The last stage is the releaseof virions from the host bacterium cell. The bac-
teriophage enzyme lysozyme breaks apart the bacterial cell wall (lysis) and the
new virus escapes. The escape of this new bacteriophage virus will then infect
neighboring cells and the cycle will continue in these cells.The Lysogenic Cycle
Some viruses do not cause lysis and ultimate destruction of their host cells which
they infect. These viruses are called lysogenic phages or temporate phages. These
bacteriophages establish a stable, long-term relationship with their host called
lysogeny. The bacterial cells infected by these phages are called lysogenic cells.
The most studied bacteriophage, which multiplies using the lysogenic cycle,
is the bacteriophage Lambda. This bacteriophage infects the bacterium E. coli.
When the bacteriophage Lambda penetrates an E. colibacterium, the bacte-
riophage DNA forms a circle. The circle recombines with the circular DNA of
the bacteria. This bacteriophage DNA is called a prophage.
Every time the bacteria host cell replicates normally, so does the prophase
DNA. On occasion, however, the bacteriophage DNA can break out of the
prophage and initiate the lytic cycle.ANIMAL VIRUSES
Animal viruses infect and replicate animal cells. They differ from bacterio-
phages in the way they enter a host cell. For example, DNA viruses enter an ani-
mal host in this way:- Attachment.Animal viruses attach to the host cell’s plasma membrane pro-
teins and glycoproteins (host cell receptors). - Penetration.Animal viruses do not inject nucleic acid into the host eukary-
otic cell. Instead, penetration occurs by endocytosis, where the virion
attaches to the microvillus of the plasma membrane of the host cell. The
host cell then enfolds and pulls the virion into the plasma membrane, form-
ing a vesicle within the cell’s cytoplasm. - Transcription in the nucleus by host RNA polymerase.
- Translation by host cell ribosomes.
- DNA replication by host DNA polymerase in the nucleus.
- Assembly of viral particles.
- Release from cell by lysis or exocytosis.
(^188) CHAPTER 12 Viruses, Viroids, and Prions