Which of the following occurs only in prokaryotes and not in eukaryotes during gene expression?
Correct answer: C. σ factor
σ factor is involved in transcription initiation by RNA polymerase in prokaryotes but not in eukaryotes. AUG serves as the start codon for both prokaryotes and eukaryotes, Okazaki fragments are formed during DNA replication in both, and rare bases in tRNA are present in both types of organisms.