DNA is a molecule that is often represented as though it were beads on a chain. There are four types of beads, usually represented by the letters C,G,T and A. Each of these letters refers to a specific type of molecule called a nucleic acid(NA). A nucleic acid has properties such that it can be polymerized, or linked with any other NA. Biologists and chemists have devised a tool kit of chemicals called restriction enzymes that can cut chains of DNA or link (ligate) them back together again.
Enzymes that cut DNA in certain sequences are called "restriction enzymes". The term "restriction" comes from the role these enzymes played in their natural host, one of many types of bacteria. These enzymes represent the "immune defenses" of bacteria because when they are invaded by phages ( virus), the enzymes will selectively clip the DNA of the invader. Scientists have taken these natural enzymes and put them to use as genetic engineering tools.
Given that scientists can isolate DNA from an organism, clip it into fragments using restriction enzymes, and ligate fragments back together again, they can create custom arrays of genes.
If we generate a circular sequence of DNA NA's which contain one gene, we say that we have cloned a gene. Useful bacteria such as E. coli ( found in warm blooded animals intestines) can be encouraged to take up a ring of DNA from it's environment, in this case a test tube. When a bacteria takes up a new gene, we say it has been "transformed".
Each sequence of DNA, or gene corresponds to a protein. A protein is a sequence of beads except that each of the beads is an amino acid. Examples of amino acids are those nutrients we are likely to find in a nutrition store. For example, lysine an amino acid and common nutrient supplement.
Proteins can be manufactured in bacteria or other hosts in this manor. A more difficult laboratory procedure is introducing a new gene into a higher animal such as a mouse. In this case, replication deficient virus particles are used to move a custom bit of DNA into a cell much the way a virulent moves it's own DNA into a cell. There are still many challenges to genetic engineering because bacterial hosts such as e. Coli are not equipped to manufacture all of the proteins that we would like to produce. Antibodies, which are common targets of the biotechnology industry are actually assembled by many enzymes which are not normally present in e. Coli. Thus, in many cases cell cultures must be used for production in the biotechnology industry instead of bacteria.