Final answer:
The repeating units of sugar and phosphate in a single DNA strand are known as the phosphodiester backbone, which form the outer structure of the DNA strand and to which the nucleotide bases are attached.
Step-by-step explanation:
Repeating sugar and phosphate units in a single DNA strand make up the phosphodiester backbone. The phosphate group of one nucleotide is covalently bonded to the sugar molecule of the next, forming a chain. These sugar-phosphate groups line up to create this backbone, which runs along the outside of each strand of DNA. The nucleotide bases, which include adenine, thymine, guanine, and cytosine, project from this backbone and pair with complementary bases on the opposite DNA strand. These base pairs are connected via hydrogen bonds, contributing to the double-helix structure of DNA, where the two strands run in opposite directions, referred to as antiparallel. This antiparallel orientation is essential for DNA replication and interactions.