How do viruses reproduce and spread?

Virus Replication

Viruses can infect most living things. Bacteriophages, also called phages, are viruses that infect bacteria. Phages have been important in the study of viruses because both phages and bacteria are easy to study in the lab. Because phages multiply rapidly, experiments with phages can be completed within days. Experiments with slower-replicating animal and plant viruses can take months to complete. Much of what is known about viruses was first found in experiments with phages. Figure below shows T4 Bacteriophage.


By Adenosine, CC BY-SA 3.0, via Wikimedia Commons

Like all viruses, phages are seemingly inanimate particles when outside a living cell. Remember, viruses do not have metabolic processes. Viruses reproduce only when inside a living cell. The reproduction of viruses is often called viral replication to distinguish it from the reproduction of cells.

When phages enter bacterial cells, they reproduce using the materials and energy of the cells. What happens to bacteria? In most cases, the bacteria die as a result of the phage infection.

Viral Replication

Most viruses reproduce in a similar way. Here we describe in detail the replication of a bacteriophage. The process in the phage is similar to replication in most viruses.

1. When a phage comes into contact with the host bacterium, it attaches to the surface of the bacterial cell through a process called attachment. Attachment occurs because the molecules in the viral tail form a close match with the molecules on the host bacteria. This matching is one of the reasons why each virus infects a particular host.

Transmission electron micrograph of multiple bacteriophages attached to a bacterial cell wall; the magnification is approximately 200,000. By Dr. Graham Beards, CC BY-SA 3.0, via Wikimedia Commons

2. After attachment, one or a series of reactions take place at the attachment point. This causes a hole in the bacterium's cell wall. The tail of the virus shrinks and injects the viral nucleic acid into the bacteria. The empty capsid remains outside.

3. When the viral nucleic acid enters the bacteria, it takes control. The host cell's DNA is either destroyed or becomes unusable. The normal activities and metabolism of the cell cease. Using the host's resources, the viral nucleic acid makes copies of itself and its proteins.

4. The newly produced viral particles are assembled.

5. When enough new virus is collected, the cell bursts and releases all the virus. Bacterial bursting is called lysis. After lysis, newly formed viruses are released to infect other host cells. They move around in their environment until they encounter other host bacteria. Then the process starts again. This whole process, from attachment to lysis, is called the lytic cycle.

A phage can complete a lytic cycle in 25 to 45 minutes. Several hundred new phages can be produced in each lytic cycle. Phages that cause their hosts to break down are called virulent phages. Virulent means very poisonous.

Lysogenic cycle

Not all phages are virulent. Phages that remain inside the host cells for a long time without causing lysis are called temperate phages. A temperate phage begins its activities in the same way as a virulent phage. Adheres to the host's surface. It then injects its nucleic acid into the cell. There the similarity ends. The temperate phage's DNA then becomes part of the host chromosome's DNA. Viral DNA acts as an extra piece of information on the bacterial chromosome. The viral DNA attached to the bacterial chromosome is called a prophage. When the bacterium copies its DNA before dividing, it also copies the prophage. The prophage can be carried over many generations without significant damage to the bacteria. Each new bacterial cell formed carries a copy of the prophage on its chromosome. Prophage-bearing bacteria are called lysogenic bacteria. The process by which viral DNA is attached to and carried by a bacterial chromosome is called lysogeny.

Even if prophage is present, normal metabolism continues in the bacterial cell. The prophage can then separate from the host chromosome. If this happens, the phage will become lethal. It will multiply and cause lysis of the host bacteria.

Animal and plant viruses engage in activities like those of lethal bacteriophages. During replication, animal and plant viruses use the materials and energy of their hosts. As with virulent phages, animal and plant viruses often cause lysis of host cells. Lysis and the resulting loss of material and energy lead to disease.

Animal and plant viruses can also behave in the same way as temperate phages do. The viral nucleic acid may be present in the host cell without causing lysis. The presence of this extra nucleic acid can cause further chemical changes in the host cell, again causing disease.

Animal viruses and phages differ slightly in their method of infection and replication. A phage simply injects its own nucleic acid into a bacterium; the capsid remains outside. After an animal virus attaches to the surface of a host cell, the entire virus is taken in. Once inside, the animal cell's enzymes dissolve the viral capsid. The viral nucleic acid can then replicate or attach to a host chromosome. An animal virus reproduces more slowly than a phage. Replication takes 6 or more hours to complete. When a newly formed virus leaves an animal cell, it may become part of the host's cell membrane. This membrane material becomes the envelope of the virus.

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