In contrast, horizontal gene transfers occur much more frequently between different species of prokaryotes. Many prokaryotes have a remarkable capacity to take up even nonviral DNA molecules from their surroundings and thereby capture the genetic information these molecules carry. By this route, or by virus-mediated transfer, bacteria and archaea in the wild can acquire genes from neighboring cells relatively easily. Genes that confer resistance to an antibiotic or an ability to produce a toxin, for example, can be transferred from species to species and provide the recipient bacterium with a selective advantage. In this way, new and sometimes dangerous strains of bacteria have been observed to evolve in the bacterial ecosystems that inhabit hospitals or the various niches in the human body. For example, horizontal gene transfer is responsible for the spread, over the past 40 years, of penicillin-resistant strains of Neisseria gonorrhoeae, the bacterium that causes gonorrhea. On a longer time scale, the results can be even more profound; it has been estimated that at least 18% of all of the genes in the present-day genome of E. coli have been acquired by horizontal transfer from another species within the past 100 million years.
sex Results in horizontal exchanges of Genetic Information Within a species
Horizontal gene transfer among prokaryotes has a parallel in a phenomenon familiar to us all: sex. In addition to the usual vertical transfer of genetic material from parent to offspring, sexual reproduction causes a large-scale horizontal transfer of genetic information between two initially separate cell lineages—those of the father and the mother. A key feature of sex, of course, is that the genetic exchange normally occurs only between individuals of the same species. But no matter whether they occur within a species or between species, horizontal gene
Figure 1–22 The viral transfer of DNA into a cell. (A) An electron micrograph of particles of a bacterial virus, the T4 bacteriophage. The head of this virus contains the viral dnA; the tail contains the apparatus for injecting the dnA into a host bacterium. (B) A cross section of an
E. coli bacterium with a T4 bacteriophage latched onto its surface. The large dark objects inside the bacterium are the heads of new T4 particles in the course of assembly. When they are mature, the bacterium will burst open to release them. (c–e) The process of dnA injection into the bacterium, as visualized in unstained, frozen samples by cryoelectron microscopy. (c) Attachment begins.
Attached state during dnA injection.
virus head has emptied all of its dnA into the bacterium. (A, courtesy of James Paulson; B, courtesy of Jonathan king and erika hartwig from G. karp, cell and molecular Biology, 2nd ed. new york: John Wiley & sons, 1999. With permission from John Wiley & sons; c–e, courtesy of Ian molineux, university of Texas at Austin and Jun liu, university of Texas health science center, houston.) transfers leave a characteristic imprint: they result in individuals who are related more closely to one set of relatives with respect to some genes, and more closely to another set of relatives with respect to others. By comparing the DNA sequences of individual human genomes, an intelligent visitor from outer space could deduce that humans reproduce sexually, even if it knew nothing about human behavior.
Sexual reproduction is widespread (although not universal), especially among eukaryotes. Even bacteria indulge from time to time in controlled sexual exchanges of DNA with other members of their own species. Natural selection has clearly favored organisms that can reproduce sexually, although evolutionary theorists dispute precisely what that selective advantage is.
The Function of a Gene can often Be deduced from Its sequence