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MCAT Biology Question — DNA Polymerase

DNA polymerase creates new DNA by adding complimentary nucleotides to a template strand from the original double-stranded DNA. If a section of the template strand had a ration of 3:2 of A:T bases, what is the ration of A:T in the newly synthesized complimentary strand of DNA?

A. 3:2

B. 1:1

C. 2:3

D. cannot be determined

Click for Explanation

DNA polymerase matches complimentary bases to each base in the template strand. If the template strand has an A:T ratio of 3:2, then the complimentary strand must have an A:T ratio of 2:3 in order to match the complimentary bases with their correct paired nucleotide. That is, each A in the original strand will be matched with a T and vice versa.

A. 3:2, incorrect, This would be true if the complimentary strand matched A:A and T:T.

B. 1:1, incorrect, Complimentary strand bases are paired in the same ratio, but with T in the place of A.

C. 2:3, correct.

D. cannot be determined, incorrect, A:T must be matched with T:A on the complimentary strand.

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