Chapter in a nutshell: Cells reproduce through the cell cycle — a long interphase (growth + DNA replication) followed by the M phase (division). Chromosomes carry the genetic material (DNA + protein); mitosis produces two identical diploid cells (growth/repair) while meiosis produces four genetically varied haploid cells (gametes), keeping the chromosome number constant across generations.
1. The Cell Cycle
The sequence of events from the formation of a cell to its division into daughter cells. It has two main phases:- Interphase (the "resting"/non-dividing but metabolically active stage) — actually the longest phase:
- M phase (Mitotic phase): karyokinesis (nuclear division) followed by cytokinesis (cytoplasmic division).
2. Structure of a Chromosome
Chromosomes are thread-like structures in the nucleus, visible during division.- Made of chromatin = DNA + histone proteins.
- A duplicated chromosome has two chromatids (sister chromatids) joined at the centromere; the centromere bears the kinetochore for spindle-fibre attachment.
- Telomeres are the protective ends.
- Types by centromere position: metacentric (middle), sub-metacentric (slightly off-centre), acrocentric (near one end), telocentric (at the tip).
3. DNA, Genes & Chromosome Number
- DNA is a double helix of nucleotides (sugar + phosphate + nitrogen base). Bases pair: A–T and G–C.
- A gene is a segment of DNA that controls a particular trait; genes are arranged linearly on chromosomes.
- Human chromosome number = 46 (23 pairs): 44 autosomes (22 pairs) + 2 sex chromosomes (XX female / XY male).
- Diploid (2n): body cells have chromosomes in pairs (human 2n = 46). Haploid (n): gametes have a single set (human n = 23).
4. Mitosis (Equational Division)
Occurs in somatic (body) cells; one parent cell → two genetically identical diploid daughter cells. Phases:| Phase | Key events |
|---|---|
| Prophase | chromosomes condense and become visible (each with 2 chromatids); nuclear membrane and nucleolus disappear; spindle begins forming |
| Metaphase | chromosomes line up at the equator (metaphase plate); spindle fibres attach to centromeres |
| Anaphase | centromeres split; sister chromatids move to opposite poles |
| Telophase | chromatids reach the poles; nuclear membranes reform; chromosomes uncoil |
- Significance: growth, repair and replacement of cells, asexual reproduction; keeps chromosome number constant.
5. Meiosis (Reductional Division)
Occurs in reproductive cells (germ cells); one diploid cell → four genetically different haploid cells through two successive divisions (Meiosis I and II) with DNA replicating only once.- Meiosis I (reductional): homologous chromosomes pair, crossing over (exchange of segments) occurs → variation; homologues separate → chromosome number halved.
- Meiosis II (equational): like mitosis; sister chromatids separate → 4 haploid cells.
- Significance: forms gametes; maintains the chromosome number generation to generation (halving before fertilisation restores 2n); introduces genetic variation (crossing over + independent assortment).
6. Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Site | body (somatic) cells | germ (reproductive) cells |
| Divisions | one | two (I and II) |
| Daughter cells | 2 | 4 |
| Chromosome number | unchanged (diploid → diploid) | halved (diploid → haploid) |
| Genetic make-up | identical to parent | genetically different (variation) |
| Crossing over | absent | present (Meiosis I) |
| Purpose | growth/repair | gamete formation |
7. Structured Examples (ICSE pattern)
Q1. Name the phase of the cell cycle in which DNA replicates. Solution: The S phase of interphase.Q2. A cell with 24 chromosomes undergoes mitosis. How many chromosomes will each daughter cell have? Solution: 24 (mitosis keeps the number unchanged).
Q3. The same cell (24 chromosomes) undergoes meiosis. How many chromosomes per gamete? Solution: 12 (meiosis halves the number).
Q4. Arrange the mitotic phases in order. Solution: Prophase → Metaphase → Anaphase → Telophase.
Q5. In which phase do chromosomes line up at the equator? Solution: Metaphase.
8. Key Terms — Quick Glossary
| Term | One-line definition |
|---|---|
| Cell cycle | sequence from cell formation to its division. |
| Interphase | growth + DNA replication stage (G₁, S, G₂). |
| Chromosome | DNA + protein thread carrying genes. |
| Chromatid | one of two identical halves of a duplicated chromosome. |
| Centromere | point joining sister chromatids; spindle attaches here. |
| Gene | DNA segment controlling a trait. |
| Mitosis | division giving 2 identical diploid cells. |
| Meiosis | division giving 4 varied haploid cells. |
| Diploid (2n) | two sets of chromosomes (body cells). |
| Haploid (n) | one set of chromosomes (gametes). |
9. Common Mistakes to Avoid
- Calling interphase the "resting phase" as if inactive — it is the most active/longest phase (DNA replicates here).
- Saying DNA replicates during mitosis — it replicates in the S phase of interphase, before division.
