The sugar-phosphate backbone is the continuous outer chain of each DNA strand, formed from alternating deoxyribose sugars and phosphate groups joined by strong covalent bonds. It provides structural stability, while the sequence of nitrogenous bases attached to the sugars stores genetic information.
How Is the Backbone Formed?
Each DNA nucleotide contains three components: a phosphate group, a five-carbon deoxyribose sugar and a nitrogenous base. During DNA polymer formation, the phosphate group of one nucleotide becomes covalently bonded to the sugar of the next nucleotide, creating a repeating sugar/phosphate/sugar/phosphate pattern.
These covalent sugar/phosphate bonds form a strong, continuous chain. The nitrogenous bases: adenine, thymine, cytosine and guanine are attached to the deoxyribose sugars and project inward from the backbone.
DNA contains two nucleotide strands. Their sugar-phosphate backbones lie on the outside of the molecule, while complementary bases pair between them: adenine pairs with thymine, and cytosine pairs with guanine. Hydrogen bonds form between these complementary bases and link the two strands together.
A common misconception is that hydrogen bonds form the sugar-phosphate backbone. This is incorrect: the backbone contains strong covalent bonds, whereas hydrogen bonds occur between complementary bases on opposite strands.
For both SL and HL, students must recognize the backbone as a continuous chain of covalently bonded sugars and phosphates. At HL, students should additionally understand directionality: each strand has distinct 5′ and 3′ ends, and the two DNA strands are antiparallel.
Exam Technique
In a labelled DNA diagram, place deoxyribose sugars and phosphate groups on the outside and bases on the inside. For an “explain” question, connect structure to function: the strong covalent backbone stabilizes each strand, while the base sequence carries genetic information. Do not state that the backbone stores the genetic code-the order of the bases does.