The cell cycle
The cell cycle is the sequence of events by which a cell copies its genome, makes its other parts, and divides into two daughter cells. DNA copying happens only in one phase, but cell growth goes on all the time. A typical human cell in culture divides about once every 24 hours; yeast takes about 90 minutes.
Two main phases
- Interphase (about 95% of the time) – the cell prepares. It is not a resting stage.
- G1 (gap 1): the cell is active, grows, but does not copy DNA.
- S (synthesis): DNA is copied. If the starting DNA is 2C, it becomes 4C, but the number of chromosomes does not change (2n stays 2n). In animal cells the centriole also duplicates.
- G2 (gap 2): proteins for mitosis are made and growth continues.
- M phase (mitosis, about 1 hour in a human cell) – karyokinesis (the nucleus divides) followed by cytokinesis (the cytoplasm divides).
G0 – the quiescent stage
Some cells stop dividing (for example heart cells in adults) and leave G1 to enter G0. They stay alive and do their work, but do not divide unless needed. In animals, mitosis happens only in diploid body (somatic) cells, but plants can divide by mitosis in both haploid and diploid cells.
Mitosis: the equational division
In mitosis the chromosome number of the daughter cells equals that of the parent, so it is called equational division. Stages:
- Prophase: chromatin condenses into chromosomes, each with two chromatids joined at the centromere. The centrosome, copied in interphase, moves to opposite poles and sends out spindle fibres (asters). By the end, Golgi, ER, nucleolus and nuclear envelope are not visible.
- Metaphase: the nuclear envelope has gone; chromosomes are fully condensed – the best stage to study their shape. Spindle fibres attach to the kinetochores (small discs on the centromere). All chromosomes line up at the equator: the metaphase plate.
- Anaphase: centromeres split, the chromatids separate and move to opposite poles, centromere first, arms trailing behind.
- Telophase: chromosomes reach the poles and loosen into chromatin. Nuclear envelope, nucleolus, Golgi and ER form again.
Cytokinesis
In animal cells a furrow appears in the membrane and deepens until the cell pinches into two. In plant cells the wall cannot pinch, so a cell plate forms in the centre and grows outward to the side walls; it becomes the middle lamella. Organelles such as mitochondria and plastids are shared between the two cells. When karyokinesis is not followed by cytokinesis, a multinucleate cell forms – a syncytium, e.g. the liquid endosperm of coconut.
Meiosis: the reductional division
Meiosis happens in diploid cells that will form gametes (or spores in plants). It has two divisions (meiosis I and II) but only one round of DNA copying. Result: four haploid (n) cells.
Meiosis I
- Prophase I (long and complex, five sub-stages):
- Leptotene – chromosomes become visible as threads.
- Zygotene – homologous chromosomes pair up (synapsis) with the help of the synaptonemal complex. A pair is a bivalent or tetrad (4 chromatids).
- Pachytene – crossing over: non-sister chromatids of homologous chromosomes swap pieces at recombination nodules. The enzyme recombinase helps. This gives recombination of genes.
- Diplotene – the synaptonemal complex dissolves and homologues start to separate, except at X-shaped points called chiasmata. In the oocytes of some vertebrates, diplotene can last months or years.
- Diakinesis – chiasmata terminalise (move to the ends); spindle forms; nucleolus and nuclear envelope disappear.
- Metaphase I: bivalents line up at the equator.
- Anaphase I: homologous chromosomes separate and go to opposite poles, while sister chromatids stay together at their centromere.
- Telophase I: nuclei may re-form; cytokinesis gives a dyad of cells. The short gap before meiosis II is interkinesis – no DNA copying.
Meiosis II
Very much like mitosis: prophase II, metaphase II (chromosomes at the equator), anaphase II (centromeres split, sister chromatids separate), telophase II. Result: a tetrad of four haploid cells.
