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The Efficacy And Advantages Of Inserting Foreign Genes Into The Chloroplastic Genome Instead Of The Nuclear Genome

This paper will discuss the efficacy and advantages of inserting foreign genes into the chloroplastic genome instead of the nuclear genome. It will also cover in depth elaboration of relevant issues by using named examples.In DNA, adjacent nucleotides are joined by the phosphate between the 5′ carbon atom of the sugar of one nucleotide and the 3′ carbon atom of the sugar in the adjoining nucleotide. In DNA, one side of the double helix terminates in a 3′ end while the other side, aligned in the opposite direction (antiparallel), terminates in a 5′ end. To these projecting 3′ ends, a short series of identical nucleotides containing adenine were attached through the activity of another enzyme. Another batch of DNA was treated in a likewise manner, except that nucleotides containing thymine were added instead of adenine.(Avril, 187-94)When these two samples of DNA were mixed, the complementary “tails” of A- and T-bearing nucleotides became joined by hydrogen bonding. This combined the once separate fragments into long, interconnected chains. DNA ligase was then added to form bonds between the sugar and phosphate groups. The two DNA strands were now one. It was certainly intriguing that one could now cut up DNA into unpredictable heterogeneous fragments and randomly stitch them back together. However, for further insights into the organization of DNA and its genes — that is, the determination of precise nucleotide sequences very specific nucleases would have to be found. The prevailing opinion was that such specific DNA-cutting capability did not exist in nature. The only clue to the possibility that more specific nucleases might exist came from observations beginning as early as 1953 that when DNA molecules from E. coli were introduced into another slightly different form of E. coli they seldom functioned genetically. They were quickly broken down into smaller fragments. This apparently was part of a system that had evolved in bacteria to protect them against the entrance of foreign DNA. In addition to all of the other more obvious forms of competition in nature, there is a constant invisible struggle played out in the microscopic world, in this case between bacteria and bacteriophages. Darwin’s natural selection is recreated here on a minute scale.(David, 131-44)First, bacteria can be grown under controlled conditions, rapidly and in enormous numbers. Overnight, a few cells will multiply into literally billions. It is very important to understand that a bacterial cell ordinarily reproduces simply by copying itself. Assuming that no mutations occur in the cells, all the descendants of that one cell are identical. Such a population of cells originating from a single cell is termed a “clone” and the process of producing that clone is referred to as “cloning” the cell. The DNA in a typical bacterial cell exists in two forms. One is the single bacterial chromosome which, unlike the chromosomes in our cells, is in the form of a circular molecule. The DNA of all other organisms can be likened to a long string. In bacteria, the ends of the string are joined, forming a circle. In addition to the DNA in the bacterial chromosome, DNA also occurs in bacteria in the form of plasmids. These, like the bacterial chromosome, are also circular DNA molecules, but much smaller. When the bacterial cell divides, the bacterial chromosome replicates and one chromosome is passed on to the new cell. Likewise, each of the plasmids replicate and half are delivered to the next generation. The plasmids are unique, independent, self-replicating DNA molecules which can exist only within the living bacterial cell.Plasmids can easily be isolated from bacteria by breaking open the cells with enzymes which break down the cell wall. The resulting mix is centrifuged.The heavier chromosomal DNA, termed “genomic” DNA, as well as cell fragments will go to the bottom. (James, 44-49) This leaves a relatively clean suspension of plasmids near the top of the centrifuge tube. These tiny circles of DNA are actually not vital to the survival of the bacterium. The plasmids can be removed from a bacterial cell and the cell will function normally. However, some plasmids do contain genes which confer a marked advantage to the cell under certain conditions. For example, the fatal poison of “lockjaw” is a product of genes in plasmids of the tetanus bacterium. E. coli has plasmids that cause one form of the infamous “traveler’s diarrhea.” Probably the most widely studied plasmid genes are the ones conferring resistance to specific antibiotics. Certain bacteria can produce enzymes coded for by plasmid genes that break down antibiotics such as penicillin, ampicillin, tetracycline, or chloramphenicol. In nature, this gives the bacteria a defense mechanism against naturally occurring antibiotics. In the tissues of an infected patient, bacteria with these plasmids may overcome the administration of therapeutic antibiotics. Such resistant infections have become a major medical problem. This seemingly esoteric description of bacterial life contains another key element in our story. These bacterial plasmids are used as the DNA molecules into which other DNA fragments cut out by a restriction enzyme can be placed. Going back to our original principle, if we cut up any DNA with a restriction enzyme and cut plasmids with the same enzyme, mix the cut plasmids and the cut DNA in the presence of DNA ligase, plasmid-foreign DNA chimeras will be formed. Gene cloning now solves these problems. To clone a gene means simply to obtain a minute, pure sample of the gene and make lots more of it, as if one had a document and made many identical ones by photocopying it. The “photocopying” of genes is accomplished by first joining a few of the genes to vectors such as plasmids and inserting the vectors, now carrying the gene, into bacteria or other suitable cells. We have introduced the basic principles of gene cloning already. In that process, so-called foreign DNA, the DNA we have removed from an organism, is inserted into the vector molecule, such as a plasmid, to create a DNA chimera. The building of such composites or artificial recombinant molecules has also been termed genetic engineering or “gene manipulation.” This procedure has also been referred to as “molecular cloning” or “gene cloning” because a population of genetically identical bacteria, all containing the desired DNA, can be grown in great numbers, thereby copying the DNA as often as the cells divide.(Roger, 34-38)What is very significant is that these chimeras within the bacterial cells may be able to copy not only themselves but actually produce a specific gene product in large amounts. This approach has already been utilized in the commercial production of human insulin, growth hormone, and the antiviral protein interferon. All of these can now be made by bacteria, because the human genes that regulate their synthesis have been isolated and cloned in bacteria. The bacterial cells, grown in vast numbers, obedient to the commands of their genes, now make a human gene product. Human gene products derived from genes isolated and cloned during the Human Genome Project will undoubtedly also be used for the benefit of humanity, particularly in the cure and prevention of disease. Consider the explicit and implicit problems. Our genes are part of the chromosomes which function within the nucleus of each cell. Every defective gene is therefore inside each diploid somatic (nonreproductive) cell and distributed at random in the haploid gametes. The aim of gene therapy is to replace or supplement the defective genetic information with normal, functional genes. How could we possibly get at those undesirable genes, hidden as they are inside the trillions of cells that constitute the human body? The actual physical removal and replacement of genes in the body’s cells is not possible. However, ample precedent exists for the introduction of normal genes into cells where they become part of the functional genome of the cell without necessitating the removal or repair of a resident nonfunctional gene.(Victor, 910-915)For there reside the genes, the chromosomal subunits in which lies the code that determines a lot more than our hair and eye color, our sex, or our height, and right- or left-handedness. They are the direct cause of many diseases such as cystic fibrosis and sickle-cell anemia, regulate our tendency toward cancer, heart attacks, or Alzheimer’s disease . . . in fact, humans are afflicted by more than 3000 known inherited diseases. Similarly a new frontier in molecular biology is upon us. We are only now approaching through the Human Genome Project and all of the research that it will inspire an intimate understanding of our genes. This will bring with it a power over human life which must be used wisely. There is much more to be said on this theme in later chapters, so let us return to the drama at hand: the revolutionary discovery of the structure and essence of the gene.

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