Vectors

After a desired segment of DNA is cut away from the donor's genome with restriction endonucleases, it is ligated into a vector DNA molecule, usually a plasmid or a bacteriophage genome. Ligases are enzymes that catalyze the formation of a phosphodiester bond between the 3'-hydroxyl group of a segment of donor DNA and the 5'-phosphate group of the vector DNA . Avector is a DNA molecule into which foreign DNA moleules are ligated and inserted into cells so that the recombinant DNA can be replicated. Plasmid vectors must also contain a marker, such as an antibiotic resistance gene, to facilitate the selection of bacterial cells that contain the plasmid. Vectors can be introduced into host cells by transformation or transduction. An expression vector is a vector that carries a gene that can be efficiently transcribed and translated by the host cell.

Characteristics of a good vector:
Stable
Self-replicating
Small
Easily isolated
Variety of single cut sites
Easily detected