DEPARTMENT OF BIOLOGY
The Basics of Evolution
Genes, Proteins, and Cellular Micromachines
To understand how mutations can change the characteristics of organisms, it is necessary to understand how genes work. In general , genes are segments of DNA that carry the information for proteins. Genes do no more than this. Inside cells, genes just sit there, waiting for their information to be used. "Using" the information means following the chemical processes by which cells produce proteins.
We will ignore, for now, the process that cells use to produce proteins. It is enough to say that the information in each gene dictates the production of a single type of protein. Human DNA is estimated to contain information for around 30,000 different proteins. Each of these proteins is a specific kind of cellular "micromachine" that has a specific function.
Proteins are produced by assembling "building blocks" called amino acids. There are 20 different amino acids that are used in cells. From these, an infinite number of different proteins can be built, depending on how many of these building blocks are strung together, and the order in which they are assembled. The differences in functions of proteins depend on the differences in amino acid sequence of the proteins.
A mutation in a gene--a change in the DNA--has the likely consequence of changing the amino acid sequence of the protein whose information that gene carries. This, in turn, can change the way that the protein micromachine works. To see the types of effects that this can have on the characteristics of an entire organism, it may be best to discuss some specific examples:
1. Eye color
Eye color in humans is determined by several different genes that produce different kinds of proteins. One gene, called EYCL3 , carries the code for an enzyme (a protein that catalyzes a chemical reaction) that produces a brown pigment. This gene is used, or "turned on" in the cells of the iris. An individual who inherits a functional gene for this enzyme from either parent will be able to produce the brown pigment in her irises, and will have brown eyes. Mutations in this gene can cause the protein not to work. An individual who inherits non-functional genes for this enzyme from both parents will be unable to produce the brown pigment. As a result, the individual will have green or blue eyes. (The green pigment also depends on a gene that produces an enzyme; the blue color is a result of the way that iris cells reflect light, and is not based upon a blue pigment).
This particular gene need not be either functional or non-functional. As with any gene, there are many, many different variations possible. One variation may produce a protein that is a very active enzyme; individuals with this version of the gene will have very dark brown eyes. Another variation produces an enzyme that works, but not very well. This enzyme cannot produce as much brown pigment. Individuals with this version of the gene will have light brown eyes.
The gene is the set of instructions for the enzyme; the enzyme produces the pigment. Variations in the gene sequence, resulting from mutations, create enzymes with varying degrees of activity. This shows up in the human population as variation in the intensity of brown color in the irises of our eyes.
2. Hair color and skin color
The story for hair color and skin color is similar to that for eye color. The genes that determine the color carry the information for enzymes that produce pigments. If we produce the enzyme, we make the pigment (brown hair, or brown skin). If we do not produce the enzyme, we do not make the pigment (blonde hair, or light skin). Different variations of the genes result in different variations in the individual's characteristics.
3. Alcohol tolerance
Some individuals cannot tolerate alcohol. This is the result of carrying a particular version of the gene, ALDH2 . It is thought that the version of the gene that produces this characteristic first arose in Asia, since the inability to tolerate alcohol is most common in Asian populations.
Alcohol is metabolized by two enzymes. The first (ADH) converts alcohol to acetaldehyde. The second (ALDH) converts acetaldehyde to acetic acid. We can then use the acetic acid in our energy-metabolism pathways.
Although alcohol itself interacts with our brain cells to make us feel giddy (among other things), acetaldehyde is toxic, and makes us feel sick (or worse). Therefore, the ability to tolerate alcohol depends, in part, on how rapidly we can convert acetaldehyde to acetic acid. The version of the ALDH2 gene that results in inability to tolerate alcohol produces a protein that prevents acetaldehyde conversion. Individuals with this genetic variation metabolize alcohol to acetaldehyde, but are unable to get rid of the acetaldehyde. As a result, they become sick very quickly, and rapidly learn to avoid alcohol. Although this may sound unfortunate, it turns out that this particular genetic characteristic provides virtually 100% protection against becoming an alcoholic.
last updated:Jan. 15, 2009