DNA doesn’t always stay the same. Often, there are changes that happen to the DNA inside of a cell because of asbestos, cigarette smoking, ultraviolet radiation, or just random chance. These changes to DNA are called mutations. Some mutations in DNA are harmless and cause no problems for the organism or its offspring. Many mutations are harmful and can cause cancers in the organism or birth defects in offspring. Even other mutations cause the death of the cell because it can’t survive any more.
There are three main types of mutations: substitutions, insertions and deletions. Substitutions are mutations where one base is substituted for another, such as G for A. These can often be harmless because the protein that the gene ends up producing can be exactly the same.
If a gene suffers from an insertion mutation, then the entire gene
can be affected or even destroyed. An insertion is when one or more bases are inserted into the gene and it shifts all of the codons down by one or more bases.
Lastly, a deletion mutation is when one or more bases are removed from the gene. This again can destroy the entire gene because it can shift all of the codons up by one or more bases.
When any of these mutations happen in a body cell, they only affect the organism itself. However, when these mutations happen in a sex cell, they can affect the offspring. This is one of the key concepts behind natural selection – yes, back to evolution! See, if it weren’t for mutations, there would be no new genes, and all life would look just like the first, simple one-celled bacteria.
Mutations are the source of new genes: it’s thought that all humans started off having brown eyes. A mutation in the gene for eye color caused some humans to have blue eyes. In the bright sun of Africa, it made no sense to have blue eyes, which are more sensitive to light. But when humans immigrated into Europe, which receives less direct sunlight, individuals with blue eyes were more fit and survived to reproduce more than the brown-eyed individuals. In fact, the emigration from Africa would have been impossible without mutations to the genes for skin color, hair type, digestion of different foods, and more!
However, the only way that these mutations were passed on from generation to generation is that the initial mutation happened in either a sperm or egg cell. If the gene for eye color had changed in a body cell, that only would have affected the individual – not its offspring!
Even though most mutations result in offspring that don’t survive to reproduce, the “good” mutations more than make up for the “bad” ones. These mutations that take hold in a population cause the genes of the population to change. To continue our example, when Africans first immigrated to Europe, almost nobody had blue eyes, and these individuals were limited to the southernmost areas of the continent. However, as time went on and the mutation for blue eyes spread through the population, the percentage went up; in some areas in northern Europe, 100% of the population had blue eyes. This change over time in the percentage of a particular gene in a population is called genetic drift. As you can see, peoples’ genes “drifted” from brown to blue eyes over time.