DNA from two different organisms can be joined together. First, the required gene must be isolated from the organism. It is then inserted into a vector (usually a plasmid or virus) and inserted into a host cell. Restriction endonucleases can be used to cut out the DNA from the DNA of the organism. Some restriction enzymes make blunt cuts, some make staggered cuts leaving exposed bases on each strand called sticky ends.Alternatively, mRNA can be isolated from the cytoplasm of a cell and the enzyme reverse transcriptase (an enzyme isolated from RNA retroviruses) and DNA polymerase can be used to make double-stranded DNA from the mRNA. Plasmids are then cut with the same restriction endonuclease to give the same sticky ends and the DNA is inserted into the plasmid and joined with DNA ligase.