This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Techniques in molecular biology, Subject name: Biochemistry
Which of the following is necessarily to be present in expression vector but not in cloning vector? 
Select and explain. 
A. Origin of replication, B. Restriction site, C. Selectable marker, D. Ribosomal entry site
D: Ribosomal entry site
Answer: D: Ribosomal entry site 
Explanation: Cloning vector Expression vector DNA of interest is replicated and cloned into multiple copies. No transcription or translation happens with the cloning vector. The DNA of interest is expressed as RNA and proteins. The promoter is needed for transcription Shine-Dalgarno sequence (ribosomal entry site) is there to be recognized by ribosomal RNAs. eg: Synthesis of insulin All the other three components mentioned in question are present in both vector.