Safe Food_ Bacteria, Biotechnology, and Bioterrorism - Marion Nestle [177]
Science has much to teach us about the biological and physical worlds we inhabit, and its methods and approaches are useful tools for investigating such matters. The basic concepts are not difficult to understand, but the methods—and especially the vocabulary—can be intimidating. Here, I extend the discussion of plant biotechnology given in chapter 5 and offer further details, still nontechnical, about the methods used to introduce new genes into plants, particularly those for synthesis of beta-carotene in Golden Rice. Let’s begin with a brief overview of basic biological principles having to do with DNA and its functions in bacteria and plants.
A (VERY) QUICK REMINDER ABOUT DNA, GENES, AND PROTEINS
DNA (deoxyribonucleic acid) is the principal determinant of the genetic characteristics of most living organisms: humans, animals, plants, bacteria, and many viruses. One of its functions is to specify the structure of proteins. The details of the processes through which DNA reproduces itself and carries out its functions appear immensely complicated—always a good sign that they are incompletely understood. They are also abstract. DNA and proteins are submicroscopic; their actions must be inferred. Furthermore, scientists (like specialists in any field) typically describe molecular actions in a vocabulary impenetrable to the uninitiated.1 Fortunately, we need to use only a few of the most familiar terms: DNA and its subunits (DNA bases, of which there are 4), and protein and its subunits (amino acids, of which there are 20).2
No matter what organism it comes from, DNA is composed of just four subunits—the DNA bases. These differ in size and shape and are arranged on the DNA molecule like beads on a string. The sequence of stringing constitutes a four-letter code that contains the genetic information of the cells that make up body organs and tissues. To summarize the basic details:
• Sequences of DNA bases (DNA segments) arranged in a specified order constitute genes.
• Some gene DNA sequences specify the structure of proteins.
• Other DNA sequences specify the structure of molecules that signal where genes begin and end.
• Gene DNA sequences specify the order in which amino acids link to make specific proteins; a sequence of three DNA bases specifies 1 of the 20 amino acids (this is the genetic code).
• Proteins are composed of various combinations of the 20 different amino acids linked in a specific order defined by the gene DNA sequence.
• Proteins do the work of cells, muscles, and other organs as structural components, signals, or enzymes.
• Enzymes catalyze biochemical reactions in the body.
• The structure of DNA is helical; its two strands are twisted around each other in a double helix.
• Proteins differ from one another in structure; they fold into specific three-dimensional shapes that depend on the sequence of their amino acids (and other components that may be introduced during or after protein synthesis).
• The structure of a protein determines its function.
These biological features operate in the same way in most organisms. Differences among species depend on the specific order of base sequences in their DNA and, therefore, in the sequence of amino acids in their proteins. When scientists extract genes from bacteria, they are taking segments of DNA that contain the same DNA bases that are already in plants—just arranged in a different sequence. The commonality of DNA bases among organisms is the main reason why many scientists are perplexed by public anxieties about genetic engineering; DNA is DNA—its base subunits are the same—no matter where it comes from or where it goes.
MORE ABOUT MAKING RICE GOLDEN: PLASMIDS
As noted in chapter 5, the genetic engineering of beta-carotene into rice represents an extraordinary technical achievement. The “foreign” genes must be identified and reproduced, inserted into the plant’s DNA, and