In Depth Tutorials and Information Complementation Tests Molecular Biology The gene is best defined as an integral unit of genetic function, in the sense that none of its parts functions normally in isolation from the others. They should not, in other words, show complementation. On the other hand, two genomes that have mutations in different genes should complement each other to produce a wild-type phenotype, provided that each mutant allele is recessive to its wild-type counterpart.
Holmes and Michael G. Replication of the genome is essential for inheritance of genetically determined traits. Genome Organization The bacterial chromosome is a circular molecule of DNA that functions as a self-replicating genetic element replicon.
Extrachromosomal genetic elements such as plasmids and bacteriophages are nonessential replicons which often determine resistance to antimicrobial agents, production of virulence factors, or other functions. The chromosome replicates semiconservatively; each DNA strand serves as template for synthesis of its complementary strand.
Mutation and Selection The complete set of genetic determinants of an organism constitutes its genotype, and the observable characteristics constitute its phenotype. Mutations are heritable changes in genotype that can occur spontaneously or be induced by chemical or physical treatments.
Organisms selected as reference strains are called wild type, and their progeny with mutations are called mutants. Selective media distinguish between wild type and mutant strains based on growth; differential media distinguish between them based on other phenotypic properties.
Exchange of Genetic Information Genetic exchanges among bacteria occur by several mechanisms. In transformation, the recipient bacterium takes up extracellular donor DNA. In transduction, donor DNA packaged in a bacteriophage infects the recipient bacterium. In conjugation, the donor bacterium transfers DNA to the recipient by mating.
Recombination is the rearrangement of donor and recipient genomes to form new, hybrid genomes. Transposons are mobile DNA segments that move from place to place within or between genomes. Recombinant DNA and Gene Cloning Gene cloning is the incorporation of a foreign gene into a vector to produce a recombinant DNA molecule that replicates and expresses the foreign gene in a recipient cell.
Cloned genes are detected by the phenotypes they determine or by specific nucleotide sequences that they contain. Recombinant DNA and gene cloning are essential tools for research in molecular microbiology and medicine.
They have many medical applications, including development of new vaccines, biologics, diagnostic tests, and therapeutic methods. Regulation of Gene Expression Expression of genes in microbes is often regulated by intracellular or environmental conditions. Regulation can affect any step in gene expression, including transcription initiation or termination, translation, or activity of gene products.
An operon is a set of genes that is transcribed as a single unit and expressed coordinately. Specific regulation induces or represses a particular gene or operon. Global regulation affects a set of operons, which constitute a regulon. All operons in the regulon are coordinately controlled by the same regulatory mechanism.
The two essential functions of genetic material are replication and expression. Genetic material must replicate accurately so that progeny inherit all of the specific genetic determinants the genotype of the parental organism.
Expression of specific genetic material under a particular set of growth conditions determines the observable characteristics phenotype of the organism.
Bacteria have few structural or developmental features that can be observed easily, but they have a vast array of biochemical capabilities and patterns of susceptibility to antimicrobial agents or bacteriophages.
These latter characteristics are often selected as the inherited traits to be analyzed in studies of bacterial genetics. Nucleic Acid Structure Nucleic acids are large polymers consisting of repeating nucleotide units Fig.
Each nucleotide contains one phosphate group, one pentose or deoxypentose sugar, and one purine or pyrimidine base. In RNA, uracil U replaces thymine. Chemically modified purine and pyrimidine bases are found in some bacteria and bacteriophages.
These asymmetric phosphodiester linkages define the polarity of the polynucleotide chain. Double-stranded DNA is helical, and the two strands in the helix are antiparallel. The double helix is stabilized by hydrogen bonds between purine and pyrimidine bases on the opposite strands.
At each position, A on one strand pairs by two hydrogen bonds with T on the opposite strand, or G pairs by three hydrogen bonds with C. The two strands of double-helical DNA are, therefore, complementary.Although diploidy provides the means for complementation testing in higher plants and animals and also in budding yeast, alternative methods, described under Complementation tests, have to be used for habitually haploid organisms, such as bacteria, viruses, and most fungi.
Although diploidy provides the means for complementation testing in higher plants and animals and also in budding yeast, alternative methods, described under Complementation tests, have to be used for habitually haploid organisms, such as bacteria, viruses, and most fungi. Complementation means that two different sources of genetic information (usually, different gene loci encoding proteins or RNAs of different function) together each provide something the other lacks.
Example: Two white-flowered plants cross to produce purple flowers, although purple is dominant.
Complementation will not occur if the mutations are in the same gene. The convenience and essence of this test is that the mutations that produce a phenotype can be assigned to different genes without the exact knowledge of what the gene product is doing on a molecular level.
Inactivation, Complementation, and Heterologous Expression ofencP, a Novel Bacterial Phenylalanine Ammonia-Lyase Gene*. Gene Complementation in Bacteria In order to perform tests for dominance or for complementation in bacteria we need a way to make the .