Cell Cycle and Cell Division covers the phases of the cell cycle (G1, S, G2 and M), mitosis and its four stages, cytokinesis in plant and animal cells, the two divisions of meiosis with crossing over, and the significance of and differences between mitosis and meiosis. It is a high-yield NEET chapter where questions test the events of each phase, the stage of DNA replication and crossing over, and how chromosome number changes.
Table of Contents
- Key Concepts - Cell cycle, interphase, mitosis, cytokinesis, meiosis I & II, significance
- Weightage in Board & Entrance Exams
- Important Definitions
- Illustrative Examples
- Important Questions for Board Exams
- Quick Revision Points
Key Concepts
1. The Cell Cycle
The cell cycle is the sequence of events by which a cell duplicates its contents (DNA, organelles) and divides into two daughter cells. It is the orderly “growth-then-division” loop that every dividing cell follows.
A typical human cell takes about 24 hours to complete one cycle, while a yeast cell may take only about 90 minutes. The cycle has two broad parts: a long interphase (preparation) and a short M phase (actual division).
Two Main Phases
- Interphase: the resting-but-busy phase where the cell grows and copies its DNA - covers about 95% of the cycle.
- M phase (Mitotic phase): the phase in which the nucleus and cytoplasm actually divide.
2. Phases of Interphase (G₁, S, G₂)
Interphase is the longest part of the cycle and is split into three sub-phases. The DNA is copied only once, in the S phase.
| Phase | What Happens |
|---|---|
| G₁ (Gap 1) | Cell is metabolically active, grows in size, makes RNA and proteins. DNA is NOT replicated. |
| S (Synthesis) | DNA replication occurs - the amount of DNA doubles (2n DNA becomes 4n DNA), but chromosome number stays the same. |
| G₂ (Gap 2) | Cell continues to grow; proteins and organelles needed for division are synthesised. |
Key idea: In the S phase the DNA content doubles, but the chromosome number does not change - each chromosome now has two sister chromatids.
The G₀ (Quiescent) Stage
Cells that do not divide further (like mature nerve cells) exit the cycle at G₁ and enter an inactive stage called G₀. They stay metabolically active but stop dividing.
3. M Phase and Mitosis (Equational Division)
Mitosis is the division of the nucleus that produces two daughter cells genetically identical to the parent, each with the same chromosome number. It is also called equational division because the chromosome number is conserved (2n → 2n).
It occurs in somatic (body) cells and is responsible for growth, repair, and replacement of worn-out cells. Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase.
4. Prophase
Prophase is the first stage of mitosis, marking the end of interphase.
- Chromatin condenses into compact, visible chromosomes, each made of two sister chromatids joined at the centromere.
- The centrioles move to opposite poles and begin forming the spindle fibres.
- The nuclear envelope and nucleolus disappear by the end of prophase.
[DIAGRAM: A cell in prophase - condensed chromosomes with two chromatids each, centrioles at opposite poles, spindle fibres beginning to form, no nuclear membrane.]
5. Metaphase
Metaphase is the stage of maximum chromosome condensation, ideal for studying chromosome morphology.
- Chromosomes line up at the centre of the cell along the metaphase plate (equator).
- Spindle fibres attach to the kinetochore on the centromere of each chromosome.
- Chromosomes are most condensed and clearly visible here.
Key idea: The alignment of chromosomes at the equatorial plate is the defining feature of metaphase.
6. Anaphase
Anaphase is the shortest stage, where the sister chromatids finally separate.
- The centromeres split, and the two sister chromatids of each chromosome separate.
- Each chromatid (now a daughter chromosome) moves towards opposite poles, pulled by the shortening spindle fibres.
- The centromere leads and the arms trail behind.
7. Telophase and Cytokinesis
Telophase is essentially prophase in reverse - the cell rebuilds two nuclei.
- Chromosomes reach the poles and decondense back into chromatin.
- The nuclear envelope and nucleolus reappear around each set of chromosomes.
- The spindle fibres disappear.
Cytokinesis
Cytokinesis is the division of the cytoplasm that completes cell division, producing two separate daughter cells.
- In animal cells: a cleavage furrow pinches the cell inward from the membrane.
- In plant cells: a cell plate forms from the centre outward (because of the rigid cell wall).
8. Significance of Mitosis
Mitosis is the basis of growth and maintenance of multicellular bodies.
