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 inheritance: a form of inheritance by which factors largely influenced by the environment of the organism that turn the genes on and off can be passed on to the next generation without influencing DNA itself
This possibility of another form of inheritance came to be called epigenetic inheritance (Hallgrímsson & Hall, 2011; Nestler, 2011). Instead of actually changing the gene itself, epigenetic modifications mark a gene. This alters how it is turned on and off. Briefly, DNA is wrapped around clusters of proteins called histones. These are further bundled into structures called chromosomes. Being tightly packed keeps genes in an inactive state by preventing access to processes that turn genes on. When action is needed, a section of DNA unfurls and the gene turns on. Whether a segment is relaxed and able to be activated or condensed resulting in no action is influenced by epigenetic marks or tags (see Figure 2.26). As a tag, histone acetylation tends to promote gene activity and is called a writer. Histone methylation and DNA methylation tend to inhibit it and are called erasers.
In a classic study, rat pups raised by actively nurturing mothers versus more passive mothers differed in epigenetic factors.
Photo by Seweryn Olkowicz, Wikipedia
epigenetic marks or tags: factors that influence whether a gene segment is relaxed and able to be activated, or condensed, and thereby inhibited
The environment can influence these writer and eraser tags. Tags help an organism respond to a changing environment. Some tags last a short time, whereas others can last a lifetime. In a now classic study, researchers observed that some rat mothers displayed high levels of nurturing behavior, licking and grooming their pups, while others were less diligent (Weaver et al., 2004). Behaviorally, the offspring of the more active mothers were less anxious and produced less stress hormone when disturbed than pups cared for by more passive mothers. Further, the females raised by nurturing mothers became nurturing mothers themselves.
The intriguing part of this study was that the offspring of the rat mothers who showed more licking and grooming differed in epigenetic factors. Pups raised by passive mothers showed more DNA methylation than aggressively groomed pups in the regulatory sequences of a gene encoding the glucocorticoid receptor, which is a protein present in most cells in the body that mediates an animal’s response to the stress hormone cortisol. This excessive methylation was detected in the hippocampus, a brain region involved in learning and memory, and this causes nerve cells to make less of the receptor. Activation of the glucocorticoid receptor in the hippocampus actually signals the body to slow production of cortisol. The epigenetic reduction in receptor number exacerbated the stress response in the animals. This made the animals more anxious and fearful. Further, these traits persisted throughout their lifetime. Overall, attentive mothers cause the methyl marks to be removed. Inattentive mothers, on the other hand, cause methyl marks to be added. Thus, rats inherit certain behaviors based on experience. The genes had not changed, but the tags had.
Figure 2.26 Epigenetic Changes Alter Gene Activity
Source: Nestler (2011).
At this point, a variety of studies have shown other examples of epigenetic mechanisms at work. For example, the diet of a mouse mother before conception can influence the hair color of her infants and even her infants’ infants (e.g., Cropley, Suter, Beckman, & Martin,
