NAD is composed of two nucleotides joined by which of the following?

NAD, or nicotinamide adenine dinucleotide, is comprised of two nucleotides that are connected through phosphate groups. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate group. In NAD, the first nucleotide has an adenine base while the second has a nicotinamide base. The link between these two nucleotides occurs at the phosphate groups, which form the backbone of the molecule.

Specifically, the two nucleotides are bonded by the phosphate group of one nucleotide to the ribose sugar of the other, which is a characteristic feature of nucleotide chains. This structure allows NAD to function in redox reactions within metabolic pathways, highlighting the significance of the phosphate group in its composition.