Practice question

Universal code designates UGA as termination codon yet selenoproteins conserve in-frame UGA at catalytic centers for Sec insertion requiring recoding. Surveys show all confirmed vertebrate, bacterial, archaeal selenoproteins use UGA exclusively; UAA or UAG never encode selenium. Reason stems from anticodon compatibility, tRNASec UCA pairs perfectly with UGA requiring single modification, other stops would need different isoacceptors. Conservation allows kinetic competition control between release factor and SelB or EFSec. Fidelity depends on SECIS presence and selenium status, ensuring efficiency regulated by nutrient availability and release factor competition dynamics.