A 13-year-old boy is evaluated for a possible inherited genetic defect. He has growth retardation, microcephaly, sun-sensitive skin, and recurrent infections. The patient is the second-born child of 2 first cousins. His parents and siblings are healthy, but 2 of his maternal cousins have similar signs and symptoms. Genetic analysis of the patient reveals a defect in the BLM gene, which codes for a DNA helicase. Which of the following is the most likely site of action of this enzyme in the DNA replication fork diagram shown below?
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This patient has Bloom syndrome, a rare autosomal recessive disorder characterized by small stature, photosensitivity, immunodeficiency, and markedly increased cancer rates. It is caused by a mutation affecting DNA helicase. Helicases are enzymes that facilitate unwinding of the double helix during DNA replication and repair; dysfunction results in DNA instability and breakage.
DNA replication normally occurs during the S (synthesis) phase of the cell cycle and is initiated as follows:
First, the origin of replication is identified and bound by a multi-subunit protein (the origin recognition complex), which locally dissociates double-stranded DNA (dsDNA) into single-stranded DNA (ssDNA).
Helicase subsequently binds to ssDNA at the origin of replication and moves into the replication fork, where it facilitates separation and unwinding of dsDNA.
Then ssDNA binding proteins bind to and stabilize the unwound strands as they move away from the replication fork, preventing them from reannealing (Choice C).
DNA unwinding induces strain on the DNA segment upstream of the helicases (positive supercoiling), and topoisomerase (DNA gyrase) relieves this tension by introducing transient single- or double-stranded nicks in the DNA (Choice A).
(Choice D) Before DNA polymerase can begin synthesizing DNA, it requires an RNA primer made up of short RNA sequences base-paired to the parent DNA. This primer is synthesized by the enzyme primase (DNA-dependent RNA polymerase).
(Choice E) DNA polymerase synthesizes new daughter DNA strands in the 5′ to 3′ direction. The leading strand is formed continuously, whereas the lagging strand is formed discontinuously (ie, multiple short DNA fragments called Okazaki fragments).
(Choice F) Okazaki fragments are ultimately bound together by DNA ligase.
Educational objective:
During DNA replication and repair, helicase mediates the continuous unwinding of double-stranded DNA at the replication fork.