DNA strands are described as antiparallel because the two strands run alongside each other but in opposite directions. Specifically, one strand runs in the 5' to 3' direction, while the complementary strand runs in the 3' to 5' direction
. This orientation is due to the chemical structure of the DNA backbone, where the phosphate group of one nucleotide connects the 3' carbon of one sugar to the 5' carbon of the next, giving each strand a directionality
. The antiparallel arrangement is crucial for DNA's function because it allows the nitrogenous bases on opposite strands to pair correctly via hydrogen bonds (adenine pairs with thymine, and cytosine pairs with guanine), maintaining a consistent double helix width and enabling proper base pairing
. It also facilitates DNA replication since DNA polymerase enzymes can only synthesize new strands in the 5' to 3' direction, requiring one strand to be read in the opposite direction to the other
. In summary, antiparallel means the two DNA strands are oriented in opposite directions (5' to 3' vs. 3' to 5'), which is essential for stable base pairing and accurate DNA replication.
