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How are nucleotides linked together in a nucleic acid chain?

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Diagram of a DNA polynucleotide chain showing linked deoxyribose sugars, phosphate groups, and nitrogenous bases (adenine, guanine, cytosine, thymine), with an inset comparing ribose and uracil in RNA. — Nucleic acid A portion of the polynucleotide chain of deoxyribonucleic acid (DNA), with an inset showing the corresponding pentose sugar and pyrimidine base in ribonucleic acid (RNA).

In a nucleic acid chain, nucleotides are linked together through their sugar and phosphate components to form a continuous backbone. This connection is achieved via phosphodiester bonds, which bridge the 5′-hydroxyl group of one sugar to the 3′-hydroxyl group of the next sugar in the sequence.1

This sugar-phosphate backbone provides the structural framework for both DNA and RNA. Because these linkages occur specifically between the 3′ and 5′ carbon atoms of the pentose sugars, each nucleic acid strand possesses a distinct chemical directionality, typically described as running from the 5′ end to the 3′ end.12