Answer:
DNA is a dynamic and adaptable molecule. As such, the nucleotide sequences found within it are subject to change as the result of a phenomenon called mutation. Depending on how a particular mutation modifies an organism's genetic makeup, it can prove harmless, helpful, or even hurtful. Sometimes, a mutation may even cause dramatic changes in the physiology of an affected organism. Of course, in order to better understand the varying effects of mutations, it is first necessary to understand what mutations are and how they occur.
Where do mutations occur?
Mutations can be grouped into two main categories based on where they occur: somatic mutations and germ-line mutations. Somatic mutations take place in non-reproductive cells. Many kinds of somatic mutations have no obvious effect on an organism, because genetically normal body cells are able to compensate for the mutated cells. Nonetheless, certain other mutations can greatly impact the life and function of an organism. For example, somatic mutations that affect cell division (particularly those that allow cells to divide uncontrollably) are the basis for many forms of cancer.
Germ-line mutations occur in gametes or in cells that eventually produce gametes. In contrast with somatic mutations, germ-line mutations are passed on to an organism's progeny. As a result, future generations of organisms will carry the mutation in all of their cells (both somatic and germ-line).
What kinds of mutations exist?
Mutations aren't just grouped according to where they occur — frequently, they are also categorized by the length of the nucleotide sequences they affect. Changes to short stretches of nucleotides are called gene-level mutations, because these mutations affect the specific genes that provide instructions for various functional molecules, including proteins. Changes in these molecules can have an impact on any number of an organism's physical characteristics. As opposed to gene-level mutations, mutations that alter longer stretches of DNA (ranging from multiple genes up to entire chromosomes) are called chromosomal mutations. These mutations often have serious consequences for affected organisms. Because gene-level mutations are more common than chromosomal mutations, the following sections focus on these smaller alterations to the normal genetic sequence.
Base substitution
Base substitutions are the simplest type of gene-level mutation, and they involve the swapping of one nucleotide for another during DNA replication. For example, during replication, a thymine nucleotide might be inserted in place of a guanine nucleotide. With base substitution mutations, only a single nucleotide within a gene sequence is changed, so only one codon is affected.
Although a base substitution alters only a single codon in a gene, it can still have a significant impact on protein production. In fact, depending on the nature of the codon change, base substitutions can lead to three different subcategories of mutations. The first of these subcategories consists of missense mutations, in which the altered codon leads to insertion of an incorrect amino acid into a protein molecule during translation; the second consists of nonsense mutations, in which the altered codon prematurely terminates synthesis of a protein molecule; and the third consists of silent mutations, in which the altered codon codes for the same amino acid as the unaltered codon.
Insertions and deletions
A schematic shows 29 nucleotides arranged to form a partially double-stranded segment of DNA, with 16 nucleotides in the top strand and 13 nucleotides in the bottom strand. Grey horizontal cylinders represent deoxyribose sugar molecules, and blue, red, green, and orange vertical rectangles represent the chemical identity of each nitrogenous base. An extra nucleotide has been added to the replicating strand because of a misalignment of base pairs.
Figure 2: During an insertion mutation, the replicating strand \"slips\" or forms a wrinkle, which causes the extra nucleotide to be incorporated.