- Growth: increases the number of cells, helping organisms grow.
- Repair and replacement: replaces worn-out cells of skin, blood, and the gut lining.
- Genetic stability: daughter cells are genetically identical to the parent (no variation).
- Cell-size restoration: keeps the nucleo-cytoplasmic ratio constant.
9. Meiosis (Reductional Division)
Meiosis is the division that halves the chromosome number, producing four haploid (n) daughter cells from one diploid (2n) parent cell. It is also called reductional division because 2n becomes n.
It occurs only in the reproductive (germ) cells during gamete formation, and it ensures that the chromosome number stays constant from generation to generation. Meiosis has two successive divisions - meiosis I and meiosis II - but DNA replicates only once, before meiosis I.
10. Meiosis I - Reductional Division
Meiosis I is where the actual reduction in chromosome number happens, when homologous chromosomes separate.
Prophase I - Five Sub-stages
Prophase I is the longest and most complex phase, divided into five sub-stages (remember: Lazy Zebras Practise Diving Daily).
| Sub-stage | Key Event |
|---|---|
| Leptotene | Chromosomes condense and become visible as thin threads. |
| Zygotene | Homologous chromosomes pair up - called synapsis; each pair forms a bivalent (or tetrad). |
| Pachytene | Crossing over occurs - exchange of segments between non-sister chromatids at the chiasmata; the chief source of variation. |
| Diplotene | Homologues begin to separate but stay joined at points called chiasmata. |
| Diakinesis | Chiasmata terminalise, nucleolus disappears, nuclear envelope breaks down - cell is ready for metaphase I. |
Rest of Meiosis I
- Metaphase I: bivalents align at the equator; spindle fibres attach to the homologous chromosomes.
- Anaphase I: homologous chromosomes separate and move to opposite poles, but sister chromatids stay together (centromeres do NOT split).
- Telophase I: nuclear membrane reappears; two haploid cells form (each chromosome still has two chromatids).
Key idea: The reduction from 2n to n happens in anaphase I, when homologues - not chromatids - separate.
11. Meiosis II - Equational Division
Meiosis II is essentially like mitosis, separating sister chromatids, but it acts on haploid cells. There is no DNA replication before it.
- Prophase II: chromosomes condense again; nuclear envelope disappears.
- Metaphase II: chromosomes line up at the equator; spindle fibres attach to kinetochores.
- Anaphase II: centromeres split and sister chromatids separate to opposite poles.
- Telophase II: nuclei reform; cytokinesis gives a total of four haploid daughter cells.
12. Significance of Meiosis
Meiosis is the engine of both genetic constancy and genetic variation.
- Maintains chromosome number: halving in gametes means fertilisation restores the diploid number - the number stays constant across generations.
- Genetic variation: crossing over (pachytene) and independent assortment of chromosomes shuffle genes, driving evolution.
- Gamete formation: produces haploid sperm and eggs.
- Basis of inheritance: the recombination here is the foundation of the laws of genetics you will study in Class 12.
13. Mitosis vs Meiosis - Comparison
| Feature | Mitosis | Meiosis |
|---|---|---|
| Where it occurs | Somatic (body) cells | Germ (reproductive) cells |
| Number of divisions | One | Two (meiosis I & II) |
| Daughter cells | 2 | 4 |
| Chromosome number | Stays the same (2n → 2n) | Halved (2n → n) |
| Crossing over | Absent | Present (pachytene) |
| Genetic result | Identical to parent | Genetically variable |
| Purpose | Growth, repair | Gamete formation, variation |
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Weightage in Board & Entrance Exams
| Exam | Typical Weightage | Most-Tested Areas |
|---|---|---|
| CBSE Board (Class 11) | 5–6 marks | Phases of cell cycle, stages of mitosis, prophase I sub-stages, mitosis vs meiosis |
| NEET | 3–5 questions | Prophase I stages, significance of meiosis, DNA content per phase, cytokinesis |
| CUET / State CETs | 2–3 questions | Definitions, comparison table, G₀ stage, crossing over |
[TABLE: Question-type split - VSA (1 mark): definitions & phase identification; SA (2–3 marks): significance, prophase I stages, cytokinesis difference; LA (5 marks): full mitosis or meiosis description, mitosis vs meiosis comparison.]
