Campbell Biology: Concepts & Connections [RENTAL EDITION] (10th Edition)

Book Preface

Although it may seem like a modern field, biotechnology, the manipulation of organisms or their components to make useful products, actually dates back to the dawn of civilizatio n. Consider such ancient practices as the use of yeast to make beer and bread, and the selective breeding of livestock, dogs, and other animals. But when people use the term biotecllnology today, they are usually referring to DNA t echnology, modern laboratory techniques for studying and manipulating genetic material. Using these methods, scientists can, for instance, extract genes from one o rganism and transfer them to another, effect ively moving genes between species as different as Escherichia coli bacteria, papaya, and fish.

In the 1970s, the field of biotech nology was advanced by the invention of m ethods for making recombinant DNA  in the lab. Recombina n t DNA is formed  when scientists combine pieces of DNA  from two different sources- often  different species- in vitro (in a test tube) to form a single  DNA molecule. Today, recombinant DNA technology is widely used forgenetic engineering, the direct manipulation of genes for practical purposes. Scientists have genetically engineered bacteria to mass-produce a variety of useful chemicals, from cancer drugs to pesticides. Scientists have also transferred genes from bacteria into plants and from one animal species in to another (Figure 12.1A).

To manipulate genes in the laboratory, biologists often use bacterial plasm ids, small, circular DNA molecules that replicate (duplicate) separately from the much larger bacterial chromosome (see Module 10.23). Plasmids typically carry only a few genes, can easily be transferred into bacteria, and are passed from one generation to the next. Because plasmids are easily manipulated to carry virtually any genes, they are key tools for DNA cloning, the production of many identical copies of a target segment of DNA. Through DNA cloning, scientists can mass produce many useful products.

Consider a typical genetic engineering challenge: A molecular biologist at a pharmaceutical compan y has identified a gene that codes for a valuable product, a hypothetical substance called protein V. The biologist wants to manufacture the protein o n a large scale. The biggest challenge in such an effort is of the “needle in a haystack” variety: The gene of interestis one relatively tiny segment embedded in a much longer DNA molecule. Figure 12.18 illustrates how the techniques of gene cloning can be used to mass produce a desired gene.

To begin, the bio logistisolates two kinds of DNA: O a bacterial plasmid (usually from the bacterium E. coli) that will serve as the vector, or gene carrier, and 0 the DNA from another organism (“foreign” DNA) that includes the gene that codes for protein V (gene V) along with other, unwanted genes. The DNA containing gene V could come from a variety of sources, such as a different bacterium, a plant, a nonhuman animal, or even human tissue cells growing in laboratory culture.
E) The researcher treats both the plasmid and the gene V source DNA with an enzyme that cuts DNA. An enzyme is chosen that cleaves the plasmid in only one place. O The source DNA, which is usually much longer in sequence than the plasmid, may be cut into many fragments, only one of which carries gene V. The figure shows the processing of  just one DNA fragment and one plasmid, but actually, millions of plasmids and DNA fragments, most of which do not contain gene V, are treated simultaneously.
The cut DNA from both sources-the plasmid and  target gene- are mixed.
The single-stranded ends of the plasmid base-pair with the complementary ends of the target DNA fragment (see Module  10.3 if you need a refresher on the DNA base-pairing rules).

The enzyme DNA ligase joins the two DNA molecules by way of covalent bonds. This enzyme, which the cell normally uses in DNA replication (see Module 10.4), is a “DNA pasting” enzyme that catalyzes the formation of covalent bonds between adjacent nucleotides, joining the strands.

The recombinant plasmid containing the targeted gene is mixed with bacteria. Under the right conditions, a bacterium takes up the plasmid DNA by transformation (see Module 10.22). 9 The recombinant bacterium then reproduces through repeated cell cycles to form a clone of cells, a population of genetically identical cells. In t his clone, each bacterium carries
a copy of gene V. When DNA cloning involves a gene-carrying segment of DNA (as it does here), it is called gen e cloning. In our example, the biologist will eventually grow a cell clone large enough to produce protein V in marketable quantities.
Q Gene cloning can be used for two basic purposes. Copies of the gene itself can be the immediate product, to be used in additional genetic engineering projects. For example, a pest-resistance gene present in one plant species might be cloned and transferred into plants of another species. Other times, the protein product of the cloned gene is harvested