The study of gene structure and function in bacteria. Genetics itself is concerned with determining the number, location, and character of the genes of an organism. The classical way to investigate genes is to mate two organisms with different genotypes and compare the observable properties (phenotypes) of the parents with those of the progeny. Bacteria do not mate (in the usual way), so there is no way of getting all the chromosomes of two different bacteria into the same cell. However, there are a number of ways in which a part of the chromosome or genome from one bacterium can be inserted into another bacterium so that the outcome can be studied. See Genetics
All organisms have diverged from a common ancestral prokaryote whose precise location in the evolutionary tree is unclear. This has resulted in three primary kingdoms, the Archaebacteria, the Eubacteria, and the Eukaryotae. All bacteria are prokaryotes, that is, the “nucleus” or nucleoid is a single circular chromosome, without a nuclear membrane. Bacteria also lack other membrane-bounded organelles such as mitochondria or chloroplasts, but they all possess a cytoplasmic membrane. Most bacteria have a cell wall that surrounds the cytoplasmic membrane, and some bacteria also contain an outer membrane which encompasses the cell wall. Duplication occurs by a process of binary fission, in which two identical daughter cells arise from a single parent cell. Every cell in a homogeneous population of bacterial cells retains the potential for duplication. Bacteria do not possess the potential for differentiation (other than spore formation) or for forming multicellular organisms. See Archaebacteria, Bacteria, Ribonucleic acid (rna)
One of the most frequently used organisms in the study of bacterial genetics is the rod-shaped bacillus Escherichia coli, whose normal habitat is the colon. Conditions have been found for growing E. coli in the laboratory, and it is by far the best understood of all microorganisms. The single circular chromosome of E. coli contains about 4.5 × 106 base pairs, which is enough to make about 4500 average-size genes (1000 base pairs each). In regions where mapping studies are reasonably complete, the impression is obtained of an efficiently organized genome. Protein coding regions are located adjacent to regulatory regions. There is no evidence for significant stretches of nonfunctional deoxyribonucleic acid (DNA), and there is no evidence for introns [regions that are removed by splicing the messenger RNA (mRNA) before it is translated into protein] in the coding regions. Very little repetitive DNA exists in the E. coli chromosome other than the seven sequence-related rRNA genes that are dispersed at different locations on the chromosome. See Chromosome, Deoxyribonucleic acid (dna), Genetic code
The first step in performing genetic research on bacteria is to select mutants that differ from wild-type cells in one or more genes. Then crosses are made between mutants and wild types, or between two different mutants, to determine dominance-recessive relationships, chromosomal location, and other properties. Various genetic methods are used to select bacterial mutants, antibiotic-resistant cells, cells with specific growth requirements, and so on.
Certain genes that have the function of modulating the expression of other genes are known as regulatory genes. Mutations that affect the action of regulatory proteins are of two types: those that occur in the genes that encode the regulatory proteins, and those that affect the genetic loci where the regulatory protein interacts to modulate the level of gene expression. Some regulatory gene mutations cause overproduction and some cause underproduction of gene products. This is the hallmark of a mutation that influences the functioning of a regulatory protein or regulatory factor-binding site; it affects the quantity but not the quality of other gene products. Furthermore, regulatory gene mutations are frequently pleiotropic, that is, they influence the rate of synthesis of several gene products simultaneously. See Gene action, Protein
Frequently, geneticists want to increase the number or types of mutants that can be obtained as a result of spontaneous mutagenesis. In such instances, they treat a bacterial population with a mutagenic agent to increase the mutation frequency. This is called induced mutagenesis. The simplest techniques of induced mutagenesis involve measured exposure of the bacteria to a mutagenic agent, such as x-rays or chemical mutagenic agents. Such procedures have a general effect on the increase in the mutation rate. More sophisticated procedures involve isolating the gene of interest and making a change in the desired location. This is called site-directed mutagenesis. The goal is usually to determine the effects of a change at a specific gene locus. The gene in question is isolated, modified, and reinserted into the organism. Discrete alterations can be made in a variety of ways on any DNA in cell-free culture, and the effect of such alterations can be subsequently tested in the organism. See Genetic engineering, Mutagens and carcinogens
Bacteria do not mate to form true zygotes, but they are able to exchange genetic information by a variety of processes in which partial zygotes (merozygotes) are formed. The first type of genetic exchange between bacteria to be observed was transformation. Naturally occurring transformation involves the uptake of DNA. This phenomenon is observed only for a limited number of bacterial species and is a relatively difficult technique to use for gene manipulation. In 1946 direct chromosomal exchange by conjugation between E. coli cells was discovered by J. Lederberg and E. Tatum, and in 1951 transduction, the virus-mediated transfer of bacterial genes, was discovered. Both conjugation and transduction provide facile, generally applicable methods for moving part of the bacterial chromosome from one cell to another. The discovery of bacterial transposons (a class of mobile genetic elements commonly found in bacterial populations) in the 1970s has been useful in marking and mobilizing genes of interest. The purely genetic approaches to mapping have been supplemented by the biochemical approaches of hybrid plasmid construction and DNA sequence analysis. See Transformation (bacteria), Transposons
At any given time, only a small percentage of the E. coli genome is being actively transcribed. The remainder of the genome is either silent or being transcribed at a very low rate. When growth conditions change, some active genes are turned off and other, inactive genes are turned on. The cell always retains its totipotency, so that within a short time (seconds to minutes), and given appropriate circumstances, any gene can be fully turned on. The maximal activity for transcription varies from gene to gene. For example, a β-galactosidase gene makes about one copy per minute, and a fully turned-on biotin synthase gene makes about one copy per 10 min. In the maximally repressed state, both of these genes express less than one transcript per 10 min. The level of transcription for any particular gene usually results from a complex series of control elements organized into a hierarchy that coordinates all the metabolic activities of the cell. For example, when the rRNA genes are highly active, so are the genes for ribosomal proteins, and the latter are regulated in such a way that stoichiometric amounts of most of the ribosomal proteins are produced. When glucose is abundant, most genes involved in processing more complex carbon sources are turned off in a process called catabolite repression. If the glucose supply is depleted and lactose is present, the genes involved in lactose breakdown (catabolism) are expressed. In E. coli the production of most RNAs and proteins is regulated exclusively at the transcriptional level, although there are notable exceptions.