ERRORS IN DNA REPLICATION

In life, nothing is always perfect. That is true about DNA replication, too. The enzymes responsible for directing the correct pairing of nucleotides during DNA replication occasionally make mistakes. A nitrogenous base may be left out. Or the wrong base may be matched up. Sometimes an extra one is added. These mistakes produce errors in the linear sequence in one strand of the DNA molecule. Such an error is called a mutation.

From what we know about the replication process, once an error occurs in a DNA strand, it may be copied again and again. The mutation in the genetic material in one cell can easily be passed on to future cells. Are these mutations good or bad? It seems a strange question to ask, because we assume that mistakes are always bad. However, what is obvious is not always true. A mutation is simply a change. Many changes in the genetic material are harmful. In fact, many of these changes make it impossible for the future cells, or even the entire organism, to continue living. Other mutations cause the organism to change in an unnoticeable fashion. Rather than harming the organism, the mutation seems to produce no effect. Sometimes the mutation gives the organism a sudden new advantage that other similar organisms lack.

Let’s consider a simple example. Imagine that all grasshoppers in a green field were brown in color. Birds could easily see the grasshoppers and eat them. Then a mutation occurred in the DNA of one grasshopper. When that grasshopper reproduced—passing on its genetic mutation— the offspring with the mutation were green, not brown. Is this a good mutation or a bad one? Being a green grasshopper in a green field is good if it makes it harder for birds to see and eat you. This color change (mutation) to green would provide a survival advantage over the more easily seen brown grasshoppers. Natural selection would make it more likely that the green grasshoppers would survive. The species would evolve in terms of body color.

Not only can mutations in DNA be good, but they are actually an important source for the genetic variations necessary for natural selection to occur. Much of the evolution of life on Earth has depended on the chance occurrence of these mutations.