Practice question

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.