The correct answer is A. Guanine – cytosine.
Adenine and thymine are complementary bases, as are guanine and cytosine. This means that adenine always pairs with thymine, and guanine always pairs with cytosine. This complementary base pairing is essential for the stability of DNA.
Adenine and thymine are both purines, which are nitrogenous bases that have a double ring structure. Guanine and cytosine are both pyrimidines, which are nitrogenous bases that have a single ring structure. The two types of bases are complementary because they have different shapes and sizes. Adenine is slightly larger than thymine, and guanine is slightly larger than cytosine. This difference in size allows the bases to fit together perfectly.
The complementary base pairing of adenine and thymine, and guanine and cytosine, is also essential for the replication of DNA. When DNA replicates, the two strands of DNA separate and each strand serves as a template for the synthesis of a new strand. The new strands are complementary to the original strands, and this ensures that the genetic information is passed on accurately.
In conclusion, the correct answer to the question “Which describes the correct pairing of DNA bases?” is A. Guanine – cytosine.