Antisense is the non-coding DNA strand of a gene. In a cell, antisense DNA serves as the template for producing messenger RNA (mRNA), which directs the synthesis of a protein.
Antisense is a term that's used to describe one of the two strands of DNA, or actually in some cases also RNA. If you imagine that there's a directional way that you read information from what's called the five prime, or the front end, to the three prime, or the back end, that's unidirectional. You can't read DNA or RNA in both directions, and so there's a sense strand for DNA, and then there's a second strand of DNA which is called the antisense strand. The sense strand has the information that would be readable on the RNA, and that's called the coding side. The antisense is the non-coding strand, but ironically, when you're making RNA, the proteins that are involved in making RNA read the antisense strand in order to create a sense strand for the mRNA. There's a second aspect of antisense, which is a fairly new discovery, called antisense RNA. These are RNAs that read in the opposite direction of the coding strand, and they actually bind to the coding strand of mRNAs and either target them for destruction or prevent them from being expressed. It's sort of a new way of gene regulation that's recently been developed.