Important Definitions
| Term | Definition |
|---|---|
| Cell cycle | Sequence of growth and division by which a cell duplicates and divides into two |
| Interphase | Preparatory phase (G₁, S, G₂) where the cell grows and replicates DNA |
| S phase | Phase of interphase in which DNA replication doubles the DNA content |
| G₀ stage | Quiescent stage in which a cell stops dividing but stays metabolically active |
| Mitosis | Equational nuclear division giving two genetically identical diploid cells |
| Cytokinesis | Division of the cytoplasm that completes cell division |
| Meiosis | Reductional division giving four haploid cells from one diploid cell |
| Synapsis | Pairing of homologous chromosomes during zygotene of prophase I |
| Bivalent (tetrad) | A paired structure of two homologous chromosomes (four chromatids) |
| Crossing over | Exchange of segments between non-sister chromatids at the chiasmata (pachytene) |
Illustrative Examples
Example 1
A cell has 24 chromosomes in G₁. How many chromosomes and how many DNA molecules will it have at the end of the S phase?
Answer: Chromosome number stays 24, but DNA doubles - so there are now 24 chromosomes each with 2 chromatids, i.e. 48 DNA molecules.
Example 2
A diploid plant cell with 2n = 16 undergoes meiosis. How many chromosomes will each of the four daughter cells have?
Answer: Meiosis halves the number, so n = 16/2 = 8 chromosomes in each of the four haploid cells.
Example 3
In which sub-stage of prophase I does crossing over occur, and what structure holds the homologues together afterwards?
Answer: Crossing over occurs in pachytene; the homologues remain held at chiasmata (visible in diplotene).
Example 4
How many cell divisions and how many daughter cells result from one round of mitosis and one round of meiosis?
Answer: Mitosis = 1 division → 2 cells. Meiosis = 2 divisions → 4 cells.
Example 5
If a human somatic cell (2n = 46) divides by mitosis, how many chromosomes will each daughter cell have?
Answer: Mitosis is equational, so each daughter cell has 46 chromosomes (the same as the parent).
Example 6
Name the stage of mitosis in which (a) chromosomes align at the equator and (b) sister chromatids separate.
Answer: (a) Metaphase - alignment at the metaphase plate; (b) Anaphase - centromeres split and chromatids move to opposite poles.
Important Questions for Board Exams
1-Mark Questions (VSA)
- In which phase of the cell cycle does DNA replication occur?
- What is the G₀ stage of the cell cycle?
- Name the structure that joins two sister chromatids.
- In which sub-stage of meiosis does crossing over take place?
- How does cytokinesis differ in plant and animal cells?
2–3-Mark Questions (SA)
- List the three phases of interphase and state the main event of each.
- Why is meiosis called a reductional division? Explain with chromosome numbers.
- State any three points of significance of mitosis.
- Describe the events of anaphase in mitosis.
5-Mark Questions (LA)
- Describe the five sub-stages of prophase I of meiosis in sequence, highlighting the events in each.
- Compare mitosis and meiosis under at least five distinguishing features.
- Explain the four stages of mitosis (prophase to telophase) and the process of cytokinesis.
Quick Revision Points
- Cell cycle = Interphase (G₁, S, G₂) + M phase; interphase is ~95% of the cycle
- DNA replicates only in S phase - DNA doubles, chromosome number stays the same
- G₀ = quiescent, non-dividing but active stage
- Mitosis: P-M-A-T → 2 identical diploid cells (equational, 2n → 2n)
- Metaphase = chromosomes at equator; Anaphase = chromatids separate
- Cytokinesis: cleavage furrow (animal), cell plate (plant)
- Mitosis is for growth, repair, and replacement; no variation
- Meiosis = 2 divisions → 4 haploid cells (reductional, 2n → n)
- Prophase I stages: Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis
- Crossing over (pachytene) + independent assortment = source of variation
- Reduction (2n → n) happens in anaphase I; chromatids separate in anaphase II
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The Cell Cycle
An ordered, checked sequence in which a cell duplicates its DNA, grows and then divides into two.
Human cell cycle ~24 h; budding yeast ~90 min.
- Two broad phases: long Interphase (growth + copying) and short M phase (splitting).
- G1 → S is the major checkpoint where the cell commits to divide.
- Quiescent cells exit into G0 (e.g. heart, nerve) — alive but non-dividing.
Interphase: G1, S, G2
The cell spends most of its life growing and copying its genome before it ever divides.
