Practice question

Dorsal axis specification after fertilization depends on stabilization and nuclear localization of beta-catenin. Cortical rotation delivers Dishevelled and GBP that locally inhibit GSK-three destruction complex, preventing beta-catenin phosphorylation and proteasomal degradation. Stabilized beta-catenin enters dorsal vegetal nuclei, complexes with TCF transcription factors to activate siamois and twin homeobox genes. These transcription factors induce Spemann organizer genes goosecoid, chordin, noggin, and Xnr3 essential for dorsal mesoderm and neural induction. Neither VegT nor Sox17 nor Smad2 directly initiates dorsal determination without beta-catenin input, confirming central role.