Cell Cycle and Cell Division — Biology Class 11 Notes (CBSE & HBSE)
Free NCERT Biology notes for Cell Cycle and Cell Division (Class 11) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Cell Cycle and Cell Division (CBSE & HBSE)
This chapter covers the cell cycle (interphase and M phase), the process of mitosis (equational division) and meiosis (reductional division), cytokinesis, and the biological significance of each. It is a high-yield chapter for NEET and board exams, with frequent questions on the events of each phase, duration of phases, and the differences between mitosis and meiosis.
Cell Cycle and Interphase
The Cell Cycle
The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises other constituents, and eventually divides into two daughter cells. It was studied in detail in human cells in culture, which divide once in approximately 24 hours.
The cell cycle is divided into two basic phases:
- Interphase — the resting phase between two successive M phases (~95% of cycle duration).
- M phase (Mitosis phase) — actual cell division.
Phases of Interphase
Interphase is the period of high metabolic activity during which the cell prepares for division. It has three sub-phases:
| Phase | Full Name | Key Events |
|---|---|---|
| G1 | Gap 1 | Cell metabolically active, grows continuously; DNA not replicated |
| S | Synthesis | DNA replication — DNA doubles (2C → 4C); chromosome number stays same (2n); centriole duplicates in animal cells |
| G2 | Gap 2 | Proteins synthesised; cell prepares for mitosis; growth continues |
NEET Trap: During the S phase the amount of DNA doubles but the number of chromosomes does NOT change. A chromosome with 2 sister chromatids is still counted as ONE chromosome.
G0 (Quiescent) Stage
Some cells (e.g. heart cells in adult animals, neurons) do not divide. They exit G1 and enter an inactive stage called the quiescent stage (G0). Such cells are metabolically active but no longer proliferate unless called to do so.
Duration in a Typical Human Cell (24 h)
- G1: ~11 hours
- S: ~8 hours
- G2: ~4 hours
- M phase: only about 1 hour
In yeast, the cell cycle can be as short as ~90 minutes.
M Phase — Mitosis and Cytokinesis
M Phase (Mitotic Phase)
Mitosis is also called equational division because the number of chromosomes in the parent and daughter cells is the same. It occurs in somatic cells and is divided into karyokinesis (division of nucleus) and cytokinesis (division of cytoplasm).
Stages of Karyokinesis
1. Prophase
- Chromosomal material condenses to form compact chromosomes (each with two sister chromatids joined at the centromere).
- Centrosomes move to opposite poles; spindle fibres begin to form.
- By end of prophase: nuclear envelope and nucleolus disappear; Golgi and ER also disappear.
2. Metaphase
- Chromosomes align at the equator (metaphase plate).
- Spindle fibres attach to kinetochores of chromosomes.
- This is the stage where chromosomes are most condensed — ideal for studying chromosome morphology/karyotyping.
3. Anaphase
- Centromeres split; sister chromatids separate and move to opposite poles.
- Each chromatid is now a daughter chromosome (with centromere leading, arms trailing).
4. Telophase
- Chromosomes cluster at poles and decondense.
- Nuclear envelope and nucleolus reappear; Golgi and ER reform.
Cytokinesis
| Feature | Animal Cells | Plant Cells |
|---|---|---|
| Mechanism | Cleavage furrow (constriction) | Cell plate formation |
| Direction | Outside → inward | Centre → outward (centrifugal) |
Tip: In some organisms karyokinesis is not followed by cytokinesis, producing a multinucleate condition called a syncytium (e.g. liquid endosperm in coconut).
Significance of Mitosis
- Produces genetically identical daughter cells (diploid → diploid).
- Responsible for growth, repair, and wound healing.
- Maintains chromosome number and cell size (nucleo-cytoplasmic ratio).
- Basis of vegetative propagation in plants.
Meiosis and Comparison with Mitosis
Meiosis — Reductional Division
Meiosis occurs in diploid germ cells and results in four haploid daughter cells. It involves two sequential divisions (Meiosis I and II) but only one round of DNA replication. It is essential for sexual reproduction and maintaining chromosome number across generations.
Meiosis I (Reductional)
Prophase I is the longest and most complex; it has 5 sub-stages:
| Sub-stage | Key Event |
|---|---|
| Leptotene | Chromosomes become visible, condense |
| Zygotene | Pairing of homologous chromosomes — synapsis; forms bivalent/tetrad (synaptonemal complex) |
| Pachytene | Crossing over occurs; recombination nodules; exchange of genetic material |
| Diplotene | Synaptonemal complex dissolves; chiasmata visible |
| Diakinesis | Terminalisation of chiasmata; nuclear envelope breaks down |
- Metaphase I: Bivalents align at equator.
- Anaphase I: Homologous chromosomes separate (sister chromatids stay together) → reduces number to haploid.
- Telophase I: Nuclear membrane reappears; cytokinesis follows (dyad of cells).
Meiosis II (Equational)
Resembles mitosis. Prophase II → Metaphase II → Anaphase II (centromeres split, sister chromatids separate) → Telophase II. Result: 4 haploid cells.
Significance of Meiosis
- Maintains constant chromosome number of a species over generations.
- Crossing over and independent assortment create genetic variation — raw material for evolution.
Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cells | 2 | 4 |
| Chromosome number | Same (2n → 2n) | Halved (2n → n) |
| Crossing over | Absent | Present (pachytene) |
| Site | Somatic cells | Germ/reproductive cells |
| Genetic outcome | Identical | Variation |
NEET Trap: Reduction in chromosome number occurs at Anaphase I (homologues separate), NOT Anaphase II. DNA replication happens only ONCE (before Meiosis I).
Frequently asked questions
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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Cell Cycle and Cell Division.