Centriole also duplicates in S phase (animal cells).
- G1: cell grows, metabolically active, no DNA replication yet.
- S (Synthesis): DNA replicates → each chromosome gains a sister chromatid.
- G2: proteins (e.g. spindle) synthesised in preparation for mitosis.
DNA Amount vs Chromosome Number
After S phase the DNA doubles, yet the chromosome number stays exactly the same.
2n = 46 human cell: G1 = 2C/46, after S = 4C/46.
- Doubling DNA just adds a second chromatid per chromosome — no new centromeres.
- Never say chromosome number doubles in S phase; only DNA content (C) doubles.
- G0 cells are NOT dead — metabolically active but non-dividing.
Mitosis (Karyokinesis) — PMAT
Equational nuclear division giving two genetically identical daughter cells with the same chromosome number.
Occurs in somatic cells; in plants, meristematic tissues.
- Prophase: chromosomes condense; spindle forms; nuclear envelope + nucleolus disappear.
- Metaphase: chromosomes align at the metaphase plate — best stage to study morphology.
- Telophase: chromosomes decondense; nuclear envelope + nucleolus reappear.
Anaphase: Centromeres Split
The single defining event of mitotic anaphase is the splitting of centromeres.
Centromere splits in Anaphase, NOT in Metaphase.
- Sister chromatids separate and move apart as independent daughter chromosomes.
- Each daughter cell ends with the same chromosome number as the parent.
- Sister chromatids are identical copies — that is why daughters are identical.
Cytokinesis: Animal vs Plant
Division of the cytoplasm that completes cell division after the nucleus has split.
Cell plate becomes the middle lamella; begins in late anaphase/telophase.
- Animal furrow deepens centripetally (inward) and pinches the cell in two.
- Plant cell plate grows centrifugally (outward) to meet the side walls.
- Karyokinesis without cytokinesis → syncytium (e.g. coconut liquid endosperm).
Meiosis I + Prophase I
First meiotic (reductional) division that halves the chromosome number and shuffles the genes.
Zygotene = synapsis (synaptonemal complex); Pachytene = crossing over.
- Bivalent / tetrad = a pair of homologous chromosomes (4 chromatids); bivalents = n.
- Crossing over in pachytene; chiasmata first visible in diplotene → variation.
- Anaphase I: homologous chromosomes separate, centromere does NOT split.
Meiosis II
Second meiotic (equational) division that separates sister chromatids, just like mitosis.
Anaphase II: centromeres split (unlike Anaphase I).
- No DNA replication in interkinesis — DNA was copied only once, before meiosis I.
- Anaphase II splits centromeres so sister chromatids separate (mitosis-like).
- 1 diploid cell → 4 haploid, genetically distinct cells.
Significance · Mitosis vs Meiosis
Mitosis gives faithful copies for growth and repair; meiosis halves and shuffles to make varied gametes.
Halving + fertilisation keep the species number constant.
- Mitosis: 1 division → 2 diploid identical cells; somatic cells; no crossing over.
- Meiosis: 2 divisions → 4 haploid variable cells; germ cells; crossing over present.
- Crossing over + independent assortment = raw material for evolution.
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Frequently Asked Questions
The cell cycle has interphase (the resting and growth phase, about 95 percent of the cycle) and the M phase (mitosis). Interphase is divided into G1 (growth, no DNA synthesis), S (DNA replication) and G2 (growth before division). A typical human cell cycle lasts about 24 hours.
DNA replication occurs only in the S (synthesis) phase of interphase. During the S phase the amount of DNA doubles (2C to 4C), but the chromosome number stays the same because the duplicated DNA remains joined as sister chromatids.
In animal cells a cleavage furrow forms in the plasma membrane and deepens inward (centripetally) until the cell pinches into two. In plant cells, because of the rigid cell wall, a cell plate forms at the centre and grows outward (centrifugally) towards the side walls.
Crossing over, the exchange of segments between non-sister chromatids of homologous chromosomes, takes place during the pachytene sub-stage of prophase I of meiosis, mediated by recombination nodules. This generates genetic variation.
Meiosis has one round of DNA replication followed by two successive divisions, and the reductional first division (meiosis I) separates homologous chromosomes, halving the chromosome number to produce four haploid cells. Mitosis is a single equational division that separates sister chromatids, keeping the chromosome number the same and giving two identical diploid cells.