Cambridge International A Level Biology Revision Guide

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Cambridge International A Level Biology 462 Chapter 19: Genetic technology Learning outcomes You should be able to: ■■ explain the principles of genetic technology ■■ ■■ ■■ describe the tools and techniques available to the genetic engineer describe the uses of the polymerase chain reaction (PCR) and gel electrophoresis explain the use of microarrays ■■ ■■ ■■ outline the use of bioinformatics in sequencing genomes describe examples of the uses of genetic technology in medicine describe some uses of genetically modified organisms in agriculture
Chapter 19: Genetic technology Crispr Until recently, genetic technology has had to rely on inaccurate methods of editing DNA. For example, a modified virus can be used to insert DNA into the human genome, but we have had no control about just where the DNA is inserted. It might be inserted in the middle of another gene, with unpredictable consequences. A technique called Crispr is changing this. Crispr has been developed from a mechanism used by some bacteria to defend themselves against viruses. It was first discovered in 1987, but its details were worked out only in 2012. Since then, it has been used successfully to edit DNA in a range of organisms, including human cells. A Crispr-associated (CAS) enzyme cuts DNA at a point that is determined by a short strand of RNA. Since the RNA can be made ‘to order’ to match any unique sequence of DNA, the genetic engineer can dictate precisely where a cut is made in human DNA to remove a ‘faulty’ allele or to insert a therapeutic allele as in gene therapy (Figure 19.1). The genetic engineer now has a tool kit that can ʻcut and pasteʼ genes with much greater precision. Figure 19.1 Crispr makes it possible to insert DNA at a precisely determined point in a chromosome. The structure of DNA, and the way in which it codes for protein synthesis, was worked out during the 1950s and 1960s. Since then, this knowledge has developed so much that we can change the DNA in a cell, and thereby change the proteins which that cell synthesises. Not only that, but we can sequence the nucleotides in DNA and compare nucleotide sequences in different organisms. It is also possible to carry out genetic tests to see if people are carriers of genetic diseases and, in a few cases, use gene therapy to treat those who have these diseases. Gene technology has brought huge benefits to many people, but may have consequences that we cannot foresee. These technologies raise social, economic and ethical issues, that we must confront. Genetic engineering The aim of genetic engineering is to remove a gene (or genes) from one organism and transfer it into another so that the gene is expressed in its new host. The DNA that has been altered by this process and which now contains lengths of nucleotides from two different organisms is called recombinant DNA (rDNA). The organism which now expresses the new gene or genes is known as a transgenic organism or a genetically modified organism (GMO). Recombinant DNA is DNA made by joining pieces from two or more different sources. Genetic engineering provides a way of overcoming barriers to gene transfer between species. Indeed the genes are often taken from an organism in a different kingdom, such as a bacterial gene inserted into a plant or a human gene inserted into a bacterium. Unlike selective breeding, where whole sets of genes are involved, genetic engineering often results in the transfer of a single gene. We will look first at the general principles involved in genetic engineering and then at some of the techniques in more detail. An overview of gene transfer There are many different ways in which a GMO may be produced, but these steps are essential. 1 The gene that is required is identified. It may be cut from a chromosome, made from mRNA by reverse transcription or synthesised from nucleotides. 2 Multiple copies of the gene are made using the technique known as the polymerase chain reaction (PCR). 3 The gene is inserted into a vector which delivers the gene to the cells of the organism. Examples of vectors are plasmids, viruses and liposomes. 4 The vector takes the gene into the cells. 5 The cells that have the new gene are identified and cloned. 463
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