A 34-year-old woman with a history of recurrent urinary tract infections comes to the physician with dysuria and increased urinary frequency. Her urine culture grows colonies of Gram-negative bacteria. The bacteria are isolated and placed in a growth-enhancing nutrient solution, where they undergo rapid cellular division. As they are actively dividing, the bacterial cells are lysed and their DNA is extracted and purified. Analysis of the partially replicated DNA fragments shows the presence of uracil. This finding is most likely mediated by which of the following enzymes?

Question

A 34-year-old woman with a history of recurrent urinary tract infections comes to the physician with dysuria and increased urinary frequency.  Her urine culture grows colonies of Gram-negative bacteria.  The bacteria are isolated and placed in a growth-enhancing nutrient solution, where they undergo rapid cellular division.  As they are actively dividing, the bacterial cells are lysed and their DNA is extracted and purified.  Analysis of the partially replicated DNA fragments shows the presence of uracil.  This finding is most likely mediated by which of the following enzymes?

Quizplus · Accepted Answer

11ec160b_36e0_4035_9249_139bf93700f1_MD0001_00 This question describes a scenario in which uracil is found in association with bacterial DNA during prokaryotic DNA replication. In general, uracil is found only in RNA, so the question essentially asks which enzyme involved in DNA synthesis catalyzes the formation of RNA strands. In prokaryotic DNA replication, primase (an RNA polymerase) is responsible for synthesizing a short RNA primer using the separated strands of DNA at the replication fork as templates. DNA replication then proceeds, with DNA polymerase using the 3' hydroxyl group of the RNA primer as a starting point for synthesis. Primase is a crucial enzyme for bacterial replication as DNA polymerase cannot initiate DNA synthesis without this short nucleic acid sequence primer. (Choice A) DNA ligase is the enzyme that repairs single-strand breaks in duplex DNA during DNA replication and repair. (Choices B and C) During replication, DNA polymerase III is the primary enzyme responsible for synthesis of daughter DNA strands; DNA polymerase I functions chiefly to replace the RNA primers with DNA segments. Unlike DNA polymerase III, DNA polymerase I has 5'→ 3' exonuclease activity that can remove RNA primers and damaged DNA segments. The 3'→ 5' exonuclease activity of DNA polymerase I and III provides a proofreading function that fixes mismatched nucleotides in the newly formed daughter strands. (Choices D and E) Helicase unwinds DNA at the replication fork. However, this process results in supercoiling of the DNA. DNA gyrase is a type II topoisomerase that helps to relieve the resultant strain. Educational objective: Primase is a DNA-dependent RNA polymerase that incorporates short RNA primers into replicating DNA.

A 34-Year-Old Woman with a History of Recurrent Urinary Tract