This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Klenow fragment lacks the activity of? 
Select and explain. 
A. 3'-5' exonuclease, B. 5'-3' exonuclease, C. 5'-3' DNA polymerase, D. 3'-5' DNA polymerase
B: 5'-3' exonuclease
Answer: B: 5'-3' exonuclease 
Explanation: Ans: B. 5'-3' exonucleaseDNA polymerase I is not the primary enzyme of replication; instead, it performs a host of clean-up functions during replication, recombination, and repair.When the 5'-3' exonuclease domain is removed, the remaining fragment (Mr 68,000), the large fragment or Klenow fragment, retains the polymerization and proofreading activities.Klenow fragment is a large protein fragment produced when DNA polymerase I from E. coli is enzymatically cleaved by the protease subtilisin.