By Barry Fox, M.D., University of Wisconsin
The term ‘virus’ is derived from a Latin word that means ‘poison’. They are the smallest of the microbes and are unique because they can only stay alive and multiply inside the cells of living beings. Viruses are at least a hundred times smaller than bacteria, and were first seen in 1931 when the electron microscope was invented.
Viruses Versus Bacteria
Smaller than bacteria, the first virus was conceptually isolated in 1895 when laboratory filters that were too small to allow bacteria to pass through were used. The smaller virus particles passed through the filter and were identified as a tobacco mosaic virus—typically found on tobacco and tomato plants.
A virus cannot strive and reproduce without a host body. They are life forms whose genetic material, either DNA or RNA, replicates inside living cells using the cells’ own synthetic machinery. This leads to the synthesis of components of the virus that are subsequently re-assembled, and then transferred to other living host cells, or sometimes to the environment.
They do not have the capacity for independent protein synthesis or for the generation of their own energy supplies. They have no cell wall and no nucleus. They reproduce by an assembly of various viral pieces and not by growth and division like bacteria do. They are totally dependent on the host cell for survival.
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What Is Viral Replication?
Viral replication is the creation of biological viruses during the infectious process in the target host cells. Steps in viral replication include an attachment to the cell, entry into the cell, early genetic expression, DNA or RNA replication, late genetic expression, assembly of the new viruses, and release of the viruses from the cell.
First, the virus attaches to the host cell and that attachment involves two minimum components—the virus attachment complex and the cellular receptors for the virus. Some cellular receptors are proteins, such as for HIV, whereas others are sugar receptors, such as sialic acid for influenza.
After a virus attaches itself to the host cell, it enters into the cells. The virus either melts and merges with the cell membrane or gets swallowed by the cell.
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Patterns of Viral Replication
Viruses utilize one of three unique patterns of replication, depending upon which genetic material they possess—DNA, RNA, or retrovirus machinery.
For DNA viruses, its DNA must enter the host cells’ nucleus to replicate. The DNA polymerase enzyme of the host cell is essentially hijacked for the viral DNA replication.
RNA is a foreign genetic material to human chromosomes since humans do not have RNA. Therefore, the first step for virus replication in RNA viruses is to trick the host cells to produce an RNA polymerase, which subsequently allows the RNA virus to be replicated.
The third type of replication involves retroviruses. Retroviruses, such as the Human Immunodeficiency Virus, or HIV, have a unique mechanism of replication. A retrovirus possesses one copy of a single-stranded RNA.
It also carries with it a unique enzyme known as ‘reverse transcriptase’. Shortly after cell entry, the single RNA strand is copied into a double-stranded DNA spiral helix—the opposite of the normal replication process. Reverse transcriptase is an important antiviral drug target for HIV.
Retroviruses also like to bury some of their genetic material into the host cell DNA. They integrate their genetic material into this host cell with a different retrovirus enzyme known as ‘integrase’.
The host cell then treats the viral DNA as part of its own genome. This new DNA will persist indefinitely. The integrase is another antiviral target for HIV infections.
A Fact of Viral Biology
It is crucial to recognize that when viruses replicate, they do so in a rapid exponential fashion that generates millions of new virus particles. This is more significant than bacteria since random genetic mutations are more frequent and, thus, lead to the ‘survival of the fittest’ in viruses.
This is particularly important for targeting anti-viral therapy—meaning combinations of medicines need to be taken to help prevent the emergence of viral resistance. After the virus’s DNA or RNA replicates in a host cell, it directs the cell to help the virus package itself in the cytoplasm, and then the virus breaks free from the host cell into a neighboring cell, or sometimes into the bloodstream.
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Classification of Viral Infections
Virus infections can be classified as either localized or body-wide, also known as disseminated. In localized infections, the virus remains at the site of entry.
Examples of local infections include respiratory viruses like the common cold, the herpes viruses that cause a cold sore, or wart viruses like a foot wart.
An example of a disseminated infection would be the varicella-zoster or chickenpox virus. After local replication in the respiratory tract, the chickenpox virus spreads throughout the body, and multiple organs can be infected, including the skin, which causes pox.
Basic Classification of Viruses
Viruses are classified by their genetic material—either DNA, RNA, or retroviruses—and by their outer component—either enveloped or non-enveloped. Enveloped viruses have different shapes, such as the rabies virus which is bullet-shaped.
It is important to know since viruses are structurally different than bacteria and no antibiotics will work in preventing virus replication.
Compared to antibiotics, the number of anti-viral medications is much less. This is another reason that vaccination against viral diseases is especially important.
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How Do Viruses Affect the Host Cells?
Some viral infections, such as Ebola, result in complete host cell shutdown, leading to cell death. Other viruses may selectively inhibit a special host function, allowing the cell to survive, but in a damaged form.
If the cells do survive in a damaged condition, there is a risk of subsequent bacterial superinfection, which can occur several days later. This is a common mechanism of disease for respiratory viruses, which may lead to secondary bacterial ear or sinus infections, or even bacterial pneumonia. It is believed that over half the deaths in the Great Influenza Pandemic of 1918 were caused by secondary bacterial infections.
Common Questions about Viruses
A virus is one of the smallest of the microbes that can only stay alive and multiply inside the cells of living beings or host cells.
The steps in viral replication include an attachment to the cell, entry into the cell, early genetic expression, DNA or RNA replication, late genetic expression, assembly of the new viruses, and release of the viruses from the cell.
Viruses are classified by their genetic material—either DNA, RNA, or retroviruses—and by their outer component—either enveloped or non-enveloped.