The genetic material of a typical bacterium, Escherichia coli, consists of a single circular DNA molecule with a molecular weight of about 2 × 109 and is composed of approximately 5 × 106 base pairs. This amount of genetic information can code for about 2000 proteins with an average molecular weight of 50,000. The DNA of the smallest free-living organism, the wall-less bacterium Mycoplasma, has a molecular weight of 5 × 108. The DNA of human cells contains about 3 × 109 base pairs and encodes about 100,000 proteins.
Note that bacteria are haploid; in other words, they have a single chromosome and therefore a single copy of each gene. Eukaryotic cells (such as human cells) are diploid, which means they have a pair of each chromosome and therefore have two copies of each gene. In diploid cells, one copy of a gene (allele) may be expressed as a protein (i.e., be dominant), whereas another allele may not be expressed (i.e., be recessive). In haploid cells, any gene that has mutated—and therefore is not expressed—results in a cell that has lost that trait.
A mutation is a change in the base sequence of DNA that usually results in insertion of a different amino acid into a protein and the appearance of an altered phenotype. Mutations result from three types of molecular changes:
The first type is the base substitution. This occurs when one base is inserted in place of another. It takes place at the time of DNA replication, either because the DNA polymerase makes an error or because a mutagen alters the hydrogen bonding of the base being used as a template in such a manner that the wrong base is inserted. When the base substitution results in a codon that simply causes a different amino acid to be inserted, the mutation is called a missense mutation; when the base substitution generates a termination codon that stops protein synthesis prematurely, the mutation is called a nonsense mutation. Nonsense mutations almost always destroy protein function.
The second type of mutation is the frameshift mutation. This occurs when one or more base pairs are added or deleted, which shifts the reading frame on the ribosome and results in incorporation of the wrong amino acids “downstream” from the mutation and in the production of an inactive protein.
The third type of mutation occurs when transposons or insertion sequences are integrated into the DNA. These newly inserted pieces of DNA can cause profound changes in the genes into which they insert and in adjacent genes.
Mutations can be caused by chemicals, radiation, or viruses. Chemicals act in several different ways.
Some, such as nitrous acid and alkylating agents, alter the existing base so that it forms a hydrogen bond preferentially with the wrong base (e.g., adenine would no longer pair ...