The Microbial Self 81“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanitysurround the internal structures and can stay dormant in this manner for
extended periods of time, even thousands of years, unharmed by harsh
treatments including boiling, radiation, desiccation, and chemical dis-
infectants. Once they find themselves in a suitable location, they will
revive and lead a normal life again.
4.1.2 Defense against invading viruses
Bacteria constantly face invasion by viruses. One way to stave off these
foreign DNAs is to destroy the viral strands with a type of enzyme called
“restriction enzymes,” like cutting a ribbon with a pair of scissors. These
are enzymes targeted against certain nucleotide sequences that are
frequently found in viruses. It is a type of innate immunity since the
enzymes are there whether or not the viruses are present. The bacteria’s
own DNA is protected by methylation and therefore is not cleaved by
the enzymes. In this instance, the microbes are able to distinguish self-
DNA from non-self-DNA.
Bacteria can also mount an acquired immune defense against
viruses. They can incorporate tiny fragments of an invading virus into
their genome, serving as a historical record of the invasion — a sort of
biochemical memory. The inserted foreign pieces of DNA can gener-
ate small RNAs corresponding to the sequence of the virus. Upon rein-
fection, these RNAs (called CRISPR-RNAs) can recognize the DNA
sequence of the invaders and bring about their destruction by an associ-
ated enzyme. This is an adaptive immunity directed specifically against
the particular invader.^2
4.1.3 ransfer of DNA and silencing of harmful T
foreign DNA
A bacterium possesses chromosomal DNA in the form of a single long
circular chain. This DNA is involved in cell division and is the main
genome of the bacterium. In addition, it may have one or more short
DNAs (called plasmids), that are not involved in division but can readily
be transferred from one bacterium to another through cell contact,