# Cell Cycle and Cell Division (NEET Biology Class 11): Mitosis, Meiosis & Prophase I Source: https://tracknation.in/story/cell-cycle-and-cell-division-neet-biology-class-11-mitosis-meiosis-prophase-i (TrackNation ā€” free UPSC notes with PYQs) > NEET Class 11 Cell Cycle and Cell Division, mined from NCERT: the cell cycle (interphase's G1/S/G2 phases, M phase), mitosis's four stages of karyokinesis plus cytokinesis (animal furrow vs plant cell plate), significance of mitosis, meiosis I in full detail (the five sub-stages of prophase I ā€” leptotene through diakinesis, synapsis, crossing over, chiasmata), meiosis II, significance of meiosis, and a full mitosis-vs-meiosis comparison table. With 12 MCQs, 7 NEET PYQs and an Active Recall promp ## The Cell Cycle ā€” Interphase & M Phase Overview šŸ“Œ **NEET priority: Important.** *2ā€“4 questions a year directly, but mitosis and meiosis are the most-referenced processes in all of NEET Biology ā€” get this chapter rock solid and it pays off across Genetics, Reproduction and Cell Biology too.* The **cell cycle** is the ordered sequence of events by which a cell duplicates its genome, synthesises other cell constituents, and divides into two daughter cells. A human cell in culture divides roughly every **24 hours**; yeast can complete a cycle in just **90 minutes** ā€” duration varies by organism and cell type. ### Two basic phases - **Interphase** ā€” the "resting" phase between two M phases, though the cell is actively preparing to divide (growth + DNA replication). Interphase occupies **>95% of the cycle** ā€” in the 24-hour human cell cycle, actual division (M phase) lasts only about an **hour**. - **M phase (Mitotic phase)** ā€” starts with **karyokinesis** (nuclear division/chromosome separation) and usually ends with **cytokinesis** (cytoplasmic division). - Cells that stop dividing (e.g. **heart cells**) exit G1 into an inactive **quiescent stage, G0** ā€” metabolically active but not proliferating, until called back into the cycle. - **Trap:** mitosis is normally restricted to diploid cells, but there ARE exceptions ā€” **male honey bees** are haploid and still divide by mitosis; plants can show mitosis in both haploid and diploid cells (e.g. gametophyte generation). ## Interphase (G1/S/G2) & Mitosis's Four Stages ### Interphase in detail ā€” G1, S, G2 - **G1 (Gap 1)**: between the end of mitosis and the start of DNA replication. The cell is **metabolically active, grows continuously**, but does **NOT** replicate DNA yet. - **S (Synthesis)**: DNA replication occurs ā€” DNA content doubles from **2C to 4C**. **Crucially, chromosome NUMBER does not change** (a diploid cell stays 2n) ā€” replication just doubles each chromosome's DNA, giving it two sister chromatids. In animal cells, the **centriole also duplicates** during S phase. - **G2 (Gap 2)**: cell growth continues and **proteins needed for mitosis are synthesised**, preparing the cell for M phase. ### Mitosis (M phase) ā€” the four stages of karyokinesis - **Prophase**: chromatin **condenses** into compact chromosomes (each with 2 sister chromatids joined at the centromere); the duplicated **centrosome** moves to opposite poles, radiating **asters** that plus spindle fibres form the mitotic apparatus. By end of prophase, the **Golgi, ER, nucleolus and nuclear envelope all disappear**. - **Metaphase**: nuclear envelope is fully gone; chromosomes (each with sister chromatids held by the centromere, with **kinetochores** as spindle-fibre attachment sites) align at the **metaphase plate** (spindle equator) ā€” the best stage to study chromosome morphology. - **Anaphase**: **centromeres split simultaneously**; sister chromatids (now daughter chromosomes) move to opposite poles, centromere leading, arms trailing. - **Telophase**: chromosomes decondense and lose individuality; a **new nuclear envelope** forms around each cluster (2 daughter nuclei); nucleolus, Golgi and ER **reform**. - **Cytokinesis**: in **animal cells**, a **furrow** in the plasma membrane deepens and pinches the cell in two. In **plant cells** (rigid cell wall), a **cell plate** forms at the centre and grows outward to meet the lateral walls. If karyokinesis is NOT followed by cytokinesis, a **multinucleate syncytium** results (e.g. the liquid endosperm of coconut). ## Significance of Mitosis & Meiosis I ### Significance of mitosis - Mitosis is an **equational division** ā€” daughter cells are **genetically identical** to the parent, same chromosome number. It drives **growth** of multicellular organisms, restores the disturbed **nucleo-cytoplasmic ratio** after cell growth, enables **cell/tissue repair** (epidermis, gut lining, blood