A mutation is simply any change in the DNA sequence. As you can imagine, more than one type of change is possible. At each position in the DNA sequence, there are one of four possible nucleotides (A, C, T, and G). The simplest change to imagine is a change where the original nucleotide in a particular position is replaced by one of the other three possibilities. An A is changed to either G, T, or C, for example. Mutations of this type involving just a single point in the DNA sequence are called point mutations. More complicated changes are also possible. Whole sections of DNA can be removed, moved from one place to the next, or duplicated. The following sections explain these types of mutations in more detail (head to Chapter 15 for information on the significance of gene duplication).
DNA is a long string of four different nucleotides that thread off in groups of three. The different three-base sequences instruct the cell to assemble different amino acids into a protein. Some three-base sequences instruct the cell where to start along the DNA sequence and where to stop. Not all sections of an organism's DNA are used to code for proteins, but for the purposes of this discussion the important sections are those that do code for proteins. (Refer to Chapter 3 for more detail about how the sequence of the DNA is used to code for the specific amino acids needed to make a given protein and Chapter 15 if you want to know more about non-coding DNA.)
In a point mutation, a single nucleotide in the DNA is replaced by some other nucleotide, resulting in a particular three-letter sequence of DNA that's different. Because of the redundancy of the genetic code, point mutations don't always result in a change in the amino acid and, therefore, don't affect the organism's phenotype, making this particular mutation selectively neutral. There is no change in fitness between the original type and the mutant type.
Sometimes, though, a point mutation results in a different amino acid being used in the production of a protein. In this type of mutation, the protein may have a different structure and may behave differently. If the organism's phe-notype is changed, this type of mutation may have fitness consequences. Or it may not — there are many examples where an amino acid change results in the production of a slightly different protein but one that works exactly as well. Changes of this sort may have fitness consequences, or they may be selectively neutral.
At other times, a point mutation could replace a three-letter sequence coding for an amino acid with a sequence that starts or stops protein production. Stopping production of a protein when only part of the amino acid sequence has been assembled is likely to result in a protein with a very different structure and is likely to have a negative affect an organism's phenotype. More often than not, making just half of a protein will be less advantageous than making the whole thing.
Larger changes also can occur in an organism's DNA. Sections of DNA can be lost or inserted into other sections. These sorts of changes are referred to as deletions and insertions, and although they don't always have a large effect, it's easy to see how they can.
Deleting a section of an organism's instructions set is not likely to be advantageous. Whatever information is eliminated may prove to be extremely important. Obviously the larger the deletion, the larger the potential problem, but even small deletions can cause major effect.
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