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Part 2: How do viruses reproduce?
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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.

Reproduksi Virus Home - Reproduksi Virus Bacteriophage binding to the cell wall of a bacterium.
Copyright Dr. Gary Kaiser. Used with permission.


Reproduksi Virus Home - Reproduksi Virus Bacteriophage injecting its genetic material into the bacterium.
Copyright Dr. Gary Kaiser. Used with permission.


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.

Reproduksi Virus Home - Reproduksi Virus The bacteriophage genome replicates.
Copyright Dr. Gary Kaiser. Used with permission.


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.

Reproduksi Virus Home - Reproduksi Virus The bacteriophage components and enzymes continue to be produced.
Copyright Dr. Gary Kaiser. Used with permission.


Reproduksi Virus Home - Reproduksi Virus The components of the bacteriophage assemble.
Copyright Dr. Gary Kaiser. Used with permission.


Reproduksi Virus Home - Reproduksi Virus Bacteriophage enzyme breaks down the bacterial cell wall causing the bacterium to split open.
Copyright Dr. Gary Kaiser. Used with permission.


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.







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