From Genes to GMOs

　　从基因到转基因生命体

　　Today, genes can be isolated, identified, and cloned (copied over and over), then inserted into other organisms to alter their traits. The process is called genetic engineering. For this technology to develop, a few tools were necessary. In the 1970s, scientists isolated bacterial plasmids. These are hula-hoop-shaped double-stranded units of DNA that can be moved easily from one cell to another. They also discovered “scissors,” called restriction enzymes for cutting the DNA into predictable, reproducible patterns. These enzymes are used to “snip apart” plasmids at very specific DNA sequences, leaving free ends that can be rejoined as the scientist chooses. Restriction enzymes occur in bacteria as part of a natural defense mechanism to guard against invading viruses. Many different types are now available, each cutting DNA at a different sequence of base pairs.

　　Once a plasmid is snipped open, a foreign piece of DNA, cut by the same enzyme scissors, can be taped, end to end, into the plasmid using another enzyme, DNA ligase. This is the glue that sticks all the pieces together. The new plasmid is inserted back into a cell, where numerous copies can be made. Introduction of specific genetic material into rapidly reproducing target bacteria can turn the cells into miniature factories for production of useful substances. For example, when the Exxon Valdez oil freighter ran aground in 1989 and spilled thirty-eight million liters of oil, oil-eating bacteria, created in just this manner, were used in the cleanup operation. The oil was broken down five times faster with help from the genetically modified organisms (GMOs).

　　Plasmid technology has also been developed for moving targeted genetic material into plants. In this technique, scientists use the plasmid from a bacterium that causes tumors on plants. In nature, this bacterium transfers genetic material into plant tissues by releasing plasmids onto damaged plant cells. The plasmids enter the plant tissue and produce a swelling, or tumor. Because of this special ability to invade plant tissue, these tumor-inducing (TI) plasmids are now used routinely as “taxi cabs” to carry target genes into a wide variety of plant cells, including, for example, corn. The European corn borer is a common pest in this economically valuable crop. When pesticides are used against them, timing is critical. If sprayed too late, the corn borer will already have made a home inside the corn stem and will not be killed. CIBA Research was the first company to develop what has become commonly known as Bt corn. It contains genes that allow it to resist infestation by the corn borer. The Bt genes came from a bacterium called Bacillus thuring ensis . It produces a protein called Bt protoxin. When an insect larva eats these bacteria, the toxin contained in the bacterium attaches to the insect’s gut and makes holes in it, and the larva starves to death. Bt corn can be grown using less pesticide, and sometimes even no pesticide.

　　如今，基因可以被分离、识别和克隆。并且，将基因插入其他生命体中还能改变它们的特征。这一过程叫基因工程。要发展这种技术，我们需要一些工具。20世纪70年代，科学家分离出了细菌的原生质。它们是形似呼拉圈的双股DNA单位，可以被轻易地从一个细胞转移到另一个细胞。科学家们还发现了被称为限制酶的“剪刀”，可以把DNA分裂成可以预测的和可以复制的形式。这些酶被用来在特定的DNA的序列中把原生质切断，留下的一端可以被科学家们自由地重新连接。作为抵抗病毒的自然防御机制的一部分，限制酶存在于细菌中。现在科学家们已经发现了许多种限制酶。它们在碱基对的不同序列中切断DNA。

　　一旦原生质被分开后，用同样限制酶剪开的外来的DNA片段和它的被分 开的那一端被粘到一起，形成了新的原生质。连接时使用的是另一种酶——DNA 连接酶。这种胶可以把所有的碎片粘到一起。新的原生质被插回细胞中，在那里可以进行多次复制。如果把特定的基因材料引入高速复制的目标细菌中，细胞就会变成生产有用物质的微型工厂。例如，当埃克森公司的瓦尔迪兹号油轮在1989年搁浅后，它泄露了3800万公升的油。用上述方法创造出的吃油的细菌被用于清理油污的行动。有了转基因生命体（GMOs）的帮助，油分解的速度提高了五倍。

　　细胞质技术还被应用于把目标基因材料转移到植物的过程中。在应用这种技术时，科学家们使用导致植物肿瘤的细菌中的原生质。通过把原生质释放到损坏的植物细胞上，这种细菌把基因材料转移到植物的组织当中。原生质进入植物组织后产生了浮肿，或者说是肿瘤。因为具有这种特殊的侵袭植物组织的能力，这些导致肿瘤的（TI）原生质被人们习惯地当成“出租车”来使用，目的是把目标基因带到多种植物细胞当中，比如说，玉米细胞。欧洲玉米螟普遍存在于玉米这种经济价值很高的农作物当中。使用杀虫剂消灭这种害虫时，时机的掌握很关键。如果杀虫剂喷洒得太晚，玉米螟早已经在玉米茎的内部安家了。这样，它们就不能被杀死。CIBA研究所是第一家研制出我们通常所称的Bt玉米的公司。这种玉米中含有能够抵御玉米螟的基因。这种Bt基因来自于一种名为图根菌的细菌。它会产生Bt原霉素的蛋白质。当昆虫的幼虫吃了这些细菌后，细菌中的霉素就会附着在昆虫的内脏，并在上面打出许多洞。这样，幼虫就会被饿死。Bt玉米在生长过程中需要很少，甚至不需要杀虫剂。