A 4-year-old boy is brought to the office for evaluation of fatigue and bone pain. Physical examination shows diffuse lymphadenopathy and purpura over his arms and legs. Laboratory analysis reveals anemia and thrombocytopenia, and a peripheral blood smear shows lymphoblasts. After further work-up, acute lymphoblastic leukemia is diagnosed, and the patient begins a chemotherapy regimen that includes doxorubicin. This agent intercalates between DNA base pairs and inhibits DNA replication, a process that normally occurs at replication forks and produces 2 distinct daughter strands. Which of the following is unique to the daughter strand that is synthesized in the opposite direction of the growing replication fork?
Show Explanatory Sources
DNA replication begins at multiple sites—origins of replication—within eukaryotic chromosomes. 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 at which 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. However, because DNA synthesis can occur in only the 5′→3′ direction, only 1 daughter strand is synthesized continuously toward the replication fork; this is the leading strand. The other strand, called the lagging strand, must be synthesized discontinuously in a direction away from the replication fork; more segments are 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 then removed and replaced with DNA, and the Okazaki fragments are subsequently joined together by DNA ligase.
(Choice A) 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 (not just the lagging strand) to help reduce replication errors.
(Choice B) DNA polymerases do not have 3′→5′ polymerase activity; all polymerases synthesize in the 5′→3′ direction.
(Choice C) During the synthesis of the lagging strand, 5′→3′ exonuclease activity is needed to remove RNA primers. However, it is also used to remove primers when joining leading strands from separate origins of replication and is therefore not a unique mechanism in the creation of the lagging strand.
(Choice D) 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 and is therefore not unique to the lagging strand.
Educational objective:
DNA synthesis occurs in the 5′→3′ direction only. During DNA replication, one daughter strand is synthesized continuously toward the replication fork (leading strand), whereas the other daughter strand is synthesized discontinuously away from the replication fork (lagging strand). The lagging strand is formed from short stretches of newly synthesized DNA separated by RNA primers (Okazaki fragments).