8/8/2019 Making Mischief
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Making Mischief
Viruses that infect bacteria are bacteriophages, or just phages.
Their life history may include
up to two cycles (figure 16.7). The lytic cycle, as the name
suggests, ends when the bacterium
lyses, or ruptures. t starts when the !irus attaches to the
surface of the bacterium and injects
its genome, and this genome, free in the cytoplasm, confiscates
the cell machinery to
replicate and assemble multiple copies. The thousands of new
copies of the !irus burst from
the host bacterium to find new bacteria to infect and, so,
"uic#ly proliferate. n the lysogenic
cycle, bacteriophages do not #ill the cell they infect, at least
not right away. They inject their
genome, called a prophage, which then becomes integrated into
the $%& of bacterium,
replicating along with the bacterium as it replicates.
Viruses that infect eu#aryotic cells may e'it peacefully, ta#ing
some of the plasma membrane
with them, or destructi!ely, causing deadly lysis of the cell.
<hough it has some
idiosyncrasies, V is an e'ample of such a !irus, structurally
resembling the flu or mumps
!irus. V is a retro!irus, so named because instead of the
transcription $%& *%&, it
re!erses this flow of genetic information ($%&+*%&),
starting with *%& that ma#es $%&.
pon first infection of the cell, the *%& genome of V
produces $%&, which enters the
nucleus and becomes integrated, as a pro!irus, into the
chromosomal $%& of the host cell.
ere, the pro!irus acts, li#e any acti!e section of $%&, to
synthesi-e products, in this case
more !iral *%& and its associated proteins. These assembled
new !iruses lea!e the cell to
infect other cell (figure 16.)
/y lysing cells, some !iruses directly #ill the host. V does
not. nstead, V wea#ens the
immune system of the host, a condition #nown as &$0. &t
first, V enters and proliferatesin macrophages, cells that assist
the immune system. ater, V enters critical cells of the
immune system, the white blood cells #nown as T lymphocytes,
where in the !irus multiplies,
filling and e!entually lysing these cells as they e'it. &s
the disease progresses, the e'ploding
numbers of the !irus infect and gradually depopulate the body of
T lymphocytes, lea!ing the
indi!idual defenseless.
ndi!iduals do not die directly from the disease, but indirectly,
from a wea#ened condition
produced by the !irus. 0o wea#ened, the indi!idual falls
prey to passing opportunistic
diseases or unchec#ed cancers that a healthy indi!idual might
otherwise ward off. & slow
death ensues.
Evolving Plagues and Pathogens
2ur future is li#ely to be a future of plagues. The body counts
will be high because our
human population is at an all time high and growing. 3onsider
just a few recent e'amples.
The 4#iller flu5 of 11 tra!eled home with the soldiers of orld
ar . orldwide, more
than 81 million people died of this !irulent flu, many more than
the numbers of soldiers who
died in the war itself. The 4&sian 9lu5 of 1:7 spread around
the world. n the nited 0tates
alone, 1;;.;;; died, and many more were infected . The 4ong