- Mixing up mitosis (2 identical diploid) with meiosis (4 varied haploid).
- Forgetting meiosis has two divisions but DNA replicates once.
- Confusing chromatid (half a duplicated chromosome) with chromosome.
10. Likely Exam Questions (with crisp answers)
- Name the two main phases of the cell cycle. → Interphase and M phase.
- In which sub-phase does DNA replicate? → S phase.
- What is a chromosome made of? → DNA and histone proteins (chromatin).
- Define a gene. → A segment of DNA that controls a specific trait.
- How many chromosomes are there in a human body cell? → 46 (23 pairs).
- Differentiate autosomes and sex chromosomes. → 44 autosomes control body traits; 2 sex chromosomes determine sex (XX/XY).
- Where does mitosis occur and what is its result? → In body cells; two identical diploid daughter cells.
- Where does meiosis occur and what is its result? → In germ cells; four genetically varied haploid cells.
- Why is meiosis called reductional division? → It halves the chromosome number.
- Name the process that introduces variation during meiosis. → Crossing over (and independent assortment).
- In which phase do sister chromatids separate in mitosis? → Anaphase.
- What is cytokinesis? → Division of the cytoplasm after nuclear division.
- Why is it important that meiosis halves the chromosome number? → So that fertilisation restores the normal diploid number, keeping it constant across generations.
- Define diploid and haploid. → Diploid = two chromosome sets (2n); haploid = one set (n).
11. DNA — Structure in Detail
- DNA (deoxyribonucleic acid) is a double helix of two strands twisted like a spiral staircase (Watson & Crick model).
- Each strand is a chain of nucleotides; a nucleotide = deoxyribose sugar + phosphate + a nitrogen base.
- Four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C). Pairing rule (complementary): A=T (2 H-bonds) and G≡C (3 H-bonds).
- The sugar–phosphate backbone runs along the outside; base pairs form the "rungs". This base sequence stores genetic information.
- RNA differs: single strand, ribose sugar, base uracil (U) in place of thymine.
12. Significance of Cell Division
- Mitosis: growth of the body, repair/replacement of worn-out cells (skin, blood), wound healing, and asexual reproduction in some organisms — all while keeping the chromosome number constant.
- Meiosis: produces gametes (sperm/egg), halves the chromosome number so fertilisation restores 2n, and creates genetic variation that drives evolution.
- Uncontrolled cell division (failure of cell-cycle control) leads to a tumour / cancer.
13. Chromosome Types by Centromere Position
| Type | Centromere position | Shape in anaphase |
|---|---|---|
| Metacentric | middle | V-shaped |
| Sub-metacentric | slightly off-centre | L-shaped |
| Acrocentric | near one end | J-shaped |
| Telocentric | at the very tip | I/rod-shaped |
14. More Structured Examples
Q6. If one DNA strand reads A-T-G-C, what is the complementary strand? Solution: T-A-C-G (A pairs T, G pairs C). Q7. A human sperm has how many chromosomes? After fertilisation, the zygote has how many? Solution: Sperm 23 (haploid); zygote 46 (diploid). Q8. Name the type of division that repairs a wound. Solution: Mitosis. Q9. How many daughter cells and of what ploidy result from one meiotic division? Solution: Four haploid cells.15. More Exam Questions (with crisp answers)
- State the base-pairing rule in DNA. → A pairs with T; G pairs with C.
- How does RNA differ from DNA? → RNA is single-stranded, has ribose sugar and uracil instead of thymine.
- What is the result of uncontrolled cell division? → Tumour/cancer.
- Why is mitosis called equational division? → Daughter cells have the same chromosome number as the parent.
- Name the longest phase of the cell cycle. → Interphase.
- What holds the two strands of DNA together? → Hydrogen bonds between complementary base pairs.
- Define homologous chromosomes. → A matching pair of chromosomes, one from each parent, with the same genes.
- Which division is essential for sexual reproduction and why? → Meiosis — it forms haploid gametes and maintains the species' chromosome number.
16. Diploid Chromosome Numbers (for reference)
| Organism | 2n |
|---|---|
| Human | 46 |
| Onion | 16 |
| Fruit fly (Drosophila) | 8 |
| Pea | 14 |
| Dog | 78 |
17. Final Quick-Revision Q&A
- Why must DNA replicate before cell division? → So each daughter cell receives a complete, identical copy of the genetic material.
- Name the structure to which spindle fibres attach on a chromosome. → The centromere (kinetochore).
- In meiosis, when does the chromosome number actually halve? → During Meiosis I, when homologous chromosomes separate.
- State one similarity between mitosis and meiosis II. → Both separate sister chromatids (equational).
- What would happen if gametes were formed by mitosis? → The chromosome number would double every generation.
- Define crossing over. → Exchange of segments between homologous chromosomes during Meiosis I, producing variation.
- Name the scientists who proposed the DNA double-helix model. → Watson and Crick.