I was very excited when I received a notification for this article on my e-mail address. Great article, as I thought. I recommend it to all visitors. Subscribe to the blog and be the first to hear about newly published articles. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Notify me of follow-up comments by email. Notify me of new posts by email. Currently you have JavaScript disabled.
In order to post comments, please make sure JavaScript and Cookies are enabled, and reload the page. These genes, which allow microbes living in mixed communities to compete for limited resources, can be transferred within a population by transformation, as well as by the other processes of HGT.
In the laboratory, we can exploit the natural process of bacterial transformation for genetic engineering to make a wide variety of medicinal products, as discussed in Microbes and the Tools of Genetic Engineering.
Viruses that infect bacteria bacteriophages may also move short pieces of chromosomal DNA from one bacterium to another in a process called transduction see Figure 3 in The Viral Life Cycle. Recall that in generalized transduction , any piece of chromosomal DNA may be transferred to a new host cell by accidental packaging of chromosomal DNA into a phage head during phage assembly.
As a result, the host may acquire new properties. This process is called lysogenic conversion. Of medical significance, a lysogenic phage may carry with it a virulence gene to its new host. Several pathogenic bacteria, including Corynebacterium diphtheriae the causative agent of diphtheria and Clostridium botulinum the causative agent of botulism , are virulent because of the introduction of toxin-encoding genes by lysogenic bacteriophages, affirming the clinical relevance of transduction in the exchange of genes involved in infectious disease.
Archaea have their own viruses that translocate genetic material from one individual to another. Paul, a year-old relief worker from Atlanta, traveled to Haiti in to provide aid following the earthquake.
After working there for several weeks, he suddenly began experiencing abdominal distress, including severe cramping, nausea, vomiting, and watery diarrhea. He also began to experience intense muscle cramping.
Because cholera is transmitted by the fecal-oral route, breaches in sanitation infrastructure, such as often occur following natural disasters, may precipitate outbreaks. The physician confirmed the presumptive diagnosis using a cholera dipstick test. He then prescribed Paul a single dose of doxycycline, as well as oral rehydration salts, instructing him to drink significant amounts of clean water.
Figure 2. A scanning electron micrograph of Vibrio cholerae shows its characteristic curved rod shape. Cholera is caused by the gram-negative curved rod Vibrio cholerae Figure 2. Its symptoms largely result from the production of the cholera toxin CT , which ultimately activates a chloride transporter to pump chloride ions out of the epithelial cells into the gut lumen.
Water then follows the chloride ions, causing the prolific watery diarrhea characteristic of cholera. The gene encoding the cholera toxin is incorporated into the bacterial chromosome of V. Thus, pathogenic strains of V. In conjugation , DNA is directly transferred from one prokaryote to another by means of a conjugation pilus , which brings the organisms into contact with one another.
The F-plasmid genes encode both the proteins composing the F pilus and those involved in rolling circle replication of the plasmid. During typical conjugation in E. Then a cytoplasmic bridge forms between the two cells at the site of the conjugation pilus.
Genes on the E. Figure 3. Although typical conjugation in E. This is because the F plasmid occasionally integrates into the bacterial chromosome through recombination between the plasmid and the chromosome, forming an Hfr cell Figure 4.
Figure 4. Because the bacterial chromosome is so large, transfer of the entire chromosome takes a long time Figure 5. However, contact between bacterial cells during conjugation is transient, so it is unusual for the entire chromosome to be transferred. As a result, prior to the age of widespread bacterial genome sequencing, distances on prokaryotic genome maps were often measured in minutes.
Figure 5. However, contact between cells during conjugation is temporary. Chromosomal genes closest to the integration site gene 1 that are first displaced during rolling circle replication will be transferred more quickly than genes far away from the integration site gene 4.
Note that it takes approximately minutes for the entire genome 4. Plasmids are an important type of extrachromosomal DNA element in bacteria and, in those cells that harbor them, are considered to be part of the bacterial genome. From a clinical perspective, plasmids often code for genes involved in virulence. For example, genes encoding proteins that make a bacterial cell resistant to a particular antibiotic are encoded on R plasmids.
R plasmids, in addition to their genes for antimicrobial resistance, contain genes that control conjugation and transfer of the plasmid. R plasmids are able to transfer between cells of the same species and between cells of different species. Single R plasmids commonly contain multiple genes conferring resistance to multiple antibiotics. Genes required for the production of various toxins and molecules important for colonization during infection may also be found encoded on plasmids.
For example, verotoxin-producing strains of E. VTEC causes severe diarrheal disease that may result in hemolytic uremic syndrome HUS , which may be lead to kidney failure and death. In nonclinical settings, bacterial genes that encode metabolic enzymes needed to degrade specialized atypical compounds like polycyclic aromatic hydrocarbons PAHs are also frequently encoded on plasmids.
Additionally, certain plasmids have the ability to move from bacterial cells to other cell types, like those of plants and animals, through mechanisms distinct from conjugation. Bees use parthenogenesis to produce haploid males drones and diploid females workers. If an egg is fertilized, a queen is produced. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced. Some vertebrate animals—such as certain reptiles, amphibians, and fish—also reproduce through parthenogenesis.
Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or marine zoos. Two female Komodo dragons, a hammerhead shark, and a blacktop shark have produced parthenogenic young when the females have been isolated from males. Sexual reproduction is the combination of usually haploid reproductive cells from two individuals to form a third usually diploid unique offspring.
Sexual reproduction produces offspring with novel combinations of genes. This can be an adaptive advantage in unstable or unpredictable environments. As humans, we are used to thinking of animals as having two separate sexes—male and female—determined at conception. However, in the animal kingdom, there are many variations on this theme. Hermaphroditism occurs in animals where one individual has both male and female reproductive parts.
Invertebrates such as earthworms, slugs, tapeworms and snails, shown in Figure Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other and both producing offspring. Self fertilization is common in animals that have limited mobility or are not motile, such as barnacles and clams. Mammalian sex determination is determined genetically by the presence of X and Y chromosomes.
The presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants. Avian sex determination is dependent on the presence of Z and W chromosomes. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects such as butterflies and moths , and reptiles use this system.
The sex of some species is not determined by genetics but by some aspect of the environment. Sex determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical periods of egg development.
This is referred to as environmental sex determination, or more specifically as temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females. In some crocodiles, moderate temperatures produce males and both warm and cool temperatures produce females.
0コメント