This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nucleotides, Subject name: Biochemistry
A dideoxynucleotide does not contain 
Select and explain. 
A. 2' & 3' OH group, B. 3' & 4' OH group, C. 4' & 5' OH group, D. 2' & 5' OH group
A: 2' & 3' OH group
Answer: A: 2' & 3' OH group 
Explanation: Dideoxy nucleotide Lacks 3' OH group also. Stop the chain elongation when they are incorporated. DNA polymerase requires 3' OH group to begin polymerisation. Use: Sanger chain termination method of DNA sequencing. Didanosine is a dideoxynucleotide used in the therapy of HIV.