped around histone proteins. This tight packaging allows every cell to store a full copy of the organism’s genome. Humans possess 46 chromosomes—23 from each parent—arranged in 22 homologous pairs and one sex chromosome pair (XX or XY).

M Phase (Mitosis) and Cytokinesis

Mitosis is divided into five distinct stages:

1. Prophase – Chromosomes condense; the mitotic spindle forms from centrosomes; the nuclear envelope disassembles.
2. Prometaphase – Spindle microtubules attach to kinetochores; chromosomes migrate toward the metaphase plate.
3. Metaphase – Chromosomes align at the cell’s equatorial plane (metaphase plate).
4. Anaphase – Sister chromatids separate and are pulled to opposite poles by spindle fibers.
5. Telophase – Nuclear envelopes re‑form around each set of chromosomes; chromosomes decondense to their interphase state.

Immediately following telophase is cytokinesis, the physical division of the cytoplasm, resulting in two daughter cells, each with a complete set of chromosomes.

The combined M phase and cytokinesis mirror binary fission in prokaryotes, but with a nucleus and a more complex spindle apparatus.