A short fragment of double-stranded DNA has the sequence 5′-A-G-C-T-3′ on one strand and 5′-?-?-?-?-3′ on the complementary strand.
(a) Write the complete complementary sequence (5′→3′) of the second strand, following standard base-pairing rules.
(b) Determine the exact number of hydrogen bonds that hold the fully base-paired fragment together. Show your reasoning for each base pair.
(c) Suppose the cytosine at position 3 of the original strand is replaced by thymine. Predict how the total number of interstrand hydrogen bonds would change and justify your prediction with reference to donor–acceptor patterns in the affected base pair.