Selecting for a gene

Plasmids are a very valuble tool for biologists. Plasmids are circular strands of DNA that are found in bacteria. Plasmids must contain an origin of replication (where DNA polymerase binds), a multiple cloning site (where you can add any gene you want), and a selectable marker (usually an antibiotic resistant gene). These plasmids can be inserted into any cell. If you want a cell to contain a certain gene, you place that gene in the plasmid, then place the plasmid in the cell. The selectable marker is usually an antibiotic resistant gene because this allows you to select for cells that contain the plasmid. You place the cells you believe to have taken up the plasmid on a plate with an antibiotic of your choosing. Only the cells containing the plasmid, with antibiotic gene and the gene of your choosing, will be able to grow on this plate.