In biochemistry, two biopolymers are antiparallel if they run parallel to each other but with opposite alignments. An example is the two complementary strands of a DNA double helix, which run in opposite directions to one another.
Nucleic acids 
Nucleic acid molecules have a phosphoryl (5') and a hydroxyl (3') end. This notation follows from organic chemistry nomenclature, and can be used to define the movement of enzymes such as DNA polymerases relative to the DNA strand in a non-arbitrary manner.
DNA Replication 
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 nucleotide sequences are complementary and parallel, but they go in opposite directions, hence the antiparallel designation. The antiparallel structure of DNA is important in DNA replication because it replicates the leading strand one way and the lagging strand the other way. During DNA replication the leading strand is replicated continuously whereas the lagging strand is replicated in segments known as Okazaki fragments.
Polypeptides have an N-terminus and a C-terminus, which refer to the ends of the polymer in a way that reflects the direction in which the polymer was synthesized. The chronological sequence of each amino acid sub-unit is the basis for directionality notation in polypeptides; a given protein can be represented as its set of unique amino acid abbreviations within an N-terminus and a C-terminus.
Beta sheet 
Many proteins may adopt a beta sheet as part of their secondary structure. In beta sheets, sections of a single polypeptide may run side-by-side and antiparallel to each other, to allow for hydrogen bonding between their carbon backbone.
See also 
- Benson, Gary. "Anti-Parellel Strands". K*Nex DNA Modeling. Boston University. Retrieved 12 December 2011.
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