Definition
The sequence of events by which a cell duplicates its genome, synthesises other constituents of the cell and eventually divides into two daughter cells is called the cell cycle.
Phases of the cell cycle
The cell cycle is divided into two basic phases:
- Interphase
- G1 phase (Gap 1)
- S phase (Synthesis)
- G2 phase (Gap 2)
- M phase (Mitosis phase)
- Karyokinesis – division of the nucleus
- Prophase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis – division of the cytoplasm
- Karyokinesis – division of the nucleus
NCERT: interphase lasts more than 95% of the duration of the cell cycle, and the M phase lasts only about an hour of a 24-hour human cell cycle. Interphase is called the resting phase, but it is metabolically very active – the cell prepares for division by growing and replicating its DNA.
In the S phase, the amount of DNA per cell doubles (2C → 4C), but the chromosome number stays the same (2n stays 2n). In animal cells, the centriole also duplicates in the cytoplasm during the S phase. Cells that stop dividing exit G1 and enter an inactive stage called the quiescent stage (G0).
Mitosis and meiosis
The M phase can be of two types. In mitosis, a cell divides once to form two daughter cells identical to the parent cell, while in meiosis, a cell divides twice to form four daughter cells with half the chromosome number.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two (meiosis I and meiosis II) |
| Daughter cells | Two | Four |
| Chromosome number | Remains the same (equational division) | Reduced to half (reductional division) |
| Crossing over | Absent | Present (pachytene) |
| Daughter cells vs parent | Genetically identical | Genetically different |
| Occurs in | Somatic (body) cells | Reproductive cells, during gamete formation |
NCERT: in animals, mitotic division is seen only in diploid somatic cells, but plants show mitosis in both haploid and diploid cells. Mitosis is responsible for growth, repair and replacement of cells, while meiosis keeps the chromosome number constant across generations in sexually reproducing organisms.
Doubling time
The time period in which the initial number of cells gets doubled is called the doubling time. Doubling time represents the duration of the cell cycle. The duration of the cell cycle, or the doubling time, generally increases with the complexity of the cell.
Examples:
- Bacteria – 20 minutes
- Yeast – 90 minutes
- Human cell – 24 hours
These are typical values for cells growing in culture. NCERT’s examples: a typical human cell in culture divides once in approximately 24 hours, and yeast takes only about 90 minutes.