Practice question

Chain growth polarity dictated by chemistry of polymerase active site containing two magnesium ions. Nucleophilic attack by 3' hydroxyl of nascent RNA on alpha phosphate of incoming nucleoside triphosphate extends chain toward distal 5' to 3' direction. Polymerase simultaneously translocates downstream along antiparallel template reading 3' to 5', preserving RNA-DNA hybrid of eight to nine bases. Because synthesis requires free 3' OH, direction fixed unidirectionally analogous to DNA polymerases. This polarity ensures nascent RNA emerges 5' end first from exit channel while template entry determines future transcription downstream trajectory.