Final answer:
The backbone of DNA is composed of alternating deoxyribose (sugar) and phosphate groups that form the external part of the double helix. Nitrogenous bases are on the interior and are paired and held by hydrogen bonds, with antiparallel orientation of the two strands.
Step-by-step explanation:
The Backbone of DNA
The backbone of DNA, which is also the outside part of the double-helix structure, consists of a repeating pattern of sugar (deoxyribose) and phosphate groups. In the iconic double helix model, this backbone forms the external framework whereas the nitrogenous bases (adenine, thymine, guanine, cytosine) are stacked in the interior, resembling the steps of a staircase. These nitrogenous bases from opposite strands pair up and are held together by hydrogen bonds, contributing to the stability and unique structure of DNA. DNA strands are antiparallel, meaning that they run in opposite directions, with one strand running 5' to 3' and the other running 3' to 5'. This orientation is crucial for DNA replication and many other nucleic acid interactions.