Biomedical Engineering Reference
In-Depth Information
alter the cell wall of the host cell, and inject the viral nuclear material into the host cell.
Figure 2.4 describes the attachment of a virus onto a host cell. Bacteriophage nucleic acids
reproduce inside the host cells to produce more phages. At a certain stage of viral reproduc-
tion, host cells lyse or break apart and the new phages are released, which can infect new host
cells. This mode of reproduction of viruses is called the lytic cycle. In some cases, phage DNA
may be incorporated into the host DNA, and the host may continue to multiply in this state,
which is called the lysogenic cycle.
Viruses are the cause of many diseases, and antiviral agents are important targets for drug
discovery. However, viruses are also important to bioprocess technology. For example,
a phage attack on an E. coli fermentation to make a recombinant protein product can be
extremely destructive, causing the loss of the whole culture in vessels of many thousands
of liters. However, phages can be used as agents to move desired genetic material into E.
coli. Modified animal viruses can be used as vectors to genetically engineer animal cells to
produce proteins from recombinant DNA technology. In some cases, a killed virus prepara-
tion can be used as a vaccine. Genetic engineering allows the production of virus-like units
that are empty shells; the shell is the capsid and all nucleic acid is removed. Such units can be
used as vaccines without fear of viral infection or replication, since all of the genetic material
has been removed. For gene therapy, one approach is to use a virus where viral genetic mate-
rial has been replaced with the desired gene to be inserted into the patient. The viral capsid
can act as a Trojan Horse to protect the desired gene in a hostile environment and then to
Lytic Cycle
Lysogenic Cycle
FIGURE 2.4 Replication of a virulent bacteriophage. A virulent phage undergoes a lytic cycle to produce new
phage particles within a bacterial cell. Cell lysis releases new phage particles that can infect more bacteria.
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