Practice question

Transformation of extended interphase chromatin into compact rod-shaped mitotic chromosomes requires condensin complexes and coordinated kinase activation. Condensin II pentamer containing SMC2, SMC4 and subunits CAP-D3, CAP-G2, CAP-H2 resides in nucleus during interphase and initiates early looping as cells enter prophase. Condensin I pentamer with CAP-D2, CAP-G, CAP-H gains access to chromosomes after nuclear envelope breakdown in prometaphase. Activation depends on mitotic kinases: CDK1-Cyclin B phosphorylates CAP-D3 Threonines and CAP-H promoting ATPase activity, Polo-like kinase 1 phosphorylates CAP-G enhancing DNA binding, and Aurora B phosphorylates CAP-H2 stimulating supercoiling activity. Using ATP hydrolysis, condensins perform loop extrusion, progressively building helical arrays of DNA loops anchored at central scaffold, while topoisomerase II alpha decatenates intertwined sisters and KIF4A motor contributes axial shortening. Resulting 10,000-fold linear compaction protects chromosomes from shear during segregation and facilitates individualization. Partial loss yields fuzzy chromosomes, anaphase bridges, chromosome decondensation failure and micronuclei, linking condensin function to genome stability and correct segregation. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.