Questions: Cell Cycle and Cell Division

Q1. Why is the cell cycle called a cycle, and what are its two main phases?

Answer (English): A cycle returns to its starting point, and the cell cycle does the same: a newly formed cell grows, copies its DNA and divides, and each daughter then repeats the same sequence. The two main phases are interphase, the preparation period, and the M phase, the division proper. Interphase is further split into G1, S and G2, and the M phase covers karyokinesis and cytokinesis. Interphase lasts more than 95 per cent of a normal human cell cycle.

Hinglish: Cycle isliye kehte hain kyunki division ke baad bani daughter cell phir wahi growth aur DNA replication ka sequence repeat karti hai. Do main phases hain: interphase (G1, S, G2) aur M phase (karyokinesis aur cytokinesis).

Q2. What happens in the S phase, and why does the chromosome number not change?

Answer (English): In S phase the cell replicates its DNA, so the DNA content rises from 2C to 4C. The two new DNA molecules stay attached at the same centromere as sister chromatids, so no new chromosome is created. A diploid cell with 2n chromosomes before S still shows 2n chromosomes after S. In animal cells the centriole also duplicates in the cytoplasm during this phase.

Hinglish: S phase mein DNA replication hoti hai aur DNA 2C se 4C ho jata hai, lekin dono copies ek hi centromere par judi rehti hain, isliye chromosome number 2n hi rehta hai. Animal cells mein centriole bhi yahin duplicate hota hai.

Q3. What is the G0 stage, and which cells enter it?

Answer (English): G0 is a quiescent stage entered by cells that leave the cycle after G1 and stop dividing. These cells keep their normal metabolism but do not replicate DNA or divide unless they receive a signal. Adult heart muscle cells and most neurons stay in G0 permanently, while liver cells remain in G0 most of the time and re-enter the cycle when the organ needs repair.

Hinglish: G0 quiescent stage hai jahan G1 ke baad cells division karna band kar dete hain par metabolism chalta rehta hai. Adult heart cells aur neurons isme permanent rehte hain, aur liver cells injury par wapas cycle mein aa jate hain.

Q4. List the four stages of mitosis with their key events.

Answer (English): In prophase the chromatin condenses into visible chromosomes, the duplicated centrosome moves to opposite poles and the mitotic apparatus forms; the nucleolus and nuclear envelope disappear by the end of the stage. In metaphase the nuclear envelope is gone, chromosomes attach to spindle fibres at their kinetochores and line up on the metaphase plate. In anaphase the centromeres split and the sister chromatids move to opposite poles. In telophase the chromosomes decondense at the poles, the nuclear envelope and nucleolus reappear, and cytokinesis usually follows.

Hinglish: Prophase mein chromatin condense hota hai aur spindle banta hai, metaphase mein chromosomes metaphase plate par align hote hain, anaphase mein centromere split hokar chromatids poles par jaati hain, aur telophase mein nuclear envelope aur nucleolus wapas bante hain.

Q5. How does cytokinesis in a plant cell differ from that in an animal cell?

Answer (English): An animal cell has a flexible plasma membrane, so cytokinesis starts as a furrow that deepens inward until the cytoplasm is pinched into two. A plant cell has a rigid wall that cannot furrow, so the division starts at the centre and grows outward as a cell plate. The cell plate becomes the middle lamella between the two new walls. In both cases mitochondria and plastids are shared between the daughters.

Hinglish: Animal cell mein membrane furrow banti hai jo andar tak deep hoti hai, plant cell mein rigid wall ki wajah se beech se cell plate banti hai jo bahar ki taraf badhti hai aur middle lamella ban jaati hai.

Q6. Why is mitosis called equational division, and where is it used in the body?

Answer (English): Mitosis is called equational division because the number of chromosomes in each daughter cell equals the number in the parent cell. It builds the body during growth, restores the nucleo-cytoplasmic ratio as cells enlarge, and replaces worn out cells. The upper epidermis, the gut lining and blood cells are renewed by mitosis, and the apical and lateral meristems of plants keep dividing to produce growth all through life.

Hinglish: Mitosis equational division hai kyunki chromosome number same rehta hai. Yeh growth, nucleo-cytoplasmic ratio ki recovery aur skin, gut lining aur blood cells ki repair karta hai, aur plants ke meristems mein life bhar chalta rehta hai.

Q7. Name the five sub-stages of prophase I and one event from each.

Answer (English): Leptotene makes the chromosomes visible. Zygotene brings homologous chromosomes together in synapsis, forming bivalents. Pachytene shows recombination nodules and crossing over between non-sister chromatids. Diplotene begins with the dissolution of the synaptonemal complex and leaves X-shaped chiasmata. Diakinesis shows terminalisation of chiasmata and assembly of the meiotic spindle, with the nucleolus gone and the nuclear envelope broken down.

Hinglish: Leptotene mein chromosomes dikhte hain, zygotene mein synapsis se bivalent banta hai, pachytene mein crossing over hota hai, diplotene mein chiasmata dikhte hain, aur diakinesis mein chiasmata terminalise hote hain aur spindle ban jata hai.

Q8. What are chiasmata and when do they become visible?

Answer (English): Chiasmata are the X-shaped points at which the two homologous chromosomes of a bivalent remain joined after crossing over. They become visible at diplotene, when the synaptonemal complex dissolves and the recombined chromosomes start to separate from each other everywhere except at the crossover points.

Hinglish: Chiasmata woh X-shaped joints hain jahan crossing over ke baad homologous chromosomes judi rehti hain. Yeh diplotene mein dikhte hain jab synaptonemal complex ghalta hai.

Q9. Why is interkinesis important in the study of meiosis?

Answer (English): Interkinesis is the short stage between meiosis I and meiosis II. Its importance is in what does not happen: there is no DNA replication in interkinesis. The whole of meiosis therefore uses only the single round of replication that took place before meiosis I, and this is the reason four haploid cells can form from one diploid parent without the DNA content rising again.

Hinglish: Interkinesis meiosis I aur II ke beech ka short stage hai jisme DNA replication nahi hoti. Isi wajah se poore meiosis mein DNA sirf ek baar replicate hota hai.

Q10. State the two main significances of meiosis.

Answer (English): First, meiosis keeps the chromosome number of a species constant across generations: gametes carry half the set, and fusion restores it. Second, it creates genetic variation through crossing over and the random separation of homologues in anaphase I. That variation is the material evolution works on, which is why meiosis is linked to both heredity and diversity.

Hinglish: Meiosis species ka chromosome number constant rakhta hai aur crossing over plus homologues ke random separation se variation banata hai. Yahi variation evolution ka raw material hai.

Quiz pointer. A five-question chapter quiz follows this page. Attempt it without looking back at the notes, then read the explanation for every question you miss, because the quiz repeats the phase and stage vocabulary used above.