Mitosis vs meiosis
| Mitosis | Meiosis |
|---|---|
| One division | Two divisions |
| 2 daughter cells | 4 daughter cells |
| 2n → 2n (equational) | 2n → n (reductional in meiosis I) |
| No pairing, no crossing over | Synapsis and crossing over in prophase I |
| Daughters identical to parent | Daughters differ (variation) |
| Body (somatic) cells | Cells forming gametes/spores |
Chromosome and DNA counts through the cycle
Rule: number of chromosomes = number of centromeres. "C" is the DNA amount of a haploid set before copying.
| Stage | Chromosomes | DNA |
|---|---|---|
| G1 | 2n | 2C |
| After S, G2, prophase, metaphase | 2n (each with 2 chromatids) | 4C |
| Anaphase of mitosis (whole cell) | 4n (chromatids now separate) | 4C |
| Each daughter after mitosis | 2n | 2C |
| Each cell after meiosis I | n (each with 2 chromatids) | 2C |
| Each cell after meiosis II | n | C |
Significance of mitosis and meiosis
Mitosis
- Keeps the chromosome number the same, so all body cells have the same genes.
- Growth of multicellular organisms.
- Repair of wounds and replacement of cells, e.g. the upper skin layer, gut lining and blood cells.
- Restores the nucleus-to-cytoplasm ratio when a cell gets too big.
- In plants, mitosis in the meristems (apical and lateral cambium) gives continuous growth.
- Asexual reproduction in many organisms.
Meiosis
- Keeps the chromosome number constant from one generation to the next: n (egg) + n (sperm) = 2n (zygote).
- Creates genetic variation through crossing over and random sorting of chromosome pairs – the raw material for evolution.
Try it: count chromosomes like a biologist
Take 4 long and 4 short pieces of wool or pipe cleaner – two colours, one for "mother" and one for "father". Each pair (one long red + one long blue) is a homologous pair, so 2n = 4. Act out mitosis: stick a copy next to each piece (S phase), line them up, pull copies apart – count 4 in each new "cell". Now act out meiosis: pair long with long and short with short, swap a small piece of wool (crossing over), pull whole pairs apart, then split again – count 2 in each of 4 cells. Check your answer with the slider in step 6 of the 3D.
Key formulas and definitions
- Cell cycle: G1 → S → G2 → M (interphase = G1 + S + G2)
- S phase: DNA 2C → 4C; chromosome number stays 2n
- Mitosis: PMAT – prophase, metaphase, anaphase, telophase; 1 cell → 2 cells, 2n → 2n
- Meiosis: 1 cell → 4 cells, 2n → n; one DNA copying, two divisions
- Prophase I: leptotene → zygotene → pachytene → diplotene → diakinesis
- Chromosomes = number of centromeres; chromatids = 2 × chromosomes after S (until anaphase)
- Bivalent (tetrad) = 1 homologous pair = 2 chromosomes = 4 chromatids
- Divisions to make N cells by mitosis from 1 cell: N = 2^k after k rounds
Worked examples
1. A plant cell has 2n = 16 in G1. Give the chromosome number and DNA amount (take G1 DNA = 2C) in (a) G2, (b) each daughter after mitosis.
(a) G2: DNA copied, so 4C; chromosomes still 16 (each with 2 chromatids). (b) Each daughter: 16 chromosomes, 2C.
2. A cell with 2n = 24 undergoes meiosis. How many chromosomes are in each cell after meiosis I and after meiosis II?
Meiosis I halves the number: 12 (each with 2 chromatids). Meiosis II separates chromatids but keeps the number: 12 in each of the 4 cells.
3. How many bivalents form in prophase I of a human cell (2n = 46)? How many chromatids is that?
Bivalents = n = 23. Each has 4 chromatids, so 23 × 4 = 92 chromatids.
4. One cell divides by mitosis 5 times in a row (all daughters divide each time). How many cells result?
Each round doubles the number: 2⁵ = 32 cells.
5. How many meiotic divisions are needed to make 100 pollen grains? (Each meiosis gives 4.)
100 ÷ 4 = 25 meiotic divisions (25 meiocytes).
6. A cell cycle lasts 24 hours. In a slide of 100 dividing cells, 4 are in M phase. Estimate how long M phase lasts.
The share of cells in a phase ≈ share of time spent in it: 4/100 × 24 h = 0.96 h ≈ 1 hour.
Common mistakes
- Calling interphase a "resting phase". The cell is busy growing and copying its DNA; it only looks quiet under the microscope.
- Thinking S phase doubles the chromosome number. It doubles DNA (2C → 4C); chromosomes stay 2n because the centromere count does not change.
- Saying sister chromatids separate in anaphase I. In anaphase I homologous chromosomes separate; sister chromatids separate in anaphase II (and in mitotic anaphase).
- Placing crossing over in zygotene or diplotene. Pairing is in zygotene, crossing over in pachytene, and chiasmata are seen in diplotene.