DEPARTMENT OF BIOLOGY
The Basics of Evolution
DNA, Genes, Mutations, and the Characteristics of Organisms
Genetic inheritance depends upon genes, which are segments of DNA, the fundamental chemical of chromosomes. DNA carries the "information" that determines how organisms grow and develop, and that determines many of their characteristics. It does not dictate all of an individual's characteristics, because many aspects of most species are shaped by the peculiarities of the environment in which they live.
Every individual of every species begins life as a single cell. In the case of humans, that single cell is the fertilized egg, which contains one set of chromosomes contributed by the mother, and one set of chromosomes contributed by the father. As the fertilized egg divides and the cells differentiate to become all of the different cell types of a human, the DNA molecules of the fertilized egg must be duplicated over and over, so that each of our trillions of cells contains an exact copy of the DNA contained in the fertilized egg. DNA replication must be tremendously accurate to ensure that every cell contains the information that it needs.
Although DNA replication is tremendously accurate, it is not 100% accurate. Occasionally, mistakes are made. If mistakes occur in the DNA of genes, then those genes are altered.
DNA is a chemical. Therefore, it follows the laws of chemistry. Consequently, DNA molecules can be damaged--chemically altered--by radiation, chemicals, cosmic rays, oxygen radicals, etc. Although DNA damage can often be repaired, it is not always repaired, and repair may be imperfect. Damage and/or imperfect repair can also alter genes.
These changes to DNA are mutations. Because they occur by normal, chemical mechanisms, it is impossible to prevent them from occurring. It is also impossible to cause them to occur in specific genes . They occur at random.
"Random mutation" does not mean "un-caused mutation." It means that the mechanisms that cause mutations cannot choose which part of a DNA molecule to affect. This is illustrated in the figure on the right, which shows an oxygen radical (a common mutagen) inside the nucleus of a cell. It is surrounded by DNA, from many different genes, but all of the DNA is chemically the same. The oxygen radical has an equal likelihood of reacting with any nucleotide in any DNA molecule near it. The probability that a base will be modified, and thus cause a mutation, is statistically random.
It is tremendously important to recognize that mutations are changes in DNA. A person cannot mutate. A turtle cannot mutate (and turn into a ninja). Why not? Because a chemical mistake in the DNA of one cell affects only that cell, and is not spread throughout the body to all of the cells. A mutation in the DNA of an adult human will not change that person [unless the mutation occurs in a gene that controls cell division, in which case the mutated cell may begin to divide uncontrollably, and become a cancer].
To change the characteristics of a whole organism, a mutation must occur in a cell in the gonads, destined to become an egg or sperm, and become incorporated into a fertilized egg, and develop into a complete individual. Only then can a new mutation, a new DNA change, become a part of every cell in an individual organism. Only then can a new mutation change the characteristics of the organism. In other words, if an individual is exposed to mutation-causing chemicals or radiation, that individual will not mutate . However, that individual's offspring may carry mutations. Once the offspring reproduce, and pass DNA changes to the next generation, then the mutations become part of the genetic diversity of that species.
If that mutation gives an individual an advantage, so that the individual is more likely to produce healthy offspring, then the numbers of individuals with that particular genetic variation will increase with each generation that passes. If a mutation gives an individual a disadvantage, so that the individual reproduces less successfully (or dies), then that particular genetic variation will be lost from the gene pool of that species.
last updated:Jan. 15, 2009