Mathematics

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1. Two lines are antiparallel with respect to an angle if the bisector of that angle intersects both lines at the same attitude but in opposite senses.
2. Two vectors are antiparallel if they are geometrically parallel, but have opposite orientations. Equivalently, each vector is a negative scalar multiple of the other.

Physics

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Antiparallel is commonly used in physics only in the second sense, applying to vectors or axial vectors, for instance forces or spins.

Chemistry and Biology

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Two antiparallel molecules run side-by-side in opposite directions.

In DNA, the 5' carbon is located at the top of the leading strand, and the 3' carbon is located at the lower section of the lagging strand. The nucleotides are similar and parallel, but they go in opposite directions, hence the antiparallel. The antiparallel structure of DNA is important in DNA replication because it unzips the leading strand one way and the lagging strand the other way.

DNA is antiparallel.

Electronics

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Two devices connected in parallel but with their polarities reversed are said to be in antiparallel or inverse parallel.