The fundamental unit of a genome that controls heredity is known as gene. The word gene was coined by earlier botanists but the clear concept of gene came out after the work of Morgan who gave "the theory of genes" for which he was awarded Nobel Prize in the year 1933. The recent work on structure of chromosomes and deoxyribonucleic acid (DNA) clearly defines the gene as "a small segment of polynucleotide chain consisting of hundreds or thousands of nucleotide". The nucleotides are the subunits of a gene.

However, in breeding programmes much work has been done on alteration of nucleotides (changing of genes) by several parasexual or conjugational methods in different organisms. Now, a large number of mutagenic chemicals are available which mutate the genes. It is likely that the changed genes may be beneficial, neutral or lethal. In recent years, protoplast fusion and somatic hybridization have also become a tool in breeding programmes for conferring genes of beneficial properties. The conventional breeding techniques take a long time to assure the changed gene in progeny and its role for human beings.

Therefore, to solve this problem in vitro recombinant DNA (rDNA) technology has focused great attention due to many more future prospects. Thus, genetic engineering (= gene cloning, rDNA technology) can be defined as "changing of genes by using in vitro processes". A gene of known function can be transferred from its normal location into a cell (that of course, does not contain it) via a suitable vector. The transferred gene replicates normally and is handed over to the next progeny. On confirmation for its presence through biochemical procedures replica of the same cell (i.e. clones) can be produced. The derivation of procedures for the reintroduction of the foreign DNA fragment into a bacterium have lead to evolution of new technology i.e. the recombinant DNA technology, gene cloning, gene manipulation or genetic engineering.

Genetic engineering has been applied for the production of valuable polypeptides, insulin, interferon, growth hormones, and in the transfer of Nif genes, control of genetic diseases, etc.

Ä Enzymes

Ä Exonucleases

Ä Endonucleases

Ä Restriction endonucleases

Ä Nomenclature

Ä Example of some enzymes

Ä SI nuclease

Ä DNA ligases

Ä Alkaline phosphatase

Ä Reverse transcriptase

Ä DNA Polymerase

Ä Foreign DNA

Ä Cloning vectors

Ä Plasmids

Ä Bacteriophages

Ä Insertion vector

Ä Replacement vector

Ä Cosmids

Ä Phasmids

Ä cDNA Clone bank

Ä Gene bank (Genomic Library)

Ä Electrophoresis