cells constantly replaced), and via meristematic tissue (apical + lateral cambium) gives plants **lifelong growth**. ### Meiosis ā€” the reduction division - Meiosis produces **haploid gametes** from specialised diploid cells ā€” **one DNA replication** (at S phase) but **two sequential divisions**, meiosis I and meiosis II, yielding **4 haploid cells**. It involves **pairing of homologous chromosomes** and **recombination** between non-sister chromatids. ### Meiosis I ā€” prophase I's five sub-stages | **Sub-stage** | **Key event** | | Leptotene | Chromosomes become gradually visible; compaction begins | | Zygotene | Homologous chromosomes pair (synapsis); synaptonemal complex forms; pair = bivalent/tetrad | | Pachytene | Four chromatids of each bivalent become distinct; crossing over occurs at recombination nodules (enzyme: recombinase) | | Diplotene | Synaptonemal complex dissolves; homologues separate except at crossover points ā€” chiasmata (X-shapes) | | Diakinesis | Chiasmata terminalise; chromosomes fully condense; nucleolus disappears, nuclear envelope breaks down | After prophase I: **Metaphase I** ā€” bivalents align at the equatorial plate. **Anaphase I** ā€” homologous chromosomes separate (sister chromatids stay together, unlike mitotic anaphase). **Telophase I** ā€” nuclear membrane/nucleolus reappear, cytokinesis gives a **dyad of cells**. A brief, DNA-replication-free **interkinesis** follows. ## Meiosis II, Significance & Why This Matters for NEET ### Meiosis II ā€” like a normal mitosis - Meiosis II starts right after cytokinesis, often before chromosomes fully re-elongate. **Prophase II ā†’ Metaphase II ā†’ Anaphase II** (centromeres split, sister chromatids separate) **ā†’ Telophase II** ā€” ending in a **tetrad of 4 haploid daughter cells**. ### Significance of meiosis - Meiosis **conserves the chromosome number** of a species across generations ā€” halving it in gametes so fertilisation restores the diploid number. Crossing over during prophase I also **increases genetic variability**, the raw material for evolution. ### Mitosis vs meiosis | **Feature** | **Mitosis** | **Meiosis** | | Occurs in | Somatic (body) cells ā€” usually diploid | Germ cells destined to form gametes | | Number of divisions | One | Two (meiosis I + meiosis II) | | DNA replication | Once, before division | Once, before meiosis I only | | Homologous pairing / crossing over | Never | Yes, during prophase I | | Daughter cells | 2, genetically identical, same chromosome number (2n ā†’ 2n) | 4, haploid, genetically variable (2n ā†’ n) | | Also called | Equational division | Reduction division | ### Why this matters for NEET - High-value one-liners: S phase changes DNA content (2Cā†’4C) but NOT chromosome number; kinetochores (not centromeres directly) attach spindle fibres; mitotic anaphase splits centromeres/sister chromatids, meiotic anaphase I separates whole homologous chromosomes (sister chromatids stay joined); crossing over enzyme = recombinase; plant cytokinesis uses a **cell plate**, animal uses a **furrow**. - **Trap:** don't confuse **chiasmata** (the visible X-shaped crossover sites, seen from diplotene onward) with **synapsis** (the pairing itself, at zygotene) or the **synaptonemal complex** (the structural scaffold that dissolves at diplotene). And a **syncytium** (e.g. coconut liquid endosperm) means karyokinesis happened WITHOUT cytokinesis ā€” many nuclei, no cell walls between them. ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** The phase of the cell cycle that occupies more than 95% of its duration is: - (a) Interphase - (b) Anaphase - (c) M phase - (d) Cytokinesis only **Answer: (a)** ā€” Interphase (G1+S+G2) makes up over 95% of a typical cell cycle; M phase (division) is comparatively brief. **Q2.** During the S phase of the cell cycle: - (a) The cell divides into two - (b) DNA content doubles but chromosome number stays the same - (c) Chromosome number doubles - (d) Proteins for mitosis are made **Answer: (b)** ā€” S phase doubles DNA content (2C to 4C) by replication, but the chromosome number (e.g. 2n) does not change. **Q3.** Cells that permanently exit the cell cycle without dividing (e.g. heart cells) enter the: - (a) G0 (quiescent) stage - (b) Anaphase - (c) G2 phase - (d) S phase **Answer: (a)** ā€” Non-dividing cells exit G1 into G0, the quiescent stage ā€” metabolically active but not proliferating. **Q4.** The disc-shaped structures on the centromere that serve as spindle-fibre attachment sites are: - (a) Kinetochores - (b) Bivalents - (c) Chiasmata - (d) Asters **Answer: (a)** ā€” Kinetochores are the attachment sites for spindle fibres, located at the centromere. **Q5.