Why are DNA strands anti-parallel?

Anti-parallel DNA strands mean strands run in opposite directions. DNA strands run parallel to each other but they have opposite alignments.

A single DNA strand has two ends. One end has 5’-OH group, where a free phosphate group is attached to deoxyribose sugar. Another end has 3’-OH group, where a free hydroxyl group is attached to a deoxyribose sugar.

The arrangement in DNA chains never has two 5’ (five prime) or 3’ (three prime) at one end. This arrangement is called the antiparallel arrangement. These strands run in opposite directions to each other where the head of one strand is laid against the tail of the other strand of DNA.

Nucleotide Strands

Each DNA molecule has two nucleotide strands that have a sugar-phosphate backbone, but sugar molecule orientation is opposite in both strands. DNA double helix is in opposite directions because of the opposite orientation of the sugar molecule in them.

This antiparallel arrangement allows base pairs to complement one another. Therefore, antiparallel DNA is structurally more stable than parallel DNA. Anti-parallel strands allow the formation of hydrogen bonds. The antiparallel arrangement of DNA plays an important in DNA replication as it replicates the leading strand one way and the lagging strand the other.

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