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What does it mean that DNA strands are antiparallel?

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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 the context of DNA structure, the term "antiparallel" means that the two strands of the double helix are oriented in opposite directions.12

The chemical structure of a DNA strand is defined by its sugar-phosphate backbone, which has a specific directionality based on the carbon atoms of the deoxyribose sugar. These are referred to as the 3′ (three-prime) and 5′ (five-prime) ends.2

Because the strands are antiparallel:
* One strand runs in the 5′ → 3′ direction.2
* The other strand runs in the 3′ → 5′ direction.2

This opposite orientation is essential for the molecular geometry of the double helix, allowing the nitrogenous bases to pair correctly via hydrogen bonds. This arrangement also dictates how DNA replication occurs, as the enzyme DNA polymerase can only synthesize new DNA in the 5′ → 3′ direction. Consequently, the cell must use different mechanisms to synthesize the "leading" and "lagging" strands simultaneously.23