DNA Replication & Repair MCAT Practice Question
A 45-year-old male with a history of colorectal polyps undergoes genetic testing and is found to have a mutation in the MLH1 gene, a DNA mismatch repair protein. His oncologist explains that defects in DNA repair mechanisms can lead to increased mutation rates. The patient asks why his cells are particularly susceptible to errors during DNA replication. A medical student reviewing his case notes that during normal DNA replication, the leading strand is synthesized continuously while the lagging strand is synthesized discontinuously as Okazaki fragments. When considering the molecular basis of replication fidelity, which of the following best explains why primase is essential for lagging strand synthesis but not for leading strand synthesis?
Answer choices
- APrimase synthesizes short RNA primers that provide the 3'-OH groups required for DNA polymerase III to initiate synthesis on the lagging strand template, whereas the leading strand requires only a single primer at the replication fork originCorrect answer
- BPrimase unwinds the DNA double helix on the lagging strand to expose the template, whereas helicase performs this function on the leading strand
- CPrimase removes the RNA primers from Okazaki fragments and replaces them with DNA, whereas DNA polymerase I performs this function on the leading strand
- DThe lagging strand template is more resistant to DNA polymerase III binding, requiring primase to create attachment sites, whereas the leading strand template has inherent polymerase recognition sequences
- EPrimase catalyzes the formation of phosphodiester bonds between adjacent Okazaki fragments on the lagging strand, a function unnecessary on the continuously synthesized leading strand
- FPrimase synthesizes DNA primers on the lagging strand while DNA polymerase III itself synthesizes the primer on the leading strand, eliminating the need for multiple priming events
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