cistrans

Cistrans is a term used to contrast two classes of interactions that appear in different scientific contexts, most commonly in chemistry and genetics. In chemistry, cis-trans isomerism (also written cis/trans or with the E/Z nomenclature) refers to stereoisomerism around a double bond or within rings. In alkenes, cis isomers have substituents on the same side of the double bond, while trans isomers have them on opposite sides. This difference can lead to distinct physical properties such as boiling points, densities, and reactivity. In cyclic compounds, cis and trans describe the relative orientation of substituents on the same or opposite faces of the ring.

In genetics, cis- and trans- acting elements describe how regulatory components influence gene expression. Cis-acting elements are DNA sequences that regulate transcription on the same DNA molecule, such as promoters or enhancers that control nearby genes. Trans-acting factors are diffusible products, typically proteins or RNAs, encoded by other genes, that can regulate target genes on different DNA molecules. The interaction of cis elements and trans factors is central to understanding gene regulatory networks and can be investigated through genetic tests that distinguish whether regulation occurs in the same chromosome (cis) or across chromosomes (trans).

Historically, the notion of cis and trans emerged from studies of gene regulation and molecular interactions, with terminology such as the cistron used to denote a unit of genetic function. Today, cis-regulatory elements and trans-acting factors are standard terms in molecular biology, though older literature may still employ the shorthand "cis-trans" or "cistrans" to describe these complementary concepts.