DNA Structure and Replication

ID #2341

Why is the statement "When DNA polymerase III begins synthesis, it covalently links a DNA base to a DNA base." considered a FALSE statement? I thought the DNA polymerase covalently linked the base in a single strand. Could you explain it to me?


For this question, it is helpful to think about what DNA Polymerase III does, and what it requires to do its job. Pol III is the major DNA dependent, DNA synthesizing enzyme. To do its job, it requires a DNA template, free dNTPs to add to the growing DNA strand, and a free 3'-OH group that will be covalently linked to the incoming nucleotide. As shown in one of the slides from Lecture 15, DNA Polymerases (including Pol III) cannot start synthesis from scratch. It requires a free 3'OH group. This free 3'OH group will be provided by the RNA primer that was synthesized by Primase. The first incoming DNA nucleotide will be covalently attached to the 3'OH group of the RNA primer, and DNA Pol III can then continue replication. 
The statement starts with "When DNA Pol III BEGINS synthesis..." so the question is asking about the very first DNA nucleotide. This nucleotide will be linked to the 3'OH group of the RNA primer (not DNA), so the statement is false. Subsequent nucleotides will be covalently linked to the previously incorporated DNA nucleotide. 

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