DNA carries the genetic code required by an organism to make proteins within its cells. As the cell makes a protein, it copies the instructions from the DNA and transcribes them to a new ribonucleic acid (RNA) molecule. These RNA, or messenger RNA (mRNA), molecules carry the instruction set to the cell's ribosomes. Genetic engineering allows for a direct modification of an organism's genome to change an organism's traits, which are an organism's features. Scientists can insert DNA into an organism that contains the genetic code for a protein not normally made by that organism. This creates recombinant DNA, which is DNA created from more than one individual, possibly of a different species. This type of recombinant DNA technology is widely used in medicine and agriculture. For instance, a gene for insect resistance can be inserted into a crop plant that is normally affected by insects, such as corn rootworm or cotton bollworm. The new crop plant will produce recombinant proteins, which are proteins made by an organism that contain a recombinant gene associated with the inserted gene, allowing the new crop plant to show greater resistance to insects. However, encouraging a modified organism to express an inserted gene can be complicated because other components that assist in the decoding of the gene must also be added. This is especially problematic when trying to get a prokaryote, which is an organism without a nucleus, to express a gene of a eukaryote, an organism with a nucleus, because the genes are not always compatible.
Once the new gene incorporates into an organism's genome, the organism's own machinery for DNA replication will automatically replicate the new gene, along with the rest of its DNA. In this way, recombinant DNA technology can be used to quickly mass-produce human, other animal, or plant genes or proteins inside naturally dividing bacterial cells. Bacteria are often used as a vector, which is a carrier of recombinant DNA, for this purpose because they are small, have relatively simple genomes, and reproduce quickly by cell division, making exact clones. Also, bacteria can naturally harbor a plasmid, which is a small, circular piece of bacterial DNA that replicates on its own and can be transferred between cells. These pieces of DNA are easily incorporated into other bacterial cells, making the plasmids the perfect DNA transportation vector. Most human insulin currently available to diabetic patients in the United States is made using bacteria.As an example of this process, assume protein A is naturally produced by a plant. This protein is believed to have potential medical applications. In order to further study it, scientists will need large amounts of the gene and the protein. One way to accomplish this is through DNA cloning, the use of recombinant DNA technology to replicate DNA within a rapidly reproducing organism. First, scientists insert gene A, which contains the genetic code for protein A, into a bacterial cell. Then, as the bacteria quickly divide, the gene is replicated too. Eventually, entire bacterial colonies will carry gene A. The copies of the gene, or expressed proteins, can be harvested from the bacteria and used for research or other applications. Obtaining the protein itself is more complicated, as bacteria may have to be coaxed into making protein A through the use of a promoter, such as a specific chemical or a certain temperature.