Genetics focuses on the nature of genes, and a major goal is to characterize their structure and function. Recombinant DNA technology has allowed individual genes to be isolated in a test tube and then characterized at the molecular level. The technology is based on restriction enzymes, which cut DNA into defined fragments. Restriction target sites can be mapped and act as DNA landmarks. Restriction fragments often have sticky ends, enabling them to be inserted into a vector capable of replicating in a bacterial cell. Such molecular hybrids are known as recombinant DNA. Bacteria amplify a single recombinant DNA molecule to form a DNA clone. Common vectors are plasmids, phages, and cosmids. An entire genome can be cloned in a set of clones known as a library. A specific clone can be found in a library by using a probe that specifically binds to the DNA or to the protein of the desired clone. Specific clones can also be isolated by their ability to transform null mutants. Tagging also is useful for cloning a gene: transforming DNA or a transposon is used to cause a mutation by insertion, and the DNA adjacent to the tag is isolated. Chromosome walking provides a way of isolating a gene by sequential isolation of overlapping clones, starting from a marker linked to the desired gene. Cloned DNA can be sequenced by several methods, including the arrest of DNA chain growth by dideoxynucleotides. The polymerase chain reaction uses primers to amplify DNA sequences. It is a way of rapidly isolating DNA whose structure is already partly sequenced and of detecting small amounts of one specific type of DNA. Gel electrophoresis separates variously sized DNA or RNA moleculres from a mixture. Probes can detect specific DNA or RNA molecules on gel, in procedures known as Southern and Northern analyses, respectively.

It is an amino acid that is nutritionally important and occurs in small amounts in proteins. It is an essential amino acid, meaning that humans and certain other animals cannot synthesize it and must obtain it from their diets. Infants require greater amounts of tryptophan than adults to ensure normal growth and development. Tryptophan is used by the body to manufacture several important substances, including the neurotransmitter serotonin and the vitamin niacin. Diets poor in tryptophan can lead to pellagra, a disease resulting from niacin deficiency; however, this disease is now rare in developed countries. In 1901 the English biochemist Frederick G. Hopkins isolated tryptophan from casein, the major protein found in milk.

The general approach of gene therapy is nothing more than an extension of the technique for clone selection by functional complementaion. The functions absent in the recipient as a result of a defective gene are introduced on a vector that inserts into one of the recipient's chromosomes and thereby generates a transgenic animal that has been genetically "cured." The technique is of great potential in humans because it offers the hope of correcting hereditary diseases. However, gene therapy is also being applied to mammals other than humans. Perhaps the most exciting and controversial application of transgenic technology is in human gene therapy, the treament and alleviation of human genetic disease by adding exogenous wild-type genes to correct the defective function of mutations. The first case of gene therapy in mammals was to "cure" a genotypically dwarf fertilized mouse egg by injecting the appropriate wild-type allele for normal growth. This technique has little application in humans, because it is currently impossible to diagnose whether a fertilized egg cell carries a defective genotype without destroying the cell. However, in an early embryo containing only a few cells, one cell can be removed and analyzed with no ill effects on the remainder.

Allele:

One of the different forms of a gene that can exist at a single locus (location).

Chimera (mosaic):

A tissue containing two or more gentically distinct cell types or an individual compsed f such tissues.

Chromosome walking:

A method for the dissection of large segments of DNA, in which a cloned segment of DNA, usually eukaryotic, is used to screen recombinant DNA clones from the same genome bank for other clones containing neighboring sequences.

Cosmid:

A cloning vector that, like a plasmid, can replicated automonomously and be packaged into phage.

DNA (deoxyribonucleic acid):

A double chain of linked nucleoties (having deoxyribose as their sugars); the fundamental substance of which genes are composed.

DNA clone:

A section of DNA that has been inserted into a vector molecule, such as a plasmid or a phage chromosome, and then replicated to form many copies.

DNA polymerase:

An enzyme that can synthesize new DNA strands from a DNA template; several such enzymes exist.

Gene Therapy:

The correction of a genetic deficiency in a cell by the addition of new DNA and its insertion into the genome.

Intron:

Intervening sequence; a segment of largely unknown function within a gene. This segment is initially transcribed, but the transcript is not found in the functional mRNA.

Plasmid:

Autonomously replicating extrachromosomal DNA molecule.

Restriction enzyme:

An endonuclease that recognizes specific target nucleotide sequences in DNA and breaks the DNA chain at those points; a variety of these enzymes are known, and they are extensively used in genetic engineering.

Transposon:

A mobile piece of DNA that is flanked by terminal repeat sequences and typically bears genes encoding transposition functions.

Vector:

In cloning, the plasmid or phage chromosome used to carry the cloned DNA segment.

Wild-Type:

The genotype or phenotype that is found in nature or in the standard laboratory stock for a given organism.