The primary difference between DNA and RNA lies in their structure and function. DNA (deoxyribonucleic acid) contains the genetic blueprint for living organisms and is typically double-stranded. It uses thymine (T) as one of its bases, whereas RNA (ribonucleic acid) is usually single-stranded and substitutes uracil (U) for thymine. Functionally, DNA serves as the repository for genetic information, while RNA plays crucial roles in protein synthesis and gene expression, acting as a messenger and translator based on the DNA's instructions.