A microbiologist performs a genetic experiment in which cultures of Escherichia coli are treated with a chemical that induces a high frequency of mutations. Individual bacterial colonies are isolated to identify a mutant strain that lacks a specific enzyme involved in DNA replication. This specific enzyme is responsible for removing short fragments of RNA that are base paired to the DNA template. Which of the following enzymes is most likely deficient in this strain of E coli?

Question

A microbiologist performs a genetic experiment in which cultures of Escherichia coli are treated with a chemical that induces a high frequency of mutations.  Individual bacterial colonies are isolated to identify a mutant strain that lacks a specific enzyme involved in DNA replication.  This specific enzyme is responsible for removing short fragments of RNA that are base paired to the DNA template.  Which of the following enzymes is most likely deficient in this strain of E coli?

Quizplus · Accepted Answer

11ec160b_36df_7ce3_9249_6bd47e428629_MD0001_00 Bacterial DNA replication is coordinated by the actions of multiple enzymes and proteins. Before DNA synthesis begins, helicase binds double-stranded DNA at the origin of replication with the assistance of DnaA protein and acts at the replication fork to unwind and separate the DNA (Choice D). Single-stranded DNA-binding protein then attaches to the separated strands to prevent their reannealing. Unwinding and separation of DNA produces positive supercoils that can lead to DNA fracture if not relieved. Topoisomerase II (DNA gyrase) relieves the tension created during unwinding by introducing negative supercoils into the DNA (Choice C). Before DNA polymerase III can begin elongating a new DNA strand, it requires an RNA primer made up of short RNA sequences base-paired to the parent DNA. This primer is synthesized by the enzyme primase (DNA-dependent RNA polymerase) (Choice F). DNA synthesis then proceeds in the 5' to 3' direction with the leading strand being formed continuously toward the replication fork and the lagging strand being formed discontinuously away from the replication fork. Replication of the lagging strand results in the formation of numerous short DNA segments called Okazaki fragments. These fragments are ultimately bound together by the enzyme ligase after their RNA primers have been removed and replaced with DNA (Choice E). The removal of RNA primers and their replacement with DNA is accomplished by DNA polymerase I, the only bacterial DNA polymerase with 5' to 3' exonuclease activity. DNA polymerase III has 5' to 3' polymerase and 3' to 5' exonuclease ("proofreading") activity; however, it cannot remove RNA primers as it lacks 5' to 3' exonuclease activity (Choice B). Educational objective: During bacterial DNA replication, DNA polymerase I functions to remove RNA primers (via 5' to 3' exonuclease activity) and replace them with DNA (via 5' to 3' polymerase activity). DNA polymerase I is the only bacterial DNA polymerase that possesses 5' to 3' exonuclease activity. __________ References: DNA replication fidelity in Escherichia coli: a multi-DNA polymerase affair. (http://www.ncbi.nlm.nih.gov/pubmed/22404288)

A Microbiologist Performs a Genetic Experiment in Which Cultures of Escherichia