To clone a gene, a vector is first prepared. To genetically engineer a plasmid or virus, restriction enzymes are used to cleave plasmid DNA and to cleave foreign DNA. The "sticky ends" produced facilitate the insertion of foreign DNA into vector DNA. The foreign gene is sealed into the vector DNA by DNA ligase. When the plasmid replicates or the virus reproduces, the foreign gene is cloned.

A genomic library can be used as a source of genes to be cloned. A radioactive or fluorescent probe is used to identify the location of a single gene among the cloned fragment of an organism's DNA.

Restriction enzymes are used to fragment DNA. If the entire genome is used for fingerprinting, the use of probes is necessary in order to see a pattern following gel electrophoresis. If just a portion of the genome is of interest or available, polymerase chain reaction (PCR) uses the enzyme DNA polymerase to make multiple copies of target DNA. Analysis of DNA segments following PCR can involve gel electrophoresis to identify the DNA as belonging to a particular organism or can involve determining the base sequence of the DNA segment.

Transgenic organisms have also been made. Genetically engineered bacteria, agricultural plants, and farm animals now produce commercial products of interest to humans such as hormones and vaccines. Transgenic bacteria also perform bioremediation, extract minerals, and produce chemicals. Transgenic agricultural plants have been engineered to resist herbicides and pests. Transgenic animals have been given bovine growth hormone. Pigs have been genetically altered to serve as a source of organs for transplant patients. Cloning of animals is now possible.

The Human Genome Project has two goals: constructing a genetic map of the chromosomes and sequencing the DNA bases. This information will probably be of assistance to carrying out gene therapy. Gene therapy is used to correct the genotype of humans and to cure various human ills.

Ex vivo therapy involves withdrawing cells from the patient, inserting a functioning gene, usually via a retrovirus, and then returning the treated cells to the patient. Many investigators are trying to develop in vivo therapy, in which viruses, laboratory-grown cells, or synthetic chemicals will be used to carry healthy genes into the patient.

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