Practice question

Ultraviolet irradiation of the vegetal hemisphere crosslinks cortical microtubules, preventing assembly of parallel microtubule array required for cortical rotation in Xenopus zygote. Rotation normally displaces maternal determinants including Wnt11 mRNA, Dishevelled and GBP toward future dorsal side, permitting local inhibition of GSK3 and nuclear accumulation of beta-catenin. UV blocks this transport, so beta-catenin remains degraded ventrally, Siamois, Twin and organizer genes fail to activate, producing ventralized embryos lacking notochord and central nervous system. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.