Question

Assume that the following single strand of DNA was synthesized using standard dATP, dGTP, dCTP, and dTTP precursors; however, the innermost phosphate (alpha phosphate) of all the dATPs was labeled with ³²P.

3' - CTAGTAT - 5'

Assume also that the strand was degraded to completion by the enzyme spleen diesterase. Spleen diesterase cleaves DNA at the covalent bond that connects the 5' carbon of the sugar to the phosphate. Which of the resulting nucleotide(s) might now carry the ³²P?

A. A, C, and G

B. A only

C. A and C

D. G and T

E. A, C, G, and T

I know the answer to the question but I to understand how and why it is the correct answer?

Answer 1

Answer: Option D

Explanation:

• Duirng DNA synthesis, only the alpha phosphate are involved in the phosphodiester linkage, the beta and gamma phosphate are removed.
• Once after the DNA is synthesized, there are only alpha phosphate in the strand.
• Here in this scenario, the ssDNA is cleaved with spleen diesterase.
• Spleen diesterase has 5'--->3' exonuclease activity.
• Exonucleolytic cleavage in the 5'- to 3'-direction yields nucleoside 3'-phosphates (instead of nucleoside 5'-phosphates).
• This is the ssDNA strand 3' - CTAGTAT - 5' .
• Now the final product after cleavage would be ( 5' to 3' direction):  Thymidine 3-Phosphate (this is the first nucleotide of ssDNA and its phosphate group at 5' end is cleaved), Deoxyadenosine 3-Phosphate, Thymidine 3-Phosphate, Deoxyguanosine 3-Phosphate, Deoxyadenosine 3-Phosphate, Thymidine 3-Phosphate, and Deoxycytosine.
• Deoxycytosine looses the 5' phosphate to the Thymidine. Since cytosine is the last nucelotide in the ssDNA, it lacks phosphate at 3' ends.
• 
• Any option which does not have Cytosine is the correct option.
• "Option B" is also not right, because we have more than one nucleotide.In this scenario, only Option D does not have cytosine. Hence it is the correct answer.
• The real answer is A, T, and G.