A research scientist purifies DNA polymerase III from Escherichia coli extract. When the enzyme is incubated in a mixture containing DNA templates, RNA primer oligonucleotides, and tagged deoxynucleotides, she finds that the enzyme possesses 3' to 5' exonuclease activity. Which of the following enzymatic actions was most likely observed during the experiment?

Question

A research scientist purifies DNA polymerase III from Escherichia coli extract.  When the enzyme is incubated in a mixture containing DNA templates, RNA primer oligonucleotides, and tagged deoxynucleotides, she finds that the enzyme possesses 3' to 5' exonuclease activity.  Which of the following enzymatic actions was most likely observed during the experiment?

Quizplus · Accepted Answer

11ec160b_3762_b694_9249_0d5e72e51733_MD0001_00 DNA replication is coordinated by the actions of multiple enzymes and proteins and requires a high degree of fidelity to ensure preservation of the genetic code in daughter cells. DNA polymerases are the primary enzymes responsible for DNA synthesis. Prokaryotes such as Escherichia coli have 3 major types of DNA polymerase: I, II, and III. All 3 prokaryotic DNA polymerases are capable of removing mismatched nucleotides via their 3' to 5' exonuclease ("proofreading") activity. Only DNA polymerase I has 5' to 3' exonuclease activity, which is used to remove the RNA primer synthesized by RNA primase (Choice D). (Choice A) In prokaryotes, topoisomerase II (DNA gyrase) temporarily cleaves both strands of the DNA double helix and introduces negative supercoils into the circular DNA to relieve tension created during strand unwinding. (Choice B) One of the major methods of DNA damage by ultraviolet (UV) light is the dimerization of adjacent pyrimidine bases to form thymidine dimers. These dimers are routinely formed after exposure to sunlight but are usually removed via nucleotide excision repair by the enzyme UV-specific endonuclease. (Choice E) Helicase promotes unwinding and dissociation of parent DNA strands at the replication fork. Educational objective: All 3 prokaryotic DNA polymerases can remove mismatched nucleotides via their 3' to 5' exonuclease ("proofreading") activity. Only DNA polymerase I has 5' to 3' exonuclease activity, which is used to remove the RNA primer synthesized by RNA primase. __________ References: DNA replication fidelity in Escherichia coli: a multi-DNA polymerase affair. (http://www.ncbi.nlm.nih.gov/pubmed/22404288)

A Research Scientist Purifies DNA Polymerase III from Escherichia Coli