Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE DIVERSITY OF GENOMES AND THE TREE OF LIFE
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(A)
(B)
100 nm 100 nm
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.
(D) Attached state during DNA injection.
(E) 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.)
(C) (D) (E)
100 nm
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
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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