74. Each strand of DNA replicates independently of every other strand
My Initial Reaction
This seems like a pretty factual-sounding statement. At this point in the semester, we're learning about DNA replication, so this is an obviously relevant fact to check. I assigned this at the end of my class where I discussed the Meselson-Stahl experiment. This was the elegant study showing that DNA replicates semi-conservatively: the two strands of the DNA double-helix each serve as the template for the contruction of a new second strand each, which duplicates the double-helix. I figured that the students would just cite this paper and we'd be done. But, as always, the students think outside the box!
Student Responses
The responses were more mixed than I had expected. Many students focused on the meaning of "independently" in the Fact. A representative response reads, "When DNA is being replicated the two individual strands are separated into what is called a replication fork and although both of the individual strands are being replicated separately by two separate, but similar, processes they occur simultaneously. This means that there is not one single strand of DNA that is replicated completely independently."
An example of a student that did agree with the Fact, as written, cites a line from the abstract of Graham et al. (2017), "Using real-time single-molecule analysis, we establish that leading- and lagging-strand DNA polymerases function independently within a single replisome."
Student Decision: Fact or Fiction?
I didn't even hold a vote. No raising of hands, electronic ballots, or tallying. The vast majority of students were hung up on the semantics of the way the Fact was stated.
However, the literature does support the essential genetic concept underlying the Fact: each strand serves as its own template for replication. In other words, we know that the way DNA replication does not occur is by an alternative (hypothetical) mechanism, "conservative DNA replication," in which both strands would be duplicated at the same time by the same single enzyme.
Literature Cited
- Meselson and Stahl (1958) "The replication of DNA in Escherichia coli" PNAS 44(7):671-682.
- Graham, Marians and Kowalczykowski (2017) "Independent and stochastic action of DNA polymerases in the replisome." Cell 169(7):1201-1213.
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