** Cytokinesis in plant cells occurs by: - (a) Formation of a furrow in the plasma membrane - (b) Formation of a cell plate that grows outward - (c) Complete absence of cytoplasmic division - (d) Formation of a syncytium always **Answer: (b)** ā€” Plant cells, having a rigid cell wall, divide via a cell plate that forms centrally and grows to meet the lateral walls. **Q6.** Synapsis, the pairing of homologous chromosomes, occurs during: - (a) Diakinesis - (b) Pachytene - (c) Zygotene - (d) Leptotene **Answer: (c)** ā€” Zygotene is defined by synapsis ā€” homologous chromosomes pairing to form bivalents. **Q7.** Crossing over between non-sister chromatids of homologous chromosomes occurs during: - (a) Leptotene - (b) Zygotene - (c) Diplotene - (d) Pachytene **Answer: (d)** ā€” Pachytene shows recombination nodules where crossing over occurs, mediated by the enzyme recombinase. **Q8.** The X-shaped structures seen where homologous chromosomes remain joined after separating is called: - (a) Synaptonemal complex - (b) Chiasmata - (c) Bivalent - (d) Tetrad **Answer: (b)** ā€” Chiasmata are the X-shaped structures marking crossover points, visible from diplotene onward. **Q9.** At the end of meiosis, the number of haploid daughter cells formed is: - (a) 2 - (b) 8 - (c) 4 - (d) 1 **Answer: (c)** ā€” Meiosis I and meiosis II together produce 4 haploid daughter cells from one diploid parent cell. **Q10.** In anaphase of mitosis, the split occurs at the: - (a) Centromere, separating sister chromatids - (b) Kinetochore only - (c) Chiasmata - (d) Nucleolus **Answer: (a)** ā€” Mitotic anaphase is marked by simultaneous splitting of centromeres, separating sister chromatids to opposite poles. **Q11.** Which of these is true about anaphase I of meiosis, unlike mitotic anaphase? - (a) No chromosome movement occurs - (b) Whole homologous chromosomes separate while sister chromatids stay joined - (c) Centromeres split - (d) Sister chromatids separate **Answer: (b)** ā€” In anaphase I, homologous chromosomes separate to opposite poles, but their sister chromatids remain attached at the centromere. **Q12.** A syncytium, such as the liquid endosperm of coconut, results from: - (a) Absence of both processes - (b) Meiosis without fertilisation - (c) Cytokinesis without karyokinesis - (d) Karyokinesis without cytokinesis **Answer: (d)** ā€” When karyokinesis (nuclear division) repeats without cytokinesis, the result is a multinucleate syncytium. **Q13.** Crossing over between non-sister chromatids of homologous chromosomes occurs during: - (a) Pachytene of prophase I - (b) Zygotene of prophase I - (c) Metaphase I - (d) Diplotene of prophase I **Answer: (a)** ā€” Crossing over occurs in PACHYTENE, mediated by the enzyme recombinase, and is visible later as chiasmata in diplotene. Synapsis (pairing) happens in the preceding zygotene stage, so the two are easily confused. **Q14.** The synaptonemal complex is formed during which sub-stage of prophase I? - (a) Diakinesis - (b) Leptotene - (c) Zygotene - (d) Pachytene **Answer: (c)** ā€” Zygotene is when homologous chromosomes pair ā€” synapsis ā€” and the synaptonemal complex forms between them. The paired complex of two synapsed homologues is called a bivalent or tetrad. **Q15.** Chiasmata become visible during which stage? - (a) Leptotene - (b) Diplotene - (c) Zygotene - (d) Pachytene **Answer: (b)** ā€” In diplotene the synaptonemal complex dissolves and the homologues separate slightly, except at the X-shaped chiasmata ā€” the physical sites where crossing over took place. **Q16.** Reduction division, halving the chromosome number, occurs in: - (a) Meiosis II - (b) Mitosis - (c) Both meiosis I and meiosis II - (d) Meiosis I **Answer: (d)** ā€” Meiosis I is the reductional division: homologous chromosomes separate, halving the chromosome number. Meiosis II is equational, separating sister chromatids just as mitosis does. **Q17.** In which phase of mitosis do chromosomes align at the equatorial plate? - (a) Telophase - (b) Anaphase - (c) Prophase - (d) Metaphase **Answer: (d)** ā€” Metaphase is defined by the alignment of chromosomes at the metaphase plate, with spindle fibres attached to the kinetochores. It is also the stage at which chromosome morphology is most easily studied. **Q18.** The stage of the cell cycle in which a cell exits the cycle and enters a quiescent state is: - (a) S - (b) G1 - (c) G2 - (d) G0 **Answer: (d)** ā€” Cells that leave the cycle enter G0, the quiescent stage, where they remain metabolically active but no longer divide unless required to. Heart cells and neurons are classic examples of cells that stay in G0. **Q19.