DNA Structure Depends on Base Pairing of Its Four Nucleotides

question 51

Multiple Choice

DNA structure depends on base pairing of its four nucleotides, A, C, T, and G.Nucleotide A pairs with T, and nucleotide C pairs with G.This forms a four-letter DNA "alphabet." Because DNA codes for amino acids in sets of three nucleotides, there are 4 cubed (4³) , or 64, possible combinations, coding for 20 different amino acids.What is the best explanation for why there is no selective advantage for DNA to have five nucleotides (e.g., A, C, T, G, and E) with C pairing with either G or functionally equivalent E?

• A) There would be a five-letter alphabet with 125 combinations, which is too numerous.
• B) It would be impossible to form the DNA molecule, because it must have an equal number of Cs and Gs.
• C) Replication would be inaccurate because sometimes C would bond with G and sometimes C would bond with E.
• D) Because G and E have the same role, there would still be four functional letters of the alphabet.
• E) It is impossible because there are not five known nucleotides in the cell.

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