In biochemistry, two molecules are antiparallel if they run side-by-side in opposite directions or when both strands are complimentary to each other.

In DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

, the 5' carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 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

Nucleotide

Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...

s are similar and parallel, but they go in opposite directions, hence the antiparallel designation. The antiparallel structure of DNA is important in DNA replication

DNA replication

DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...

 because it unzips 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 fragment

Okazaki fragment

Okazaki fragments are short molecules of single-stranded DNA that are formed on the lagging strand during DNA replication. They are between 1,000 to 2,000 nucleotides long in Escherichia coli and are between 100 to 200 nucleotides long in eukaryotes....

s.