What model of DNA is supported by the observation that thymine equals adenine and guanine equals cytosine?

The observation that thymine equals adenine and guanine equals cytosine is a key finding that supports the Watson and Crick model of DNA. This model describes DNA as a double helix structure, where the bases are paired specifically: adenine forms hydrogen bonds with thymine, and guanine pairs with cytosine. These specific pairings are fundamental to the double helical structure proposed by Watson and Crick, as they ensure that the two strands of DNA are complementary to one another.

This pairing concept also provides the basis for DNA replication. When the DNA helix unwinds, each strand serves as a template for the formation of a new complementary strand, maintaining the genetic information. The observation regarding the proportional equality of the bases, known as Chargaff's rules, is integrated into the Watson and Crick model, as it relies on the complementary nature of base pairing. Thus, the consistency of base pairing directly supports the structural integrity and functionality described in the Watson and Crick model, making it the correct choice in this context.