The cell cycle
The cell cycle is the sequence of events by which a cell grows, duplicates its DNA, and divides into two daughter cells. A typical human cell in culture divides about once every 24 hours.
The cycle has two main parts:
Phase | What happens |
|---|---|
Interphase (about 95% of the cycle) | the cell grows and prepares to divide |
M phase (mitotic phase) | the nucleus divides (karyokinesis), then the cytoplasm divides (cytokinesis) |
Interphase
Interphase is not a "resting" phase. The cell is very active.
G₁ (gap 1): the cell grows and is metabolically active, but its DNA does not replicate.
S (synthesis): DNA replicates. The amount of DNA per cell doubles (from 2C to 4C), but the number of chromosomes stays the same: each chromosome now has two sister chromatids.
G₂ (gap 2): proteins needed for mitosis are made, and the cell continues to grow.
Some cells stop dividing and leave G₁ to enter a quiescent stage called G₀. Mature nerve and heart muscle cells, for example, rarely divide.
Mitosis (equational division)
Mitosis produces two daughter cells that are genetically identical to the parent, with the same chromosome number. It is divided into four stages.
Stage | Key events |
|---|---|
Prophase | chromatin condenses into visible chromosomes; the spindle begins to form; the nucleolus and nuclear envelope disappear by the end |
Metaphase | chromosomes line up at the cell's equator (the metaphase plate); spindle fibres attach to the kinetochores at the centromeres. This is the best stage to study chromosome shape |
Anaphase | the centromeres split and the sister chromatids are pulled to opposite poles |
Telophase | chromosomes reach the poles and decondense; the nuclear envelope and nucleolus re-form |
Cytokinesis
In animal cells, a furrow forms in the cell membrane and deepens until the cell is pinched in two.
In plant cells, which have a rigid wall, a cell plate forms in the centre and grows outwards to become the new wall.
Significance of mitosis
growth of multicellular organisms,
repair and replacement of cells (for example, skin cells and the lining of the gut),
asexual reproduction in some organisms,
keeping the chromosome number constant in the cells of the body.
Uncontrolled mitosis can lead to cancer.
Meiosis (reductional division)
Meiosis takes place in reproductive cells and produces gametes. One diploid (2n) cell undergoes two successive divisions to give four haploid (n) cells, each genetically different.
Meiosis I (reduces the chromosome number)
Prophase I is long and complex. It is divided into five sub-stages:
Leptotene: chromosomes become visible.
Zygotene: homologous chromosomes pair up (synapsis), forming bivalents. The synaptonemal complex forms.
Pachytene: crossing over occurs: segments are exchanged between non-sister chromatids of homologous chromosomes. The enzyme recombinase is involved.
Diplotene: homologues begin to separate but stay joined at the cross-over points, the chiasmata.
Diakinesis: chiasmata move to the ends (terminalisation), and the nuclear envelope breaks down.
Metaphase I: bivalents line up at the equator. Anaphase I: homologous chromosomes separate and move to opposite poles. The sister chromatids stay together. Telophase I: two haploid cells form.
Meiosis II (like mitosis)
The two haploid cells divide again. In anaphase II the sister chromatids separate. The result is four haploid cells.
Mitosis and meiosis compared
Mitosis | Meiosis | |
|---|---|---|
Number of divisions | 1 | 2 |
Daughter cells | 2, diploid (2n) | 4, haploid (n) |
Genetic make-up | identical to the parent | different from the parent and each other |
Crossing over | no | yes (prophase I) |
Occurs in | body (somatic) cells | reproductive cells |
Significance of meiosis
It keeps the chromosome number constant from one generation to the next, because fertilisation of two n gametes restores 2n.
Crossing over and the random arrangement of homologues create genetic variation, which is the raw material for evolution.