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Solution


Q.

Which of the following options gives the correct sequence of events during mitosis  [2017]
1 Condensation → Nuclear membrane disassembly → Arrangement at equator → Centromere division → Segregation → Telophase  
2 Condensation → Crossing over → Nuclear membrane disassembly → Segregation → Telophase  
3 Condensation → Arrangement at equator → Centromere division → Segregation → Telophase  
4 Condensation → Nuclear membrane disassembly → Crossing over → Segregation → Telophase  

Ans.

(1)

Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. During prophase, the indistinct and intertwined DNA molecules condense to form elongated chromosomes. The nuclear membrane disintegrates during prometaphase. During metaphase, the chromosomes align themselves at the equatorial plate. During anaphase, the centromere of each chromosome divides into two so that each chromosome comes to have its own centromere. Chromatids move towards opposite poles along the path of their chromosome fibres. Finally, during telophase, two chromosome groups reorganise to form two nuclei. Nuclear envelope reappears, Golgi complex and endoplasmic reticulum are reformed. Option (c) also gives the correct sequence of events but it misses step II (nuclear membrane disassembly). Hence, it is ruled out as the best appropriate answer is option (a)