Which of the following is needed for DNA synthesis within a bacterial cell but is not needed during PCR in a test tube?

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BlackTom AI · Accepted Answer

To answer this question, let's examine what is required for DNA synthesis in a living bacterial cell versus during a PCR reaction in a test tube.
Option 1: dNTPs. In both bacterial DNA replication and PCR, deoxynucleotide triphosphates (dNTPs) provide the building blocks for DNA synthesis. Without dNTPs, neither process can proceed. Therefore, dNTPs are needed for both reactions, so this option is not the unique differentiator.
Option 2: DNA polymerase. Both processes require a DNA polymerase enzyme to catalyze the addition of nucleotides to the growing DNA strand. In bacteria, DNA polymerase catalyzes replication; in PCR, a thermostable DNA polymerase (like Taq) performs amplification. Hence, DNA polymerase is necessary in both scenarios, so this does not identify something unique to in vivo synthesis.
Option 3: DNA gyrase. In a living bacterial cell, DNA replication introduces supercoiling ahead of the replication fork, which must be relieved to prevent tangling and to allow replication to proceed efficiently. DNA gyrase (a type II topoisomerase) introduces negative supercoils and untangles the DNA as part of normal metabolism. In a PCR reaction in a test tube, there is no real chromosomal topology challenge to solve; the DNA template is relaxed and circular, and the reaction occurs under denaturation/annealing/extension cycles with no need for enzymes to modulate supercoiling during replication. Therefore, the need for DNA gyrase is specifically tied to in vivo DNA synthesis and is not required in PCR.
Option 4: Primers. Primers are short DNA sequences that provide starting points for DNA synthesis in both processes. In PCR, primers are essential for defining the region to amplify; in bacterial chromosomal replication, primers (RNA primers laid down by primase) are needed to initiate DNA synthesis on the lagging strand. However, the question asks for what is needed for DNA synthesis within a bacterial cell but is not needed during PCR in a test tube. Since primers are required in PCR, but RNA primers (or the concept of primers) are part of in vivo replication as well, the contrast here is not as strong as with gyrase. Moreover, in PCR you supply DNA primers, while in vivo replication uses RNA primers and other factors; this makes the distinction less clear-cut for this question's wording.
Option 5: All of these are needed for both reactions. Given the previous considerations, at least one component (DNA gyrase) is not needed in PCR, so this all-encompassing statement is incorrect.

MOLGEN 4500E AU2025 (25000) Exam 4- Final Exam- Requires Respondus LockDown Browser

Which of the following is needed for DNA synthesis within a bacterial cell but is not needed during PCR in a test tube?

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