Lecture 10 February 12 2003 R.Jones Chapters 10.6; 11-11.9; and 12
Cloning and Genetic Engineering
1. Cloning: The process of duplicating exactly an organism or a molecule. Propagation of plants often occurs by cloning and among the largest organisms in the world are clonal stands of willow trees and of certian fungi that grow on the surface of the soil or below ground. Replication of exact copies of DNA is known as molecular cloning and exact duplication of animals, mammals and humans is also referred to as reproductive cloning-Dolly the sheep the most prominent example in mammals, but the technique had been succesfully used for more than 20 years to clone amphibians experimentally.
2. The original purpose of early cloning experiments with plants and amphibian was to provide definitive evidence that the chromosomes of somatic cels carried and specified all of the traits of a normal adult. This was found to the case for carrot and frog cells (see Fig, 11.3). This concept was extended in the case of Dolly the sheep. . In the propagation of organisms by cloning a nucleus from a somatic cell (or a somatic cell) is used to replace the nucleus in an egg. The egg's haploid (n) nucleus is removed and replaced with a 2n somatic nucleus or cell. The diploid "zygote" is then grown in culture and after several rounds of mitosis it is implanted in the womb. It's genotype is that of the donor somatic cell and it will be an exact replica of this cell.
3. Stem cells are cells that have retained thier embryonic potential, what researchers call undifferentiated cells. They can be derived by reproductive cloning procedures and are often obtained from blastocysts as a result of in vitro fertilization (see Introduction Ch. 11) or from fetal umbilical cord blood.
4. In molecular cloning bacterial plasmids and restriction enzymes allow DNA to be cut and pasted and the cloning, i.e. multiplication of DNA to be carried out, for example, in bacteria or by PCR. Cloning of DNA simply means making multiple exact copies of the DNA. By inserting a modified plasmid carrying a foreign gene into a bacterium it is also possible to translate that gene in bacteria and accumulate the protien product. Porcine insulin is made this way in bacteria and is used to treat diabetes.
4. PCR-polymerase chain reaction allows multiple copies of DNA to be made in the test tube. PCR requires primers (naked strands of DNA cannot be duplicated by DNA polymerase without a primer, i.e., the replication process must first be initiated before replication can continue). Heat resistant DNA polymerase (Taq polymerase) allows DNA to be heated, uncoiling the DNA which is then replicated by Taq polymerase and primers, this is then heated to unwind DNA that is then replicated etc (see Fig. 12.12 of the textbbok).
5. Plants can be genetically modified in several different ways. Breeding can introduce new traits by crossing different parents carrying desirable characteristics. This is the so-called Mendelian approach and this is how most of today's agrcultural commodities were bred. There are many extreme example of crosses in plants such as in the new cereal triticale, a cross between wheat and rye. As a result of breeding animals and plants now used in agriculture are much larger, often very diifferent (e.g. Triticale) and more productive than progenitor organisms.
6. Using recombinant DNA technology, i.e. genetic engineering, its is possible to introduce specific new genes into aninals ands plants that may confer benefits. One way that this is done in plant is to use a plasmid from the bacterium Agrobacterium tumefaciens. The most recent example of beneficial transformation (introcution of a new gene or genes into a plant) was reported in rice. So-called golden rice has been made where a gene encoding vitamin E synthesis has been introduced into rice. Rice grains harboring this gene have higher vitamin E levels. Vitamin E is a major vitamin deficiency in Asia and this strategy is aimed at relieving vitamin deficiency especially in children. Recombinant DNA technology can also be abused and like all technologies there are overwhelmingly negative and positive aspects.