Gene library and gene bank

1. Gene Library

Setting Up of Gene Library. A chromosome of an organism is broken into small fragments com­prising one or a few genes with the help of restric­tion endonucleases. Each fragment is joined to a suitabe vehicle DNA, forming recombinant DNAs of different nature. These are then introduced into a host (bacterial, yeast, plant or animal) cell. The foreign DNA fragment replicates as the host cell divides. This produces a group of cells, each con­taining a foreign DNA fragment comprising one or a few genes. The daughter cells identical to the parent cell are referred to as a clone of cells. The process is called DNA cloning. Many clones of the cells, each clone containing a different fragment of the same foreign DNA, are thus formed. The various clones representing, an organism are referred to as gene library of that organism.

Complementary DNA (c-DNA) formed from an RNA. template by using the enzyme reverse transcriptase may be cloned as explained above for setting up a gene library. The collection of c-DNA fragments is termed c-DNA library.

Use of Gene Library. From a gene library, it is possible to identify a clone containing the specific gene. This gene can be multiplied by growing the relevant clone in a culture. The base sequence in this gene can be found as the methods for this purpose are known. From the base sequence, the sequence of amino acids in a polypeptide can be worked out on the basis of the triplet code.

2. Gene Bank

Gene bank is a place for the storage of clones of known DNA fragments, genes, gene maps, seeds, spores, and frozen spermatozoa, eggs and embryos. A gene bank provides materials for use in genetic engineering and breeding experiments, particularly concerning the extinct species. Preserved human sperm, ova and embryos can also be used for fertility purposes in human females.

Transgenic organisms

What are Transgenic Organisms ?

The organisms which contain functional foreign genes experimentally introduced into their genome by genetic engineering from another species are called transgenic organisms, also trans­genics. The foreign gene alters the host cell geneti­cally, and is termed transgene. The production of transgenic organisms is known as transgenesis.

The transgenic organisms are also called genetically modified organisms (GMOs). The lat­ter may also be formed by introduction into them an extra copy of their own gene or a cloned and modified gene. Transgenics may be animals, plants or microorganisms.

Transgenic Animals

The first transgenic animals were produced in 1982. These were mice having in them the growth hormone gene (rGH) from rat, such transgenic mice grow to double the normal size. Since then many transgenic animals have been produced. These include the following.

1. Transgenic Sheep. Transgenic sheep having different foreign genes have been produced.

1) Female sheep having human anti-haemophiliac factor IX gene in their genome have been produced. They secrete human clotting factor IX in their milk. The factor IX can be separated from the milk, and administered to the haemophiliac patients. The sheepflock can now provide enough clotting factor to supply all the world's haemophiliacs.

2) Sheep containing bacterial genes which code for the enzymes needed for the synthesis of amino acid cysteine have been produced. They give higher wool yield. The amino acid cysteine is an important com­ponent of die protein keratin of wool. 3) Sheep with gene for growth hormone have also been formed. They yield more mutton.

2. Transgenic Goats. Goats having a human gene that codes for a blood clot-dissolving protein (tissue plasminogen activator, TPA) have been formed. They secrete this protein in the milk. This protein is used for treatment of coronary throm­bosis.

3. Transgenic Cows. Cows containing cDNA coding for the antibacterial human iron-binding protein, lactoferrin, have been produced.

4. Transgenic Pigs. Pigs having bovine growth hormone (bGH) gene or human growth hormone (hGH) gene have been formed. Such pigs grow a little faster, and have greatly reduced subcutaneous fat. They can, thus, provide leaner pork with lower fat content.

Transgenic pigs having human genes carry human antigens in their organs. The organs (heart, kidney, pancreas) of such pigs can be transplanted into humans without the risk of rejection.

5. Transgenic Fish. Transgenic fish of many species, such as common carp, catfish, salmon, goldfish, have been produced. They contain human growth hormone (hGH) gene. They attain a size twice of that shown by nontransgenic fish.

6. Transgenic Mouse. One strain of transgenic mouse contains a human gene that causes breast cancer. This enables the researchers to study the very early development of cancer.

Transgenic mouse carrying a gene for human growth hormone (hGH) grows to more than twice the size. A plasmid carrying hGH gene is injected into the egg that develops into a super-mouse. The hGH gene produces human growth hormone in the cells of the mouse that, therefore, grows to an unusual size.