DNA is antiparallel because its two sugartophosphate backbones run in opposite directions: one strand runs 5′ to 3′, while the complementary strand runs 3′ to 5′. This orientation allows complementary bases to align correctly and form stable hydrogen bonds.
This is required at both SL and HL in A1.2 Nucleic acids; the detailed significance of 5′ to 3′ directionality is Additional Higher Level content.
The Mechanism
Each DNA strand has directionality because the nucleotides are joined by covalent bonds between the phosphate attached to the 5′ carbon of one sugar and the hydroxyl group on the 3′ carbon of the next sugar. The two ends are therefore chemically different.
| End of strand | Defining feature |
|---|---|
| 5′ end | Has a phosphate associated with the sugar’s 5′ carbon |
| 3′ end | Has a hydroxyl group on the sugar’s 3′ carbon |
When two strands form a DNA molecule, their backbones must point in opposite directions. This positions the nitrogenous bases so that complementary base pairing can occur: adenine pairs with thymine, and cytosine pairs with guanine. Hydrogen bonds form between these correctly aligned bases, producing a stable, uniform DNA structure.
Antiparallel orientation is also essential during DNA replication. DNA polymerases add nucleotides to the 3′ end of a growing strand, so new DNA is synthesized only in the 5′ to 3′ direction. At HL, this explains why replication is continuous on the leading strand but discontinuous on the lagging strand.
A common misconception is that “antiparallel” means the strands contain opposite base sequences. It refers to opposite backbone directions, not to the order or identity of the bases.
Exam Technique
For an explain question, state both directions and link antiparallel orientation to correct complementary base pairing. In a DNA diagram, label the ends accurately and ensure that if one strand is 5′ to 3′, the other is 3′ to 5′.