BIOLOGY Volume 3 - A Guide to General Biology - 2004

25. APPLIED GENETICS

In the second half of the 20th century, biology entered its "Golden Age." In the period between the Discovery of the Introduction/20.html">DNA Structure in 1953 and the relatively recent ability to decipher the human Genetic Code, a powerful new branch of science—biotechnology—emerged at the crossroads of genetics and molecular biology. Its significance is underscored by the fact that in the USA, biotechnology accounts for nearly half of all funding allocated to academic research. Biotechnology finds Applications in industry, medicine, agriculture, and many other fields. While its achievements can be directed toward the good of humanity, they also carry the potential to bring untold harm. The physics community faced the exact same dilemma in the first half of the 20th century when the discovery of atomic structure made it possible to use nuclear energy for both peaceful and destructive purposes. Francis Crick and Maurice Wilkins, who shared the Nobel Prize with James Watson for elucidating the DNA structure, were physicists and had worked on weapons development before turning to biology. This Background made these scientists deeply mindful of the ethical dimensions of their research. Consequently, when doubts arose in the late 1970s regarding the ethics and safety of Genetic Engineering, Crick and Wilkins temporarily halted their work. As future biologists, you too will one day bear responsibility for decisions made during discussions on new and controversial issues that are bound to arise as our understanding of Molecular Genetics deepens. The more informed we are when forming our own viewpoints, and the more people willing to discuss these issues, the greater the likelihood that our decisions will be the right ones.

Genetic Engineering

The first part of this chapter focuses on genetic engineering, the most powerful tool in the arsenal of applied genetics and biotechnology. It allows scientists to study and modify the genetic instructions encoded in the Chromosomes of plants and animals. Humanity has gained The ability to modify other organisms for our own benefit and (potentially) to treat Hereditary diseases (Gene Therapy). The major milestones in The Development of genetic engineering are presented in Table 25.1.

Class="center">Table 25.1. Major Milestones in the development of genetic engineering. (From The Encyclopaedia of Molecular Biology, ed. J. Kendrew, 1994, Blackwell Science.)

25.1. Bacterial Genetic Engineering

The fundamental Methods of Genetic Engineering were developed in the early 1970s. The core principle of these methods involves introducing a new gene into an organism. Such a gene can be synthesized de novo or transferred from another organism. If a gene encoding a specific protein is inserted into a bacterial genome, the bacterial cell transforms into a living factory for producing that protein. As an example, let us examine The transfer of genes encoding human insulin, human Growth Hormone, and bovine somatotropin (BST) into bacterial Cells (Sections 25.2.1–25.2.3).

25.1.1. Overview

Obtaining a copy of any gene is presently a quite straightforward task. In some cases, a single starting copy is all that is required. Generating multiple identical copies is known as cloning. Plasmids or Bacteriophages are traditionally used as cloning vectors (i.e., vehicles to transport the DNA that needs to be cloned). Plasmids are small, circular DNA molecules found in certain bacteria. They are separate from the main (chromosomal) DNA and can replicate independently of it (Section 2.3.1). Bacteriophages (or phages for short) are Viruses that can inject their DNA into a bacterial cell, where this DNA replicates (Fig. 2.19). The DNA fragments to be cloned are joined to either plasmid or phage DNA. The resulting "construct," consisting of DNA fragments from different organisms, is called recombinant DNA. When such DNA is introduced into a bacterial cell, the number of recombinant DNA copies increases with each Cell Division cycle. A foreign gene embedded in the bacterial genome can be utilized to produce useful Proteins in industrial quantities, such as human insulin, which is not normally synthesized by bacterial cells.

The cloning process is schematically illustrated in Fig. 25.1 and described in more detail below. Introducing new genes into PLANT AND ANIMAL embryos to create so-called transgenic organisms (organisms capable of passing these genes on to their offspring) is a more challenging task. This topic will be discussed in Sections 25.3–25.5.

Bacterial genetic engineering can be divided into five stages.

Stage 1. Isolating a copy of the target gene from all other GENES OF THE organism.

Stage 2. Inserting the target gene into a vector.

Stage 3. Using the vector to introduce the target gene into a recipient cell.

Stage 4. Selecting cells that have successfully taken up the foreign (donor) DNA.

Stage 5. Cloning the gene.

The sequence of steps outlined above is the most straightforward. However, in some cases, the order of Procedures may vary; for instance, an entire group (or library) of genes can be inserted into a vector and cloned first, followed by the Isolation of the specific gene of interest.