MCAT Question of the Day
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    Q:

    		 Enzymatic DNA sequencing reactions are prepared primarily by adding DNA Polymerase, deoxynucleotide phosphates (dNTPs), double-deoxynucleotide phosphates (ddNTPs), and a DNA template. A ddNTP serves to terminate the polymerization reaction once it is added to the growing chain. Since this occurs randomly, various fragment lengths are generated which are then observed using PCR. What is the most likely mechanism by which ddNTPs terminate the polymerization reaction?
    		 

    A

    		 ddNTPs lack a 5' OH in addition to a 2' OH. Thus, ddNTPs cannot be added to the growing DNA molecule, and polymerization terminates.

    B

    		 ddNTPs lack a 5' OH in addition to a 2' OH. Thus, they can only be added to the growing end of the DNA molecule via their 3' OH, and polymerization terminates because DNA synthesis can only occur 5' to 3'.

    C

    		 ddNTPs lack a 3' OH in addition to a 2' OH. Thus, once they are added to the growing end of the DNA molecule, there is no OH on which to add new nucleotides and polymerization terminates.

    D

    		 ddNTPs lack a 3' OH in addition to a 2' OH. Thus, once they are added to the growing end of the DNA molecule, only ddNTPs can continue to be added. The reaction terminates due to the lack of further ddNTPs present in the reaction mixture.



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