Final answer:
The backbone of DNA is composed of sugars and phosphate groups, which are parts of the nucleotides that make up DNA, a type of nucleic acid.
Step-by-step explanation:
The substances that compose the backbone of DNA are sugars and phosphate groups. DNA and RNA are types of nucleic acids, which are built from monomers called nucleotides. Each nucleotide consists of three parts: a nitrogenous base, a pentose sugar, and a phosphate group. In the DNA structure, the sugar and phosphate groups link together to form the backbone of the polynucleotide chain. This backbone is essential for the structural integrity of DNA, and it supports the sequence of nitrogen-containing bases that encode genetic information.