How Viruses Replicate

The easiest way to understand how viruses replicate is to study the life cycles of viruses called bacteriophages. Bacteriophages replicate by either a lytic cycle or a lysogenic cycle. The difference in these two cycles is that the cell dies at the end of the lytic cycle and remains alive in the lysogenic cycle.

The first two scientists to observe bacteriophages were Frederic Twort of England and Felix d'Herelle of France in the early 1900s. The name bacteriophage is credited to d'Herelle and means "eaters of bacteria."

Lytic Cycle

The most studied bacteriophage is the T-even bacteriophage. The virions of T-even bacteriophages are big, complex, and do not contain envelopes. The T-even bacteriophages are composed of a head-and-tail structure and contain genomes of double-stranded DNA. The lytic cycle of replication begins with the collision of the bacteriophage and bacteria, called attachment. The tail of the bacterio-phage attaches to a receptor site on the bacterial cell wall. After attachment, the bacteriophage uses its tail like a hypodermic needle to inject its DNA (nucleic acid) into the bacterium. This is called penetration. The bacteriophage uses an enzyme called lysozyme in its tail to break down the bacterial cell wall, enabling it to inject its DNA into the cell. The head or capsid of the bacteriophage remains on the outside of the cell wall. After the DNA is injected into the host's bacterial cell's cytoplasm, biosynthesis occurs. Here the T-even bacteriophage uses the host bacterium's nucleotides and some enzymes to make copies of the bacteriophage's DNA. This DNA is transcribed to mRNA, which directs the synthesis of viral enzymes and capsid proteins. Several of these viral enzymes catalyse reactions that make copies of bacteriophage DNA. The bacteriophage DNA will then direct the synthesis of viral components by the host cell.

Next maturation occurs. Here the T-even bacteriophage DNA and capsids are put together in order to make virions.

The last stage is the release of virions from the host bacterium cell. The bacteriophage 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. coli bacterium, 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.

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