ความคิดเห็นที่ 12
Story of Human Involvement in the Modification of Food
With plants perhaps more than any other kingdom of organisms, humans have been involved in modifying their characteristics through the careful selection of the male and female plant that would participate in the sexual union to give rise to the next generation of plants. Some of the genetic diversity that we see today in the market, the incredible diversity from artichokes to zucchini, from bananas to yams, resulted from this involvement in the process of genetic exchange among plants. The parents chosen to give rise to the next generation were selected based on their characteristics, plants with a greater yield or better disease resistance characteristics were chosen in order to generate progeny with both characteristics. Progeny were screened to find those that had the desirable characteristics from both parents. In addition, individuals also looked for individual changes that occurred naturally during the outgrowth of the next generation. These small changes are responsible for the navel orange and the seedless grape, for example. It was processes like these that led over time to the conversion of ancient ancestors of the lowly carrot to the modern varieties that we eat today and from this rather unlikely ancestor of corn to the modern hybrid corn we see today. Over the years, mostly in the last century, these selections and changes led to incredible increases in the productivity of crops like corn, cotton, wheat and soybean.
What is actually going on when the two plants are bred to give rise to a new generation or when the change occurs to give rise to the navel orange. Plants and humans are composed of many individual units or cells. Our bodies contain around a trillion cells and inside each cell there is genetic information, also called DNA, which tells the plant (or human) what to do? That information is written in a common language shared among all living things. The entire set of information, contained in what is called a genome, contains all of the rules and commands that tell the cell what to do. In the human it decides whether you have blue or brown eyes, curly or straight hair. In the plant, it tells the cell whether it is a part of a leaf or a root. The directions are found in the DNA.
The DNA is made up of chemical units, strung together like beads on a string and it is the arrangement of those units that determines what characteristics a plant (or any other organism, for that matter) has. It is said that, if you were to stretch the DNA in your cell or that in a wheat plant, it would reach somewhere around 6 miles! If one then were to represent each of the units in that long string by an alphabetic letter, it would take 1.7 million pages to contain all the information in a wheat plant, for example.
What happens then when you cross two wheat plants to get an improved variety? Genetic rules state that we can't end up with 3.4 million pages, but rather the information in that new fertilized cell can only contain 1.7 million pages. This means that there is random retention of certain volumes in the progeny from each parent and the individual making the cross has no control over which volumes are kept and which ones are lost. In addition to that uncertainty, volumes often get rearranged during the normal fertilization process of the egg and sperm, resulting in different parts of the "text" being juxtaposed in progeny. The breeder involved in this genetic exchange can only observe the process without really knowing how the content of the new volumes will be arranged; the breeder can only chose progeny in the end with the desired characteristics.
How is this similar to or different from the process used with genetic engineering? A striking difference between the two methods of genetic exchange is in the amount of DNA that is manipulated. In the molecular approach the amount is relatively small, comparable to a half page of information in the analogy that was just described. This is in contrast to the classical breeding situation where all of the information in both parents is shuffled around to arrive at the new progeny. In addition, the "text" in that half page is known to the scientist, who can "read it" and understand its function before moving it into its host organism. A similarity is that in general the same enzymatic machinery is used in both cases, but in the case of classical breeding it occurs inside the cell; in the case of genetic engineering the DNA manipulations occur in the laboratory, but in both cases the makeup of the genetic material is changed in the progeny.
A second significant difference is that, in contrast to the classical breeding approach, the source of the DNA need not be related as it is with sexual crosses. The DNA can be from the same plant, an unrelated plant, a bacterium or even a mammal. All living organisms share the same language and often share a significant portion of the same text. For example, you likely share 99.9% of the information in your cells with the person sitting next to you. Perhaps more surprisingly, you probably share some 50-60% of the information in your cells with a tomato plant. That genetic information in the tomato is fully capable of functioning in a new host, like a different tomato plant or in a corn plant. A third difference with genetic engineering technologies, compared to classical breeding is that it is possible to control precisely in which tissues the linked gene is expressed. If one is looking to improve the nutrition of a grain, for example, it is possible to target expression of the new characteristic to a particular compartment of the grain, like the endosperm. It is very difficult, often impossible, to accomplish this through classical methods.
Aus Biotechnologist
จากคุณ :
Aus Biotechnologist 
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30 ก.ค. 47 09:22:53
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