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In the first part of this series about viruses, we looked at virus structure and function. As is true in most organisms, structure and function are thoroughly intertwined. This week we look at a fascinating subject -- virus replication.
Replication
A single virus particle (virion) is in and of itself essentially inert. It lacks needed components that cells have to reproduce. Viruses are intracellular obligate parasites which means that they cannot reproduce or express their genes without the help of a living cell.
Once a virus has "infected" a cell, it will "marshal" the cell's ribosomes, enzymes and much of the cellular machinery to reproduce. Unlike what we have seen in mitosis and meiosis, viral reproduction produces many, many progeny, that when complete, leave the host cell to infect other cells in the organism.
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Bacteriophage binding to the cell wall of a bacterium. |
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Bacteriophage injecting its genetic material into the bacterium. |
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The exact nature of what happens after the host is infected varies depending on the nature of the virus. In most cases, the process depends on the form of the genome. The process for double-stranded DNA, single-stranded DNA, double-stranded RNA and single-stranded RNA will differ.
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The bacteriophage genome replicates. |
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Self-Assembly
Interestingly enough, once the viral progeny components are produced by the cellular machinery, the assembly of the viral genome and the viral capsids is a non-enzymatic process. It is usually spontaneous.
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The bacteriophage components and enzymes continue to be produced. |
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The components of the bacteriophage assemble. |
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Bacteriophage enzyme breaks down the bacterial cell wall causing the bacterium to split open. |
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Specificity
Ah, the beauties of structure and function! Viruses typically can only infect a limited number of hosts (also known as host range). The "lock and key" mechanism is the most common explanation for this range. Certain proteins on the virus particle must fit certain receptor sites on the particular host's cell surface.
Share Your Opinions
What do you think? How does the structure of a virus help determine its reproductive functions? Would you classify a virus as living or nonliving? Come on over to the Biology Forum and share your thoughts, opinions and feelings.
Next Page > Bacterial Viruses
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