ABSTRACT
To be certain that the desired gene has been purified, it might be necessary to purify each of the individual DNA fragments in the digested DNA sample. Restriction enzymes have been used to delete or remove this central "non-essential" region of lambda to construct vectors that allow the insertion of additional DNA fragments. Replacement vectors take advantage of the fact that the vector arms ligated together without an insert are too small to be packaged in empty phage heads, while lambda arms ligated with an extra DNA fragment can be large enough to allow efficient packaging. These replacement vectors preferentially select for recombinants containing certain size classes of DNA fragments and can accept total inserts as large as 20,000 base pairs, a size that is difficult with other vectors. Vectors have also been designed that allow cloning of DNA fragments in eukaryotes. For example, expression vectors allow the over-production of a desired gene product from a cloned DNA fragment.
