Essential Cell Biology 5th edition

Mobile Genetic Elements and Viruses
315
organisms cannot be grown under laboratory conditions. Thus it is only
through the analysis of DNA sequences, obtained from around the globe,
that we are beginning to obtain a more detailed understanding of all life
on Earth—knowledge that is less distorted by our biased perspective as
large animals living on dry land.
MOBILE GENETIC ELEMENTS AND VIRUSES
The tree of life depicted in Figure 9−23 includes representatives from life’s
most distant branches, from the cyanobacteria that release oxygen into
Earth’s atmosphere to the animals, like us, that use that oxygen to boost
their metabolism. What the diagram does not encompass, however,
are the parasitic genetic elements that operate on the outskirts of life.
Although these elements are built from the same nucleic acids contained
in all life-forms and can multiply and move from place to place, they do
not cross the threshold of actually being alive. Yet because of their prevalence
and their penchant for propagating themselves, these diminutive
genetic parasites have major implications for the evolution of species
and for human health.
We briefly discussed these mobile genetic elements, earlier in the chapter,
and here we consider them in greater detail. Known informally as
jumping genes, mobile genetic elements are found in virtually all cells.
Their DNA sequences make up almost half of the human genome.
Although they can insert themselves into virtually any region of the
genome, most mobile genetic elements lack the ability to leave the cell in
which they reside. This is not the case for their relatives, the viruses. Not
much more than strings of genes wrapped in a protective coat, viruses
can escape from one cell and infect another.
In this section, we discuss mobile genetic elements and viruses. We
review their structure and outline how they operate—and we consider
the effects they have on gene expression, genome evolution, and the
transmission of disease.
Mobile Genetic Elements Encode the Components They
Need for Movement
Mobile genetic elements, also called transposons, are typically classified
according to the mechanism by which they move or transpose. In
bacteria, the most common mobile genetic elements are the DNA-only
transposons. The name is derived from the fact that the element moves
from one place to another as a piece of DNA, as opposed to being converted
into an RNA intermediate—which is the case for another type
of mobile element we discuss shortly. Bacteria contain many different
DNA-only transposons. Some move to the target site using a simple cutand-paste
mechanism, whereby the element is simply excised from the
genome and inserted into a different site. Other DNA-only transposons
replicate before transposing; in this case, the new copy of the transposon
inserts into a second chromosomal site, while the original copy remains
intact at its previous location (Figure 9−24).
Each mobile genetic element typically encodes a specialized enzyme,
called a transposase, that mediates its movement. These enzymes recognize
and act on unique DNA sequences that are present on the mobile
genetic elements that code for the transposase. Many mobile genetic
elements also harbor additional genes: some mobile genetic elements,
for example, carry antibiotic-resistance genes, which have contributed
greatly to the widespread dissemination of antibiotic resistance in bacterial
populations (Figure 9−25).