In DNA, which base pairing arrangement is correct?

• A. Adenine pairs with Thymine via two hydrogen bonds, and Guanine pairs with Cytosine via three hydrogen bonds.
• B. Adenine pairs with Cytosine via two hydrogen bonds, and Thymine pairs with Guanine via three hydrogen bonds.
• C. Adenine pairs with Uracil via two hydrogen bonds, and Guanine pairs with Cytosine via three hydrogen bonds.
• D. Adenine pairs with Thymine via three hydrogen bonds, and Guanine pairs with Cytosine via two hydrogen bonds.

Answer: (A)

DNA base pairing uses complementary rules: adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three hydrogen bonds. This A–T and G–C pattern follows Chargaff’s rules (A = T, G = C). Uracil is used in RNA instead of thymine and pairs with adenine. The difference in bond numbers also affects stability, with GC pairs being more thermally stable than AT pairs. So the arrangement where A pairs with T via two hydrogen bonds and G pairs with C via three hydrogen bonds is correct for DNA.