21.2 Virus Infections and Hosts

Group IV viruses use dsRNA intermediates to make copies of the genome.

The negative-stranded genome can be converted to messenger RNA.
The templates for the production of the negative-stranded genome are made from full-length positiveRNA strands.

The host genome can be transcribed by the viral DNA.

The examples of each group are summarized in Table 21.3 in the Baltimore classification.

Viruses are parasites.

A virus needs to attach to a living cell before entering it.

The invading virus needs to copy and make its own genes.

The host cell must allow the progeny virions to escape so that they can spread.
Only certain types of hosts and certain types of cells can be affected by a Viruses.
The specificity is usually due to a surface molecule on the host cell surface.
The virus needs a specific viral receptor to attach.
Differences in metabolism and host-cell immune responses are likely factors in determining which cells a virus may target.

The host-cell processes of a virus are used to replicate.

Dramatic biochemical and structural changes in the host cell can be caused by the viral replication cycle.
The symptoms of viral diseases are caused by cell damage caused by the virus and by the immune response of the virus, which attempts to control and eliminate the virus from the body.

The cell does not die immediately during the budding process.

Even though the cells remain alive for a period of time, damage to the cells may make it impossible for them to function normally.
A virus can attach to a specific site on the host cell by attaching to the capsid or by attaching to the viral envelope.
The specificity of this interaction determines the host and the cells within it.
Think of several keys and several locks, where each key will fit only one lock.

The video explains how flu attacks the body.

Viruses can enter a cell with or without the capsid.

The capsid is left outside the cell as the nucleic acid enters.
You may recall that plant and animal viruses can enter through endocytosis.
The viral envelope can enter the cell by itself.
Once inside the cell, the viral capsid degrades, and then the viral nucleic acid is released and becomes available for replication and transcription.

The replication mechanism is dependent on the viral genome.

Host-cell proteins and enzymes are usually used by DNA viruses to replicate the viral DNA.
The RNA core is a template for the synthesis of viral genes.
The host cell is directed by the viral mRNA to synthesise and assemble new viruses.

There are exceptions to this pattern.

If the host cell doesn't provide the necessary enzymes, the viral genes give the information to synthesise the missing proteins.
Retroviruses, such as HIV (group VI of the Baltimore classification scheme), have an RNA genome that must be reverse transcribed into DNA, which is incorporated into the host cell genome.
The retroviruses must have genes that are specific to the virus and can be used to convert RNA into DNA.
The reverse transcriptase is derived from the expression of viral genes in the host cells.
Researchers have been able to develop drugs that don't affect the host's metabolism because HIV produces some of its own enzymes.

This approach has led to the development of a variety of drugs used to treat HIV and has been effective at reducing the number of infectious viruses in the blood.

The last stage of viral replication is the release of the new virions produced in the host organisms, where they are able to spread to adjacent cells and repeat the replication cycle.

When the host cell dies, some viruses can be released, and others can leave the cells with Viruses can leave the cells with Viruses can leave the Cells with Viruses can leave the Cells with Viruses can leave the Cells with Viruses can leave the Cells with Viruses can leave the Cells with Viruses can leave

The virus is engulfed after this.

New virions are made with the help of new genes.

Influenza virus is packaged in a viral envelope.

The virus can leave the cell without being killed.

You can watch a video on viruses, identifying structures, modes of transmission, and more.

You've learned that viruses can be very specific to the hosts and cells within them.

The feature of a virus makes it specific to one or a few species of life.
On the other hand, there are so many different types of viruses on Earth that almost all living organisms have their own set.
The smallest and simplest cells can be attacked by different types of viruses.
In the next section, we will look at some of the features of the prokaryotic cells.
The viruses that cause the infections are called bacteriophages.
The Archaea have their own viruses.

There are Bacteriophages attached to a cell.

The tails are used as a passageway for the transmission of the phage genome.

DsDNA viruses use host genes for their functions.

The genome must be inserted into the cell.
T4 is an example of a lytic bacteriophage.
Sometimes a virus can remain in the cell.
The l (lambda) virus is an example of a lysogenic bug.
Plants and animal cells can sometimes get infections from Viruses that aren't producing enough virions for a long time.
The cause of oral and genital herpes in humans is the animal herpesviruses.
Although there are similarities between lysogeny and latency, the term lysogenic cycle is usually reserved to describe bacteriophages.

