A 4-year-old boy is brought to the physician for fatigue and persistent bone pain. Physical examination shows diffuse lymphadenopathy and multiple purpura over his arms and legs. Laboratory analysis reveals anemia and thrombocytopenia, and a peripheral blood smear shows lymphoblasts. After further workup, he is diagnosed with acute lymphoblastic leukemia and started on a chemotherapy regimen that includes doxorubicin. This agent intercalates between DNA base pairs and inhibits DNA replication, a process that normally occurs at sites known as replication forks. As the replication forks move across the DNA molecule, 2 distinct daughter strands are formed. Which of the following is unique to the daughter strand that is synthesized in the opposite direction of the growing replication fork?

Question

A 4-year-old boy is brought to the physician for fatigue and persistent bone pain.  Physical examination shows diffuse lymphadenopathy and multiple purpura over his arms and legs.  Laboratory analysis reveals anemia and thrombocytopenia, and a peripheral blood smear shows lymphoblasts.  After further workup, he is diagnosed with acute lymphoblastic leukemia and started on a chemotherapy regimen that includes doxorubicin.  This agent intercalates between DNA base pairs and inhibits DNA replication, a process that normally occurs at sites known as replication forks.  As the replication forks move across the DNA molecule, 2 distinct daughter strands are formed.  Which of the following is unique to the daughter strand that is synthesized in the opposite direction of the growing replication fork?

Quizplus · Accepted Answer

11ec160b_3761_5701_9249_c1eede951b2f_MD0001_00 DNA replication begins at multiple sites within eukaryotic chromosomes called origins of replication. At these sites, the parent DNA double helix is separated and unwound in a process facilitated by the helicase enzyme and single-stranded DNA-binding proteins. The locations where unwound DNA meets the double helix are known as replication forks. Replication forks travel bidirectionally away from the origin of replication as DNA polymerase synthesizes complementary daughter DNA strands. 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. (Choice B) 5'→3' exonuclease activity is needed during the synthesis of both daughter strands to remove RNA primers (although there are many more primers on the lagging strand, primers must also be removed when joining leading strands from separate origins of replication). In addition, 5'→3' exonuclease activity is necessary to correct DNA replication errors that occur on both strands. (Choice C) DNA polymerases have 3'→5' exonuclease activity (proofreading function) that allows them to reverse direction and remove incorrectly placed bases. This process occurs on both newly formed daughter strands to help reduce replication errors. (Choice D) DNA polymerases do not have 3'→5' polymerase activity; all known polymerases synthesize in the 5'→3' direction. (Choice E) Before DNA polymerase can initiate DNA synthesis, RNA primers must first be synthesized by the enzyme primase (DNA-dependent RNA polymerase). This process is necessary for synthesis of both daughter strands (even though it occurs much more frequently on the lagging strand). Educational objective: DNA synthesis can occur only in the 5'→3' direction. Okazaki fragments are short stretches of newly synthesized DNA that are separated by RNA primers. They are formed by the discontinuous synthesis of DNA on the lagging strand during replication. __________ References: Reconstitution of eukaryotic lagging strand DNA replication. (http://www.ncbi.nlm.nih.gov/pubmed/20178844) Timing, coordination, and rhythm: acrobatics at the DNA replication fork. (http://www.ncbi.nlm.nih.gov/pubmed/20382733)

A 4-Year-Old Boy Is Brought to the Physician for Fatigue