What term describes the alternating chain of sugar and phosphate in DNA and RNA?

The term that describes the alternating chain of sugar and phosphate in DNA and RNA is the sugar-phosphate backbone. This structure is fundamental to the formation of the nucleic acid polymer, providing structural support and integrity to the molecule. The sugar component is typically ribose in RNA and deoxyribose in DNA, linking together through phosphate groups. This arrangement creates the robust framework that allows the nitrogenous bases to extend outwards, where they can interact and form the genetic coding necessary for biological functions.

In contrast, the other options refer to different components of nucleic acids or protein structures. The nitrogenous base refers specifically to the molecule that pairs with another base to encode genetic information. A polypeptide chain refers to a sequence of amino acids linked together by peptide bonds, which forms proteins. The gene sequence indicates the specific order of nucleotides within a segment of DNA that codes for a particular trait or protein. Therefore, the correct answer is indeed the sugar-phosphate backbone, as it accurately captures the essential structural component of both DNA and RNA molecules.