Methylation: From Genome to Phenotype
Have you ever played a flight simulator video game? The game may have offered various realism settings that allow gamers to choose just how "real" their gaming experience will be. Often, you can switch midair collisions on and off, or decide whether you can run out of ammo or gas. The default settings may fall somewhere between pure simulator and arcade shoot-'em-up, but the game has the potential to be more realistic, depending on whether you flip on the appropriate options.
As it turns out, our genes work in a very similar way. If our accumulated genetic material (or genome) serves as our program, our gaming experience is our phenotype, an organism's observable characteristics. A host of factors, in turn, causes the epigenetic processes that flip different genes on and off.
Scientists first coined the term "epigenetic" (which literally means "above the genome") in the 1940s as a way of classifying changes that occurred between genome and phenotype. For instance, why would only one identical twin develop cancer and not both? In a quest to understand what was happening, scientists looked more closely at the relationship between DNA and cellular development.
DNA resides inside the nucleus of a cell, a master program in the center of every minute piece that makes us who we are. Enzymes attach carbon and hydrogen bundles (CH3) called methyl groups to the DNA, often near the beginning of a gene -- the same place where proteins attach to activate the gene. If the protein can't attach due to a blocking methyl group, then the gene usually remains off. Scientists call this particular epigenetic process methylation. The arrangement of these bundles can change drastically in the course of a lifetime, but also can set permanently during embryo development. It all depends on the various factors that can affect the distribution of methyl groups.
While epigenetic scientists have devoted most of their research to methylation, they've identified many different types of epigenetic processes. Chromatin modification figures heavily among these processes. Inside the nucleus, DNA coils around bundles of histone proteins to form chromatin, which in turn forms chromosomes. Alter the structure of the chromatin and you alter gene expression. Various chemical groups achieve this end by attaching to the histones.
How does all this influence the nature versus nurture debate? Find out on the next page.