What does the primary structure of DNA refer to?

The primary structure of DNA is defined as the sequence of nucleotides that make up the DNA molecule. This sequence determines the genetic information carried by the DNA, which is crucial for various biological functions including protein synthesis and regulation of gene expression. Each nucleotide consists of a phosphate group, a sugar (deoxyribose in the case of DNA), and a nitrogenous base (adenine, thymine, cytosine, or guanine). The specific ordering of these nucleotides encodes the genetic instructions.

Understanding the primary structure is essential, as it serves as the foundation upon which higher-order structures, such as the double helix, are built. The helical structure itself is a result of the interactions between these sequences and does not define the primary structure. Similarly, while the sugar-phosphate backbone and hydrogen bonds between bases are important for the overall stability and structure of DNA, they do not represent the primary sequence that determines genetic encoding. Therefore, the unique sequence of nucleotides is what truly characterizes the primary structure of DNA.