** DNA replication occurs during which phase, and what happens to chromosome number? - (a) S phase; both DNA and chromosome number double - (b) G2 phase; the DNA doubles - (c) S phase; the DNA doubles but the chromosome number stays the same - (d) G1 phase; the chromosome number doubles **Answer: (c)** ā€” In S phase the DNA content doubles from 2C to 4C, but the chromosome NUMBER is unchanged at 2n ā€” each chromosome now simply has two sister chromatids. Separating DNA amount from chromosome number is the classic trap here. **Q20.** In plant cells, cytokinesis occurs by: - (a) Cell plate formation, proceeding from the centre outwards - (b) Constriction of the nuclear envelope - (c) A cleavage furrow, proceeding from the periphery inwards - (d) Cell plate formation, proceeding from the periphery inwards **Answer: (a)** ā€” Plant cells build a cell plate that grows from the centre towards the walls ā€” the reverse direction from the animal cleavage furrow, which pinches inwards from the periphery. The rigid cell wall is why plants cannot use a furrow. **Q21.** The stage of mitosis shown in S is - (a) Prophase - (b) Metaphase - (c) Anaphase - (d) Telophase **Answer: (b)** ā€” In S the chromosomes are lined up on the equatorial (metaphase) plate with spindle fibres attached to their centromeres from both poles. **Q22.** Splitting of the centromeres and movement of sister chromatids to opposite poles is shown in - (a) R - (b) S - (c) Q - (d) P **Answer: (d)** ā€” P shows anaphase: the centromeres have split and the daughter chromosomes, led by their centromeres, are pulled towards the two poles. **Q23.** The correct sequence of the stages shown, from earliest to latest, is - (a) P ā†’ Q ā†’ R ā†’ S - (b) Q ā†’ P ā†’ S ā†’ R - (c) S ā†’ Q ā†’ P ā†’ R - (d) Q ā†’ S ā†’ P ā†’ R **Answer: (d)** ā€” Q is prophase (condensing chromosomes, nuclear envelope breaking down), S is metaphase, P is anaphase and R is telophase (nuclei re-forming, cell dividing). ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q24.** The average duration of the cell cycle in a mammalian cell is about: (NEET PYQ) - (a) 90 minutes - (b) 7 days - (c) 24 hours - (d) 1 hour **Answer: (c)** ā€” A typical mammalian (e.g. human) cell in culture divides roughly once every 24 hours. **Q25.** Which enzyme mediates crossing over during pachytene? (NEET PYQ) - (a) DNA polymerase - (b) Recombinase - (c) Helicase - (d) Ligase **Answer: (b)** ā€” Crossing over is an enzyme-mediated process carried out by recombinase. **Q26.** Mitosis is called equational division because: (NEET PYQ) - (a) It reduces chromosome number by half - (b) It only occurs in haploid cells - (c) It involves two divisions - (d) The chromosome number of daughter and parent cells is the same **Answer: (d)** ā€” Mitosis is 'equational' since parent and daughter cells retain the same chromosome number. **Q27.** Meiosis is referred to as reduction division because: (NEET PYQ) - (a) It has no DNA replication - (b) It occurs only in plants - (c) It halves the chromosome number to produce haploid gametes - (d) It doubles the chromosome number **Answer: (c)** ā€” Meiosis reduces the chromosome number by half, producing haploid gametes from diploid cells. **Q28.** The stage between meiosis I and meiosis II, with no DNA replication, is called: (NEET PYQ) - (a) Prophase II - (b) Interkinesis - (c) Diakinesis - (d) G0 phase **Answer: (b)** ā€” Interkinesis is the brief, DNA-replication-free gap between meiosis I and meiosis II. **Q29.** Which of the following best describes onion root tip cells used to study mitosis? (NEET PYQ) - (a) Only used to study meiosis - (b) 16 chromosomes per cell - (c) 46 chromosomes per cell - (d) No chromosomes visible **Answer: (b)** ā€” Onion root tip cells, a classic mitosis study material, have 16 chromosomes per cell. **Q30.** The centriole duplicates during which phase of the cell cycle in animal cells? (NEET PYQ) - (a) G2 phase - (b) S phase - (c) M phase - (d) G1 phase **Answer: (b)** ā€” In animal cells, centriole duplication occurs during the S phase, alongside DNA replication. ### Active Recall Prompt Write everything you can recall about **Cell Cycle and Cell Division**, naming each part first: the **cell cycle** (duration, interphase vs M phase, G0); **interphase** (G1, S, G2 ā€” what happens in each); **mitosis** (the four stages of karyokinesis in order, then cytokinesis in animal vs plant cells); the **significance of mitosis**; **meiosis** (why it happens, two divisions one replication); **meiosis I** (the five sub-stages of prophase I, then metaphase/anaphase/telophase I); **meiosis II**; and the **significance of meiosis**. Begin each fact with its topic and end it with a full stop.