encode a protein, influencing its production. Proteins, which do the work of the body, are involved in a variety of processes. Functionally, proteins in the form of enzymes are able to make metabolic events speed up, whereas structural proteins are involved in building body parts. Similar proteins in insects are involved in creating such structures as spider webs and butterfly wings. Proteins are diverse and complex and are found in the foods we eat as well as made by our cells from some 20 amino acids. Proteins serve as signals for changes in cell activity as illustrated by hormones. Proteins are also involved in health and disease as well as in development and aging.
Although the cells in the body carry the full set of genetic information, only a limited amount is expressed at any one time related to the function of the cell. That is to say, although a large variety of proteins could be produced at any one time, there is selectivity as to what is produced relative to internal and external conditions. Further, the location of the genes makes a difference in that cells in the brain produce different proteins from those in the muscles, or liver, or heart.
A gene is turned on (produces the protein) or turned off (does not produce the protein) relative to specific events. Just because a person has a specific gene does not mean that it will necessarily be expressed. The environment in which a person develops and lives plays an important role in gene expression. Even identical twins with the same genotype can display different phenotypes if their environmental conditions differ during their development. For example, if one was to grow up in a high mountain range and the other in a desert below sea level, important physiological differences such as lung capacity and function would be apparent. There are few factors other than blood type in terms of human processes that can be explained totally by genetic factors alone. It is equally true that few human processes can be explained totally by the environment.
DNA
With the discovery of the structure of DNA by Watson and Crick in 1953, specifying the method by which genetic material was copied became possible. Deoxyribonucleic acid (DNA) provides information necessary to produce proteins. Proteins can be viewed as a link between the genotype (complete genetic composition of an organism) and the phenotype (an organism’s observable characteristics). Moving the genotype to the phenotype initially begins in two steps. First, the information in DNA is encoded in ribonucleic acid (RNA). Second, this information in RNA determines the sequence of amino acids, which are the building blocks of proteins. Technically, the DNA synthesis of RNA is called transcription, whereas the step from RNA to protein is called translation. RNA is like DNA except its structure is a single strand, whereas DNA has a double strand. Once encoded, the RNA goes to a part of the cell capable of producing proteins. Proteins are produced by putting together amino acids.
deoxyribonucleic acid (DNA): a molecule that provides information necessary to produce proteins, which are involved in growth and functioning
genotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment
phenotype: an organism’s observable characteristics
ribonucleic acid (RNA): DNA information is carried as RNA, which determines the sequence of amino acids, the building blocks of proteins; it is made up of single strands rather than the dual strands in DNA
To be more specific, DNA represents the chemical building blocks, or nucleotides, that store information. There are only four types of bases for this coding. DNA molecules are composed of two strands that twist together in a spiral manner. The strands consist of a sugar phosphate backbone to which the bases are attached. Each strand consists of four types of nucleotides that are the same except for one component, a nitrogen-containing base. The four bases are adenine, guanine, thymine, and cytosine. These are generally referred to as A, G, T, and C. To give you some sense of size, each full twist of the DNA double helix is 3.4 nanometers (i.e., one billionth of a meter). Said in other terms, if we took the DNA in the 46 chromosomes of a single human cell and stretched it out, it would be around 6 feet long. This measurement gives you some idea of the thinness of DNA.
DNA, which is the information storage molecule, transfers information to RNA, which is the information transfer molecule, to produce a particular protein. Further, change in the rate at which RNA is transcribed controls the rate at which genes produce proteins. The expression rate of different genes in the same genome may vary from 0 to approximately 100,000 proteins per second. Thus, not only do genes produce proteins, but they do so at different rates. The crucial question becomes what causes a gene to turn on or off.
Genome is the name given to the complete set of genes in a given cell. The Human Genome Project was started in 1990 by the United States with the goal of mapping all the genes of the human body. It was an international project that was declared complete in 2003. The estimation at that time was that there are approximately 20,500 genes in a human cell.
How Do Genes Influence Behavior?
In terms of behavior and experience, the production of proteins can be transitory. For example, touching a cat’s whiskers causes changes in gene expression in the cells of the sensory cortex of the brain (Mack & Mack, 1992). This is just a momentary change. Changes can also be long term. Turning on one set of genes may have lasting influence on the ability of other genes to produce specific proteins. For example, when a songbird first hears the specific song of its species, a particular set of genes comes into play, which, once set, determine the song produced by that bird for its entire life. This process has been mapped by a number of researchers (see, for example, Mello, Vicario, & Clayton, 1992; Ribeiro & Mello, 2000). Likewise, raising mice in an enriched environment—that is, one with lots of toys and stimulation—will cause increased gene expression in genes that are associated with learning and memory (Rampon et al., 2000).
When a songbird first hears the specific song of its species, a particular set of genes comes into play, which, once set, determine the song produced by that bird for its entire life.
©iStockphoto.com/Paul Tessier
How do we know which genes are involved? In the Rampon et al. (2000) study, the genes of mice in enriched environments were compared with those of control mice who did not have this experience. Another way to know which genes are involved in a process is to actually change the genes in a particular organism. So-called “knockout” mice are genetically engineered to have particular genes turned off by breeding them in specific ways. Research shows that simple genetic changes made experimentally in animals can result in protein changes that influence social behavior. Some examples of such behaviors are increased fear and anxiety, increased grooming, hyperactivity, and even increased alcohol consumption when stressed.
Epigenetic Processes
One basic idea from Mendelian genetics was that genes are not changed by experience. What is passed on, except in the case of damage to the gene, is exactly the same gene that was received by the organism from its parents. This came to be called the central dogma of molecular biology as described by Francis Crick. He basically stated that information flow was one-directional. That is, it went from the gene to the protein. What came to be called reverse translation was seen as impossible. Thus, the gene could not be influenced or changed by changes in proteins. This was the basic view from the 1950s until very recently.
As researchers became interested in how genes turn on and off and what factors influence this, it became apparent that the story was more complicated. It was discovered that the processes that determine which genes turn on and off could themselves be passed on to the next generation. Of course, which factors turn the genes on and off are largely influenced by the environment of the organism. Thus, although DNA itself could not be influenced by the environment, it was possible for the environment to influence future generations through its changes to those processes that turn genes on and off.
epigenetic