To understand how genes are cloned, we need introduce three terms.

This means "recombined" DNA, or DNA from more than one separate source. Recombinant DNA is mixed DNA.

No, this is not a hot car! A vector is a carrier. What has to be carried? We have to somehow "carry" the mixed DNA into the cells and this is exactly what the vector does; it carries recombinant DNA into cells.

One of the most common vectors are plasmids. Plasmids are tiny circular pieces of DNA that are commonly found in bacteria. It turns out that they make good carriers. After plasmids are extracted from bacteria, a gene from an outside source is, in turn, inserted into the plasmids. Now things get very interesting. If the bacteria is modified properly, it will actually allow plasmids to enter and take up residence – even plasmids with those foreign genes! In its new setting, the plasmid can start to duplicate. Whenever the bacteria reproduces, each new cell will contain at least one new plasmid with the changed gene. As this duplication is multiplied many times, eventually you get many copies, many clones, of the outside gene!

How Is foreign DNA Actually Inserted Into a Plasmid?

Enzymes that "cut" and "glue" are needed.

This first step is to cut open the Plasmid’s DNA and in the resulting gap, the outside DNA is placed. To accomplish this, there are two different types of enzymes that are needed: an enzyme that will cut and another that will glue. To actually open up the DNA a restriction enzyme is used. In nature these enzymes will protect their parent bacteria by cutting up potentially dangerous intruding DNA from other sources. They are called restriction enzymes because they "restrict" the development of viral DNA. There are over a hundred different restriction enzymes that have so far been identified. Each of these biological scissors cut the DNA at a specific place called a restriction site. The result is a set of double-stranded DNA pieces with single-stranded ends. (See Diagram B.) These ends that jut out are not only "sticky" but they have gaps that can be now be filled with a piece of foreign DNA. For DNA from an outside source to bond with an original fragment, one more enzyme is needed: DNA ligase. DNA ligase is a bacterial enzyme that seals any breaks in the DNA molecule. Once this enzyme is applied, the foreign DNA bonds to the original and the DNA has been spliced, or recombined.

Another way of copying genes or a specific piece of DNA is the polymerase chain reaction. The name "polymerase" comes from the DNA polymerase; this is the enzyme responsible for copying a cell’s DNA. In this procedure the DNA is heated with both the enzyme DNA polymerase and special "primers." Primers are pieces of nucleic acid – sequences of about 20 bases. However, these sequences must match the arrangement of bases on either side of the DNA which is to be duplicated. How are the copies actually produced? These primers attach themselves to the DNA by a process called "complimentary base pairing." (See diagram B.) After the primers are securely bound to the DNA, the replication begins and the target DNA is copied.