Cell cycle · Interphase · Mitosis · Meiosis · Significance
The cell cycle is the ordered sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell, and eventually divides into two daughter cells. It is a self-controlled, tightly regulated process.
Human cells in culture divide roughly once every 24 hours. The time varies hugely between organisms — yeast can complete a cycle in about 90 minutes.
Interphase is misleadingly called the "resting phase" — the cell is actually extremely busy preparing for division. It is the longest phase and is split into three sub-phases:
| Phase | What happens | DNA |
|---|---|---|
| G₁ (Gap 1) | Cell is metabolically active, grows continuously, makes RNA & proteins. No DNA replication. | 2C · 2n |
| S (Synthesis) | DNA replication — DNA amount doubles. In animal cells the centriole also duplicates in the cytoplasm. | 2C→4C · 2n |
| G₂ (Gap 2) | Proteins (esp. tubulin for spindle) are synthesised; cell grows and gears up for mitosis. | 4C · 2n |
Cells that do not divide further exit the cycle from G₁ and enter G₀. Such cells remain metabolically active but stop proliferating unless specifically signalled. Examples: heart muscle cells and nerve cells (neurons).
The M phase represents the actual division and consists of two overlapping events: karyokinesis (division of the nucleus) followed by cytokinesis (division of the cytoplasm).
Mitosis is an equational division — the two daughter cells are genetically identical to the parent and to each other, and the chromosome number is conserved (2n → 2n). Karyokinesis has four stages:
| Stage | Key events |
|---|---|
| Prophase | Chromatin condenses into compact chromosomes (each = 2 chromatids at a centromere); centrosome (with 2 centrioles) has duplicated and the two move to opposite poles, initiating spindle formation; nuclear envelope, nucleolus, Golgi and ER disappear. |
| Metaphase | Chromosomes are most condensed and align at the metaphase plate (equator); spindle fibres attach to the kinetochores on the centromeres. |
| Anaphase | Centromeres split, sister chromatids separate and move as daughter chromosomes toward opposite poles (centromere leading, arms trailing). |
| Telophase | Chromosomes reach the poles and decondense; nuclear envelope, nucleolus, Golgi and ER reappear; two daughter nuclei form. |
| Animal cell | Plant cell |
|---|---|
| A cleavage furrow appears in the plasma membrane and deepens outside → inward until the cell pinches in two. | A cell plate forms in the middle and grows centre → outward (inside-out) toward the wall, because the rigid cell wall cannot pinch. |
Mitosis is the basis of growth and maintenance of the body. Its biological roles:
Meiosis is the division that halves the chromosome number: a diploid cell (2n) gives four haploid (n) cells. It occurs in the meiocytes (gamete mother cells / germ cells) during gamete or spore formation.
| Sub-stage | Key event |
|---|---|
| Leptotene | Chromosomes become gradually visible under the light microscope; they begin to condense. |
| Zygotene | Synapsis — homologous chromosomes pair, forming a bivalent (tetrad); the synaptonemal complex forms between them. |
| Pachytene | Crossing over — exchange of segments between non-sister chromatids at recombination nodules, catalysed by the enzyme recombinase. Bivalents now clearly appear as tetrads. |
| Diplotene | The synaptonemal complex dissolves; homologues separate slightly but stay joined at chiasmata (X-shaped sites of crossing over). |
| Diakinesis | Terminalisation of chiasmata; chromosomes fully condense; spindle assembles; nucleolus & nuclear envelope disappear — marks the end of Prophase I. |
| Stage | Event |
|---|---|
| Metaphase I | Bivalents line up at the equator; spindle fibres from opposite poles attach to the paired homologues. |
| Anaphase I | Homologous chromosomes separate and move to opposite poles — but sister chromatids stay joined at their centromeres. (This is the actual reduction step.) |
| Telophase I | Nuclear membrane & nucleolus reappear; cytokinesis follows giving a dyad of cells. |
Meiosis II starts immediately after Meiosis I (no S phase in between) and resembles a normal mitosis — an equational division on the already-haploid cells.
| Stage | Event |
|---|---|
| Prophase II | Nuclear membrane disappears again; chromosomes condense. |
| Metaphase II | Chromosomes align at the equator; spindle fibres attach to kinetochores of the two chromatids. |
| Anaphase II | Centromeres split — sister chromatids finally separate and move to opposite poles. |
| Telophase II | Two nuclei form at each pole; result overall = four haploid cells. |
| Feature | Mitosis | Meiosis |
|---|---|---|
| Where | Somatic cells / meristems | Meiocytes (germ / spore mother cells) |
| Divisions | One | Two (I & II) |
| DNA replication | Once before division | Once, before Meiosis I only |
| Daughter cells | 2 | 4 |
| Ploidy of products | Diploid (2n) — same as parent | Haploid (n) — halved |
| Pairing / synapsis | Absent | Present (Zygotene) |
| Crossing over | Absent | Present (Pachytene) |
| Genetic result | Identical to parent | Genetically variable |
| Purpose | Growth, repair, replacement | Gamete formation, variation |
| Point in time | DNA (C) | Chromosome no. |
|---|---|---|
| G₁ | 2C | 2n |
| After S / G₂ | 4C | 2n |
| End of Mitosis (each cell) | 2C | 2n |
| End of Meiosis I (each cell) | 2C | n |
| End of Meiosis II (each cell) | 1C | n |