单项选择题
| A new biotechnology procedure that
could become commercially available in as little as two to four years is
"transgenosis", which permits scientists to create an animal with specific
traits by adding, removing, inactivating, or repairing genes in an embryo. The
additional genes can come from any source. For example, if a gene of interest
occurs in mosquitoes—say, one that codes for resistance to a certain disease—it
can be removed and placed in the embryo of a farm animal, the several strains of
commercially useful transgenic farm animals that will probably emerge in the
next few years could include leaner pigs, poultry resisting to influenza or
other deadly diseases, sheep with wool that is easier to wash, and goats that
produce valuable pharmaceuticals in their milk. The simplest way to make transgenic animals is to inject a gene into a one-cell embryo and then implant the embryo in another animal. Under the right conditions, the new gene joins one of the embryo’s strands of genes. Each cell created as the embryo divides gets a copy of the new gene. An alternative technique is to incorporate the gene into a type of virus known as a retrovirus that has been modified so it cannot reproduce itself after entering a cell. The virus, which cannot cause disease, delivers the gene to the cell’s nucleus. Often this method is better than gene injection because a retrovirus always delivers just one gene, and the gene is always undamaged and complete. |