Which components alternate to form the DNA backbone?

• A. Nitrogenous bases and sugars
• B. Phosphate groups and sugars
• C. Water and salts
• D. Nucleotides pairs between strands

Answer: (B)

DNA’s blueprint is built with a sugar–phosphate framework running along the outside. The backbone is formed by the sugar (deoxyribose) of one nucleotide linked to a phosphate group of the next nucleotide, creating phosphodiester bonds that alternate along the strand. This arrangement gives the molecule its sturdy, repetitive spine while the nitrogenous bases extend inward to pair with the opposite strand. So the components that make up the backbone are phosphate groups and sugars.

Nitrogenous bases work as the interior rungs that pair across strands, not as part of the backbone. Water and salts are important for the molecule’s environment but aren’t structural components of the backbone. Nucleotide pairs between strands describe base pairing between the two strands, not the backbone itself.