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#chromosomes

42 public questions tagged with this topic.

Cell Division

Individual chromosomes are clearly visible in which phase?

Visibility of individual chromosomes under light microscope depends on degree of condensation regulated by condensin complexes and histone modifications across cell cycle. Interphase chromatin exists as 10 nanometer beads-on-string fiber further folded into 30 nanometer fiber and looped domains anchored to nuclear matrix, occupying distinct chromosome territories where active transcription and replication occur, but overlapping extensively making individual chromosomes unresolvable. Entry into M phase, triggered by cyclin B-CDK1 rising activity, activates condensin I and II ATPases that extrude DNA loops and supercoil, producing 300 to 700 nanometer thick highly compact rod-shaped chromatids, while topoisomerase II decatenates sister intertwines. Phosphorylation of histone H3 serine 10 and histone H2A contributes. Resulting condensed chromosomes with distinct morphology including centromere constriction become individually discernible during prophase, prominently aligned at metaphase plate, ideal for karyotyping after hypotonic swelling. In G0, G1, S, G2, chromosomes remain decondensed diffuse, not recognizable as separate entities, so clear visualization exclusive to M phase.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17: Mitotic Chromosome Condensation.

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Cell Division

What happens during prometaphase?

Prometaphase is transitional stage converting interphase nuclear architecture to mitotic spindle structure, initiated by rising cyclin B-CDK1 activity. CDK1 phosphorylates lamins A/C and B at serine residues within tail domain, causing depolymerization of filamentous lamina and fragmentation of nuclear envelope into vesicles merging with endoplasmic reticulum. Condensed chromosomes consisting of two sister chromatids embraced by cohesin are released into cytoplasm. Concurrently, kinetochores assembled at centromeres containing CENP-A nucleosomes fully mature, forming outer plate with KMN network composed of KNL1 scaffold, Mis12 complex, and Ndc80 complex that directly binds microtubule plus ends with high affinity. Motor proteins CENP-E kinesin and dynein mediate initial lateral capture of highly dynamic microtubules emanating from duplicated centrosomes, followed by conversion to stable end-on attachment capable of generating force via depolymerization-coupled pulling. Chromosome congression begins and spindle assembly checkpoint proteins Mad1, Mad2, Bub1, BubR1 accumulate at unattached kinetochores to delay anaphase. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 17, Prometaphase Events.

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