plasmids can be good cloning vectors because they carry an origin of replication and are therefore able to replication independently within a cell
Most plasmids used as vectors also encode some type of selectable marker such as the gene for resistance to ampicillin
Ig the host cells are ampicillin sensitive,the only host cells that can grow on a medium containing ampicillin are those that have taken up the plasmids
Vectors must also have a small sequence of base pairs that can be recognized by a restriction enzyme.when this enzyme opens the circular plasmid,foreign DNA can be incorporated
When the plasmids vector and foreign DNA are both cut with the same restriction enzyme and mixed together,not all molecules will join to form recombinants
Some vector molecules will reanneal without incorporating foreign DNA.to identify cells that contain plasmids that have incorporated foreign DNA,a second marker gene is needed on the vector
This second marker contains the restriction enzyme site within its nucleotide sequence.if foreign DNA is inserted,the second marker is inactivated.this is referred to as insertional inactivation
A common second marker is the lac Z gene,which codes for the enzyme,beta galactosidase
Bata galactosidase can cleave a colorless chemical called x-gel ,to form a blue compound
Therefore colonies of cells that harbor the intact vector,but no new recombinant DNA,can make beta galactosidase and form a blue color in the presence of x-gel
However,colonies that contain new recombinant DNA cannot make beta galactosidase and are white