The plasmid: a tiny DNA ring
Bacteria carry their main chromosome, but many also hold small extra circles of DNA called plasmids. A plasmid copies itself independently and gets passed along when the cell divides. That makes it the perfect delivery container: cut it open with a restriction enzyme, drop in your gene, seal it with ligase, and you have a recombinant plasmid ready to ride into a cell.
Any DNA molecule used to carry a gene into a cell is called a vector. Plasmids are the most common, but viruses and other carriers serve as vectors too. A good vector needs three things: a way to copy itself in the host, a place to insert your gene, and a selectable marker — often an antibiotic-resistance gene — that lets you tell which cells took up the vector.
Transformation: getting DNA into a cell
Transformation is the step where a bacterium takes up the recombinant plasmid from its surroundings. Cells do not normally swallow DNA easily, so we coax them — a common trick is a quick heat shock after chilling them in a salt solution, which briefly makes the membrane leaky enough for the plasmid to slip in.
Only a few cells in the dish actually take up a plasmid. Here the selectable marker earns its keep: spread the cells on a plate containing the antibiotic, and only the transformed cells survive and grow. Each survivor founds a colony, and every cell in that colony carries copies of your gene.
Cloning: one gene becomes billions
Molecular cloning means making many identical copies of a piece of DNA. Because a single bacterium divides roughly every 20–30 minutes, one transformed cell becomes millions overnight — each carrying your plasmid and replicating it. By morning you have effectively limitless copies of the gene, all from one starting molecule.
- Cut the gene and an open plasmid vector with the same enzyme; ligate them into a recombinant plasmid.
- Transform bacteria so a few cells take up the plasmid.
- Select on antibiotic plates so only plasmid-carrying cells grow into colonies.
- Grow a colony in liquid culture; the cells multiply and copy the gene billions of times.