written by: J.Sace
• edited by: Leigh A. Zaykoski
• updated: 11/22/2008
How do viruses reproduce? This article explains one of two strategies used by all viruses, the lytic cycle, so-named because it ends with the lysis of the host cell. The T-even bacteriophage, which preys on E. coli bacteria, is used as an example.
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Although there are different ways on how a virus enters and exits a host cell, the basic mechanism of viral multiplication is alike for all viruses. Among viral life cycles, the best understood are those of the bacteriophages. There are two alternative mechanisms by which these bacteriophages (phages for short) can multiply: the lytic cycle or the lysogenic cycle. Lysis and death of host cells terminate the lytic cycle whereas the host cell remains alive in the lysogenic cycle. This article will only discuss the lytic cycle using the T-even bacteriophage infecting its host, E. coli, as an example for the lytic cycle.
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T-Even Bacteriophages: The Lytic Cycle
The virions (fully developed viral particle) of T-even bacteriophages are complex, large, nonenveloped, with head-and-tail structural characteristic. It is interesting to note that even if the length of the DNA contained in the bacteriophage is only about 6% of that contained in E. coli, the phage has adequate DNA for more than 100 genes. The multiplication cycle of the bacteriophage is composed of five distinct stages: attachment, penetration, biosynthesis, maturation, and release. These stages are alike to all viruses.
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This initial stage happens after a chance collision between a bacteria and phage particles. Attachment could only become possible if the attachment site in the viral surface interacts with a complementary receptor site in the bacteria. Interaction between the two sites is through weak chemical bonds. Fibers at the end of the tail of the T-even bacteriophage are used as attachment sites while the complementary receptor sites are on the cell wall of the bacteria.
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Immediately after attachment, the T-even bacteriophage injects its DNA into the bacterium. This is accomplished when the bacteriophage’s tail secretes the enzyme lysozyme, whichbreaks down a section of the bacterial cell wall. When a hole is already created in the cell wall, the phage will then begin to transfer its DNA from its core towards the bacterial cytoplasm. Only the genetic materials are injected while the capsid remains outside the bacterial cell. To better understand this stage; just imagine a phage particle acting like a hypodermic syringe to inject its DNA into the bacterial cell.
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Biosynthesis of viral nucleic acids and proteins immediately occur after the viral DNA reached the bacterial cytoplasm. The virus will induce the degradation of the host DNA and viral proteins hamper transcription and translation processes.
At first, the phage utilizes the host cell’s nucleotides and a number of its enzymes to synthesize many copies of phage DNA. Almost immediately, the biosynthesis of viral proteins starts. All mRNA in the cytoplasm are directly transcribed from the phage DNA, and they are used for the biosynthesis of phage enzymes and capsids proteins.The enzymes, ribosomes, and amino acids of the host are used for translation.
A few minutes after infection, complete phages cannot be found in the host cell. Just separate components – DNA and protein – can be observed.
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The maturation stage is marked with the assembly of bacteriophage DNA and capsids into complete virions. The viral components basically assemble into a viral particle spontaneously, eradicating the need for many nonstructural genes and gene products. The bacteriophage tails and heads are separately assembled from protein subunits, and the head is packed with DNA and attached to the tail.
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This final stage of viral multiplication is characterized by the release of virions from the host cell. Successful release of the virions occurs after the breakage or the lysis of the bacterial plasma membrane. Bacterial lysis happens when the mature capsids inside release lysozymes which are capable of breaking the cell wall. The released bacteriophages infect other vulnerable cells in the surrounding area, and the viral multiplication cycle is repeated within those cells.