This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. of vectors, parallel but of opposite direction

adj. describing the orientations of the two strands of DNA

adj. describing two beta strands comprising a protein's secondary structure that are aligned and hydrogen bonded but the vector describing the amino- and carboxy-terminal ends of each strand are oriented 180° relative to one another.

n. A line that forms equal angles with two other lines, but in opposite directions.

adj. Running in a contrary direction.

Running parallel but in a contrary direction.

n. In geometry, one of two or more lines which make equal angles with two other lines, but in contrary order.

adj. (especially of vectors) parallel but oppositely directed

"For example, noting that in Watson-Crick base pairing the paired bases have the same face-orientation in forming the antiparallel double helix tells us much of what we need to know to build the correct structure."

parallel   

varactor    series-parallel    liesand    nonuniform    head/neck