The Cloning Technology I. The difference between a 【T1】______colony and 【T1】___

The Cloning Technology
   Today, we are going to talk about the cloning technique. First, we will see the difference between a single cell colony and cloning a mammal. Then we will focus on cell cloning process and its scientific use.
   In March 1997, the world said "hello" to Dolly, the first mammal to be cloned from an adult cell. We asked ourselves: Would the world soon be populated with human clones? Did you know that cloning isn’t new? In fact, scientists have been cloning genes for many years in laboratories around the world. Of course, cloning a gene and cloning a whole mammal are quite different in terms of process and product. But just how do you get from a single cell to a clone?
   We generally think of cloning as producing a duplicate of something. Scientists, however, usually use the term clone in referring to cells. A clone is population of cells produced by one ancestor. Because of the way cells multiply, all the cells in a clone have exactly the same genetic makeup. Gene cloning uses a process known as "recombinant DNA technology". By contrast, cloning a mammal, such as the sheep Dolly, involves a technology known as "nuclear transfer". Although the end result is the same, the actual processes of gene cloning and mammalian cloning are quite different. But we need to start from the beginning to understand how these complicated and amazing bioengineering technologies evolved.
   Now, we will study on what genes, chromosomes, and the genome are.
   Within the body of a mammal, tissues are made of cells. Inside each cell is a long molecule called DNA. Its shape enables it to divide easily. Arrangements of chemical groups within DNA form genes. Genes direct cells to produce proteins, and thus determine what function a given cell will have. For example, skin cells produce proteins which are present in skin tissue, and blood cells produce a protein present in blood. Genes essentially tell each cell what type it is. Genes are linked together to form chromosomes. The entire collection of chromosomes in each of your cells is called genome. All the cells in your body contain the same genome. Once you realize that all cells contain the information for making the whole organism, you know that cloning is theoretically possible.
   But what makes a skin cell different from a blood cell? The chromosomes within the nucleus of each cell are folded in various ways. Genes that are buried within the chromosomes are inactive, or switched off, while genes on the surface of the chromosomes are active. In a skin cell, the genes for producing skin proteins are active, while in a blood cell, the gene is active. Gene cloning is the process of producing a population of cells all of which contain a specific gene. Using these identical cells, scientists can study the entire genome and obtain clues to how genes are switched on and off.
   Cloning genes also enables scientists to mass-produce proteins that can be used to treat a variety of diseases. For example, the cells in the pancreas produce a protein called insulin which is important in helping the body maintain appropriate blood sugar levels. Some people have a disease called diabetes because their cells don’t produce enough insulin. Cloning the insulin gene has provided a way of producing large amounts of human insulin which is used to treat diabetes.
   Then, we are going to disclose the mystery of cloning a gene. To clone a gene, scientists remove the DNA from a cell, isolate the specific gene of interest, and then get it to multiply. Sound simple? Not so fast. Let’s look at each step.
   First the DNA must be removed from the cell. Over the years scientists have perfected chemical methods for doing this, based on the physical properties of DNA molecules. The DNA is then cut into pieces using special proteins. You may think of these special proteins as little knives programmed to cut the DNA in specific places. By using the correct special proteins, scientists can isolate whatever gene they want.
   Once the gene is isolated, it can then be duplicated. Mammalian genes do not reproduce by themselves, however. To replicate them, scientists attach them to pieces of non-mammalian DNA that do replicate on their own. The most common procedure uses small circular pieces of DNA called plasmids that come from bacteria. Plasmids have two useful characteristics. They are easily incorporated into bacteria; they multiply by themselves inside bacteria. The gene of interest is combined with a plasmid and some proteins. The proteins open the plasmid circle and stitch in the new gene. The result is a recombinant DNA molecule. The plasmid carries the gene into a host cell. Once inside the host, the plasmid multiplies, making lots of copies of itself and the gene it contains.
   When the host cell divides, copies of the recombinant DNA molecule are passed to the cell’s offspring. As the bacterial cells continue to divide, a colony of identical cells is produced. Each cell contains one or more copies of the recombinant DNA molecule. The gene is now considered cloned.
   Let’s go back now to the example of the insulin gene. Prior to cloning this gene, scientists purified insulin from animal sources, like pigs or cows. Scientists isolated the insulin gene from the DNA of human pancreatic cells. Then they attached the insulin gene to a plasmid and were able to get bacterial cells to incorporate the recombinant DNA. The result was a colony of bacteria that continually produced human insulin—a little insulin producing factory!