Final answer:
Birds and dogs use DNA to build their bodies because they share the same basic biochemical components and genetic structure, despite being different species. DNA sequences reflect evolutionary relationships, and organisms that share a common ancestor have more DNA in common.
Step-by-step explanation:
Birds and dogs both use DNA as instructions for building their bodies because, despite the apparent differences, they share the same biochemical building blocks, such as amino acids and nucleotides that make up proteins and DNA respectively. All living organisms, from bacteria to mammals, have DNA that determines their development, and this DNA is composed of the same basic structure across the diverse array of life.
The genetic code contained within DNA is practically universal, and the degree of similarity between the DNA sequences of different species can reflect their evolutionary relationships. For example, even though whales and sharks are both aquatic animals, a whale actually shares more DNA sequences in common with a horse than with a shark, due to their closer evolutionary ties as mammals. The similar construction of appendages in humans, dogs, birds, and whales further indicates that these organisms share a common ancestor.
Just as members of the same human family have more DNA in common with each other than with unrelated individuals, the same principle applies across species. Organisms that are more closely related will have a greater amount of DNA alignment, sharing more characteristics and behaviors.