Question

Below is a figure comparing CTP on the left and CMP on the right. Based on these structures, and your understanding, which statement(s) below is/are CORRECT? (Choose all that apply.) +++ CMP Deoxyetidine Thosphate (CT) Assuming a C was matched incorrectly to an A, the proofreading function of DNA polymerase would release a CMP as a product Both CTP and CMP may be incorporated into a growing DNA strand by DNA polymerase. Assuming a C was matched incorrectly to an A, the proofreading function of DNA polymerase would release a CTP as a product. Only CMP may be incorporated into a growing DNA strand by DNA polymerase. Only CTP may be incorporated into a growing DNA strand by DNA polymerase.

Answer 1

DNA polymerase is an enzyme that aids in the replication of DNA strands. It acts by adding an incoming deoxyribonucleoside 5′-triphosphates (dNTPs) to the growing strand of DNA in the 5' to 3' direction. The dNTPs have three phosphates labeled as alpha, beta, and gamma.

a) Assuming a C was matched incorrectly to an A, the proofreading function of DNA polymerase would release a CMP as a product.

The given statement is CORRECT.

Explanation: DNA polymerase attaches to the template DNA strand and adds new dNTPs complementary to that of the template. While attaching the incoming dNTP, the beta and gamma phosphates are removed as a diphosphate and only the alpha-phosphate gets incorporated in the DNA strand with the nucleotide. Thus, the nucleotide is now in the form of a monophosphate. Hence, if a base is mismatched as given in this scenario (i.e. C mismatched with A), the proofreading activity of DNA polymerase removes the attached deoxyribonucleotide monophosphate. Thus, in this case, the proofreading function of DNA polymerase would release a CMP (cytidine monophosphate) as a product. Hence, the given statement is CORRECT.

b) Both CTP and CMP may be incorporated into a growing DNA strand by DNA polymerase.

The given statement is INCORRECT.

Explanation: As explained in the previous answer, during the addition of dNTP to a growing DNA, the beta and gamma phosphates of the dNTP are removed. Thus, only dNMP gets incorporated in the growing DNA strand. In this case, only CMP will get incorporated into a growing DNA strand by DNA polymerase. Hence, the given statement is INCORRECT.

c) Assuming a C was matched incorrectly to an A, the proofreading function of DNA polymerase would release a CTP as a product.

The given statement is INCORRECT.

Explanation: In reference to the explanation provided above (in answer to the question 'a'), the addition of the dNTPs is in the form of monophosphate. Hence, the proofreading activity of DNA polymerase will remove the nucleotide as a monophosphate Thus, in this case, cytidine will be removed in the form of cytidine monophosphate (CMP), and NOT cytidine triphosphate (CTP). Note that the CMP could be later phosphorylated to CTP by the enzyme kinase. Hence, the given statement is INCORRECT.

d) Only CMP may be incorporated into a growing DNA strand by DNA polymerase

The given statement is CORRECT.

Explanation: In reference to the previous explanations, since the beta and gamma phosphates are removed from a dNTP, during the addition of nucleotides to a growing DNA strand, only nucleotide monophosphates (CMP, in this case) are incorporated by DNA polymerase. Hence, the given statement is CORRECT.

e) Only CTP may be incorporated into a growing DNA strand by DNA polymerase

The given statement is INCORRECT.

Explanation: Since the phosphates are removed from dNTPs during addition by DNA polymerase to a growing DNA strand, the nucleotides are added as dNMPs. Thus, in this case, it would be added as CMP and NOT CTP. Hence, the given statement is INCORRECT.