Q.2 During mitosis, disappearance of the nucleolus is a hallmark of
Disappearance of the nucleolus marks prophase as a hallmark event in mitosis. This occurs due to chromosome condensation and nuclear envelope breakdown, preparing the cell for chromosome segregation.
Option Analysis
Metaphase (A): Chromosomes align at the equatorial plate, with spindle fibers attached to kinetochores. The nucleolus has already disappeared earlier, so this is not the hallmark.
Prophase (B): Chromatin condenses into visible chromosomes, the nucleolus disappears, and the nuclear envelope fragments. This disassembly is a defining early sign of mitotic entry.
Anaphase (C): Sister chromatids separate and move to opposite poles. No nucleolar changes occur here, as it vanished previously.
Telophase (D): Chromosomes decondense, nuclear envelopes reform, and nucleoli reappear around NORs. This reverses prophase events.
The disappearance of nucleolus during mitosis signals the start of prophase, a critical hallmark for CSIR NET Life Sciences aspirants studying cell division. This event, alongside chromosome condensation, ensures accurate genetic distribution in daughter cells.
Mitosis Stages Overview
Mitosis divides the nucleus into prophase, metaphase, anaphase, and telophase. Prophase initiates with nucleolus disassembly due to phosphorylation halting rRNA synthesis.
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Early prophase: Chromatin coils; nucleolus shrinks.
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Late prophase/prometaphase: Full disappearance; nuclear envelope breaks.
Metaphase aligns chromosomes; anaphase separates chromatids; telophase reforms nucleoli.
Why Prophase?
Nucleolus, site of ribosome biogenesis, dissolves via Cdc2 kinase action on nuclear proteins. This “open mitosis” allows spindle access to chromosomes.
| Stage | Nucleolus Status | Key Events |
|---|---|---|
| Prophase | Disappears | Condensation, envelope breakdown |
| Metaphase | Absent | Alignment at equator |
| Anaphase | Absent | Chromatid separation |
| Telophase | Reappears | Decondensation, envelope reform |
