Mutation and Recombination Mutation is a heritable change in DNA sequence that can lead to a change in phenotype. By definition, a mutant differs from its parental strain in genotype, the nucleotide sequence of the genome. Selectable mutations are those that give the mutant a growth advantage under certain environmental conditions and are especially useful in genetic research. If selection is not possible, mutants must be identified by screening.
Replica plating Although screening is always more tedious than selection, methods are available for screening large numbers of colonies in certain types of mutations. For instance, nutritionally defective mutants can be detected by the technique of replica plating (Figure 10.2).
Molecular Basis of Mutation Mutations, which can be either spontaneous or induced, arise because of changes in the base sequence of the nucleic acid of an organism's genome. A point mutation, which results from a change in a single base pair, can lead to a single amino acid change in a polypeptide or to no change at all, depending on the particular codon involved (Figure 10.3).
Molecular Basis of Mutation In a nonsense mutation, the codon becomes a stop codon and an incomplete polypeptide is made. In a missense mutation, the sequence of amino acids in the ensuing polypeptide is changed, resulting in an inactive protein or one with reduced activity. Deletions and insertions cause more dramatic changes in the DNA, including frameshift mutations, and often result in complete loss of gene function
Mutagenesis Mutagens are chemical, physical, or biological agents that increase the mutation rate. Mutagens can alter DNA in many different ways, but such alterations are not mutations unless they can be inherited. There are several classes of chemical mutagens, one being the nucleotide base analogs.
Genetic Recombination Homologous recombination arises when closely related DNA sequences from two distinct genetic elements are exchanged and/or combined into the different sequences (Figure 10.9). Recombination is an important evolutionary process, and cells have specific mechanisms for ensuring that recombination takes place.
Genetic Exchange in Prokaryotes There are several modes of genetic exchange in prokaryotes. Transformation Transduction Conjugation Transposition
Transformation The discovery (by Fred Griffith) of transformation was one of the seminal events in biology because it led to experiments demonstrating that DNA is the genetic material (Figure 10.13).
Transformation Certain prokaryotes exhibit natural competence, a state in which cells are able to take up free DNA released by other bacteria. Incorporation of donor DNA into a recipient cell requires the activity of single-stranded binding protein, RecA protein, and several other enzymes. Only competent cells are transformable.
Transduction Transduction involves the transfer of host genes from one bacterium to another by bacterial viruses. In generalized transduction (Figure 10.15), defective virus particles incorporate fragments of the cell's chromosomal DNA randomly, but the efficiency is low.
Transduction In specialized transduction (Figure 10.16), the DNA of a temperate virus excises incorrectly and takes adjacent host genes along with it; transducing efficiency in this case may be very high.
Plasmids: general principles Plasmids are small circular or linear DNA molecules that carry any of a variety of unessential genes. Although a cell can contain more than one plasmid, they cannot be closely related genetically. Figure 10.18 shows a genetic map of the F (fertility) plasmid, a very well characterized plasmid of Escherichia coli.
Types of Plasmids and Their Biological Significance The genetic information that plasmids carry is not essential for cell function under all conditions but may confer a selective growth advantage under certain conditions. Examples include antibiotic resistance (Figure 10.20), enzymes for degradation of unusual organic compounds, and special metabolic pathways. Virulence factors of many pathogenic bacteria are often plasmid-encoded.