In the next section, Latency will be described.

There are two lytic and lysogenic cycles in a temperate bacteriophage.

The host cell is lyses by the phage in the lytic cycle.

In the lysogenic cycle, the host genome is passed on to subsequent generations.

Exposure to toxic chemicals may cause the prophage to excise and enter the lytic cycle.

New phages are released into the environment during the lytic cycle.

In the lysogenic cycle, the host genome is changed.

The tobacco mosaic virus is one of the most common plant viruses.

There are also plant viruses in other virus categories.
Plants do not have active mechanisms for delivering the viral genome across the protective cell wall.
Some type of mechanical damage is required for a plant virus to enter a new plant.

This damage can be caused by weather, insects, animals, fire, or human activities.

Plants can be moved from cell to cell by modification of plasmodesmata.
Plants may carry diseases from parent plants.
There are a variety of ways in which plant viruses can be transmitted.

There are different symptoms of viral diseases according to the virus and its host.

Black streaks on the stems of the plants, altered growth of stems, leaves, or fruits, and ring spots, which are circular or linear areas of discoloration found in a leaf are some of the symptoms of plant viruses.

Our food supply can be seriously affected by plant viruses.

Poor crop quality and quantity can bring about huge economic losses.
Plants used in landscaping may be damaged by other viruses.
The name of the plant that the virusesinfecting it, such as tomato spotted wilt virus, bean common mosaic virus, and cucumber mosaic virus, are included.
The peony ring spot and rose mosaic virus are some of the most common viruses in landscaping plants.
There are too many plant viruses to discuss in detail, but symptoms of bean common mosaic virus result in lowered bean production.
The rose mosaic disease causes wavy yellow lines on the plant's leaves.

The viruses of animals don't have to penetrate a cell wall to get into a cell.

The host cell may cooperate with the virus.
Animals may enter cells in two different ways.
During the normal cell process of endocytosis, the viral capsid can be taken inside the cell via a vesicle.
An alternative method of cell penetration used by non-enveloped viruses is for capsid proteins to undergo shape changes after binding to the receptor.
The host cell is then "injected" with the viral genome through these channels.

Some nonenveloped viruses enter the cell by endocytosis in a similar fashion.

fusion only occurs when there is a certain amount of the same Viruses.
The envelope of these viruses is fused with the cell's nucleus to release the genome and capsid of the virus into the cell's cytoplasm.

After copying their genomes, animal viruses assemble and leave the cell.

As we have already discussed using the example of the influenza virus, animal viruses can bud from the cell's nucleus as they assemble themselves, taking a piece of the cell's nucleus in the process.

On the other hand, non-enveloped viral progeny, such as rhinoviruses, accumulate in cells until there is a signal for lysis or a cell death.

In the next module, you will learn that animal viruses are associated with a variety of human diseases.

The common cold and flu are examples of acute viral diseases.

The hepatitis C virus causes low levels of damage to the body.

Since many of the symptoms of viral diseases are caused by immune responses, a lack of symptoms is an indication of a weak immune response.

This allows the virus to escape elimination by the immune system and persist in individuals for years, all the while producing low levels of progeny virions in what is known as a chronic viral disease.
The risk of developing cancer from chronic infections of the liver by this virus can be as high as 30 years after the initial infections.

For months, even years, the herpes simplex virus can remain in a state in the nervous tissue.

As the virus hides in the tissue, there is nothing for the immune response to act against, and the immunity to the virus slowly declines.
Under certain conditions, including physical and psychological stress, the herpes simplex virus can be reactivated and cause skin diseases.
Once the immune response is stimulated again, the immune system can destroy the viruses in the skin in a few days or weeks.
As a result of this type of replicative cycle, cold sores and genital herpes outbreaks only occur intermittently, even though the viruses remain in the nervous tissue for life.
The varicella-zoster virus is a common cause oftent infections.
The varicella-zoster virus can remain in the body for many years and can cause a painful condition known as "shingles" in adults.

Its double-stranded DNA genome can be incorporated in the host DNA and can be reactivate after a period of time.

The viruses interfere with the normal regulation of the host cell cycle either by introducing genes that stimulates unregulated cell growth or by interfering with the expression of genes that prevent cell growth.
There are two types of oncogenic viruses: DNA orRNA.
There are several types of cancer that can be caused by viral infections.