===Preparation of vector DNA===The cloning vector is treated with a restriction endonuclease to cleave the DNA at the site where foreign DNA will be inserted.
Če pa je DNA, ki jo kloniramo, izjemno velika (več sto tisoč ali milijonov baznih parov), potem za vektor pogosto izberemo umetne kromosome bakterij ali umetne kromosome kvasovk.