A 65-year-old woman with chronic obstructive pulmonary disease and type II diabetes mellitus comes to the emergency department due to profound fevers and malaise. After initial evaluation, she is hospitalized for septicemia. Blood cultures plated on lactose-containing media grow rapidly dividing gram-negative bacteria. Replication of these microbial cells requires synthesis of two daughter strands of DNA using the parent strands as templates. Which of the following processes will differ the most between the 2 daughter strands formed at each replication fork?

Question

A 65-year-old woman with chronic obstructive pulmonary disease and type II diabetes mellitus comes to the emergency department due to profound fevers and malaise.  After initial evaluation, she is hospitalized for septicemia.  Blood cultures plated on lactose-containing media grow rapidly dividing gram-negative bacteria.  Replication of these microbial cells requires synthesis of two daughter strands of DNA using the parent strands as templates.  Which of the following processes will differ the most between the 2 daughter strands formed at each replication fork?

Quizplus · Accepted Answer

11ec160b_3761_cc32_9249_8721941d74d1_MD0001_00 DNA replication is similar in prokaryotes and eukaryotes, with DNA polymerases I and III being the main polymerase enzymes involved in prokaryotic DNA replication. For DNA replication to begin, DNA helicase must first unwind the DNA double helix and separate the parent strands (Choice A). The unwound single-stranded DNA is stabilized by the binding of single-stranded DNA-binding proteins to prevent spontaneous reannealing (Choice B). Synthesis of the daughter strands occurs simultaneously from both parent strands. Because DNA synthesis can occur only in the 5'→3' direction, one daughter strand is synthesized continuously toward the replication fork (leading strand). However, the other strand must be synthesized discontinuously in a direction away from the replication fork (lagging strand), with more and more segments being added as the replication fork moves across the DNA double helix. This results in the formation of Okazaki fragments, short stretches of newly synthesized DNA that are separated by RNA primers. These primers are removed and replaced with DNA, and the Okazaki fragments are subsequently joined together by DNA ligase. Because of the discontinuous nature of DNA synthesis on the lagging strand, DNA ligase acts many more times on the lagging strand than on the leading strand. (Choice D) DNA polymerases I and III have proofreading ability (ie, 3'→5' exonuclease activity), and the proofreading function of these polymerases is not restricted to either the leading or lagging strand. (Choice E) Topoisomerase II produces negative supercoiling in the DNA helix ahead of the replication fork to reduce the strain produced by unwinding, which causes positive supercoiling. Educational objective: DNA replication occurs in the 5'→3' direction on both strands. In contrast to the continuous synthesis of the leading strand, lagging strand synthesis occurs discontinuously and is composed of short stretches of RNA primer plus newly synthesized DNA segments (Okazaki fragments). As a result, lagging strand synthesis requires the repetitive action of DNA primase and DNA ligase. __________ References: Timing, coordination, and rhythm: acrobatics at the DNA replication fork. (http://www.ncbi.nlm.nih.gov/pubmed/20382733)

A 65-Year-Old Woman with Chronic Obstructive Pulmonary Disease and Type