DNA, the nucleic acid that contains our genes, is bicatenary, that is to say, it is made out of two complementary molecular chains. These two chains, united by extremely weak links, are placed in their resulting double helix in opposing senses –reason why they are called antiparallels–, one going from terminal 5' to 3' and the other from 3' to 5', where the numbers refer to the position of the carbon atoms in the sugar sub molecule (see image). The opposition of senses is important because the polymerase, the enzyme in charge of stringing up new nucleotides during replication, must always work in the 3' to 5' sense of the pattern being replicated, having therefore an easier task on one of the two chains (known thus as the sense chain). The arrows in the graph indicate the sense in which the polymerase is constrained to work to form each chain, always assembling a new 5' terminal at a 3' terminal of a nucleotide already installed.
In the antiparallel chain (known as the antisense chain), a different molecule of polymerase must work in a discontinuous way, jumping a little forward, stamping a few nucleotides and jumping again when it runs into the sequence already stamped. The result is the successive creation of small nucleotide sequences known as fragments of Okazaki, later glued together by a different type of enzyme known as ligase. The polymerase molecule that works in the preferred sens, not having to jump, gains time; therefore, the corresponding strand –the sense one– is also called leading strand; that of the antisense, consequently, the lagging strand.
The messenger RNA, in charge of transmitting messages from the nucleus to the cytoplasm, is monocatenary (made up of only one chain of nucleotides). It is always built on the DNA leading strand, having thus a continuous expeditious transcription.