Molecular Focus
Herbicide-resistant soybeans, insect-resistant maize crops, and cows treated with the BST gene to increase milk production are just a few examples of GMOs, the products of genetic engineering. Genetic engineering is the process of taking genes or a gene from one organism and inserting it into another organism’s DNA by artificial means. There are several ways in which this is done. However, in order to understand this, we must first understand what genes are.
Figure 2
Genes are particular segments, or stretches, of the molecule DNA. DNA is a molecule found in the nucleus of an organism’s cells that contains the hereditary genetic information necessary for that organism to function. For simplicity’s sake, DNA can be thought of as a manual containing the instructions for an organism’s growth and development. These instructions are encoded as a sequence of nucleotides. Nucleotides are made up of nucleobases (cytosine, guanine, adenine, thymine), a phosphate group, and a five-carbon sugar. DNA looks much like a spiral staircase, with two strands of the DNA wrapped around each other and connected by the nucleobases through hydrogen bond. The sides of the ladder are composed of the phosphate group and deoxyribose sugar. The rungs of the ladder are made up of two complementary nucleobases, one from each DNA sequence. Figure 2 illustrates how the nucleobases are complementary because they only bond in base pairs: cytosine bonds with guanine and adenine with thymine.
Protein Synthesis
Figure 3
Genes contain information about the cell, including instructions for the synthesis of proteins. The proteins that are produced are responsible for the expression of that gene. For example, certain genes code for the synthesis of proteins that create blonde hair. Two processes are responsible for protein synthesis: transcription and translation; Figure 3 shows both processes as they occur in the cell. During transcription, the messenger molecule, mRNA, copies the DNA strand, carrying the complementary strand out of a cell’s nucleus and to the ribosomes that are found in the cell’s cytoplasm. During translation, the protein tRNA uses the code from mRNA to form an amino acid chain. tRNA binds one amino acid per three bases; these triplets of nucleotides are called codons. Each codon codes for one of the 20 amino acids necessary to synthesize the proteins.
Figure 4
Scientists employ a wide variety of methods when cutting and synthesizing DNA in order to isolate genes. They also have a number of different ways used to insert those genes into future GMOs. Scientists use restriction enzymes to cut DNA into fragments and separate these fragments in order to isolate genes. Through gel electrophoresis, the fragments are run through a gel that separates fragments based on the size of the fragment (See Figure 4). In the technique of Biolistics, DNA is used to coat gold or magnesium tungsten particles, which is then shot into plant cells with gene guns. At the high velocity the particles move, the genes are stripped from them and remain in the cell after the particles collide with the cell wall. In another technique, Agrobacterium tumefaciens, a type of bacteria, is commonly used to transfer genes to plants and fungi. Naturally, Agrobacterium tumefaciens binds to the cells of wounded plants. T-DNA, a small part of Agrobacterium’s plasmids (small circular DNA structures), is transferred to the plant. Genes in the T-DNA region direct the synthesis of plant hormones and amino acid compounds beneficial to the bacteria. Scientists exploit the natural talents of the Agrobacterium by incorporating foreign genes into their plasmids for transfer into other organisms. A more passive technique that scientists use is known as gene silencing, which works by preventing mRNA from being formed in a cell or preventing it from arriving at the ribosomes, which blocks protein synthesis and prevents gene expression. Most genes inserted into animals code for growth hormones. BST, or Bovine Somatotropin, is one such hormone, inserted into cows to increase milk production.