Viruses

All business nothing personal

Virus

Microscopic, non-cellular particle made of genetic material (DNA or RNA) and protein that can invade a living cell.

Viruses are …

Obligate intracellular parasites = can only reproduce by invading a host cell and using enzymes and organelles of the host cell to make more viruses.

Size range of viruses

10 nm to 400 nm. Can only be seen with an electron microscope. Compared to bacteria viruses are many times smaller.

Typical virus two parts

1. Nucleic acid core (DNA or RNA)

2. Protective outer protein coat or capsid

Draw and label of “typical” virus structure

Nucleic acid core

Either DNA or RNA (never both)

Nucleic acid strand may be single or double, linear or circular.

Enables the virus to reproduce new viruses exactly like itself.

Protective outer protein coat (capsid)

Approx. 95% body of virus

Capsids built from a large number of protein subunits capsomeres.

Some proteins make up body of virus are enzymes

Protects the nucleic acid core

4 main virus shapes

helical, polyhedral, enveloped, complex

Helical

Rod-shaped viruses made of repeating units of protein in a spiral/helical arrangement, surrounds a helical strand of nucleic acid runs the length of the virus

Example of helical viruses

Tobacco Mosaic Virus, Rabies

Polyhedral

Capsid proteins form a polyhedron with 20 triangular faces and 12 corners.

Example of polyhedral viruses

Adenovirus, Poliovirus

Enveloped

Have a capsid that is covered by an envelope consisting of protein, lipids and carbohydrates. Has

Example of enveloped viruses

Influenza virus, HIV

Complex and example

Made up of a combination of other forms

e.g. Bacteriophage (only invade bacteria) have a polyhedral head (surrounds nucleic acid strand) and a hollow helical tail.

Tail fibers: structures by which the virus attaches itself to a bacterium.

Viral specificity

Viruses are very specific in host cells (host range)

Results from evolution of virus recognition system, by a “lock-and-key” fit between the proteins on the virus surface and specific receptor molecules on the cell’s surface.

Some virus have broad host ranges …

West Nile virus infect mosquitoes, birds, and humans.

Equine encephalitis virus can infect mosquitoes, birds, horses, and humans.

Other viruses have host ranges …

So narrow thay they infect only a single species, or single tissues within that species.

Tobacco Mosaic virus only attacks specific tobacco plants and attacks only the leaf tissues.

Infection by viruses of multicellular …

Eukaryotes is usually limited to particular tissues. Human cold viruses infect only the cells lining the upper respiratory tract, rabies virus and poliovirus infect only nerve cells, HIV binds to specific receptors on certain white blood cells (helper-t cells).

Characteristics that support the scientist who argue that viruses are alive

Contain genetic material (DNA or RNA), protein coat, small amount of enzyme.

Display diversity and evolve and ± reproduce

Characteristics that support the scientist who argue that viruses are not alive

Non-cellular, can’t live independently

Do not undergo aerobic cellular respiration (ATP production)

Do not respond to stimuli

Do not grow (unless infect living cell)

Reproduce only within living cells (Do not directly come from pre-existing viruses)

Can solidified into crystals (placed in solution active again)

“Life” cycle (reproductive cycle) of viruses

Must invade or infect a living host cell to reproduce.

Isolated viruses are merely packaged sets of genes in transit from one host cell to another.

General features of viral reproductive cycles

1)

A viral infection begins when the genome (DNA or RNA) of a virus makes it way into a host cell.

2 & 3)

Once DNA ( or RNA ) is inside, can “hijack” the host cell to copy the viral nucleic acid and manufacture viral proteins.

4)

Viral nucleic acid and proteins (capsomeres) are assembled into new viruses.

The simplest type of viral reproductive cycle ends with …

the exit of hundreds or thousands of virses from the infected host cell, often damages or destroys the cell. The viral progeny (virus offsprings) have the potential to infect additional cells, spreading the viral infection.

Viruses can reproduce by …

two alternative mechanisms: the lytic cycle and the lysogenic cycle

Lytic cycle (Lytic infection)

Process in which a host cell is invaded, lysed (burst), and destroyed by the virus.

“Lytic” refers to the breaking apart of cells.

Lytic cycle (bacteriophage)

Steps in a phage attack on a host bacterial cell.

Viruses invade, reproduce, and exit immediately.

A bacteriophage that reproduces only by lytic cycle is a virulent phage

Lysis used as a tool to detect viruses

If a culture of bacteria is inoculated with bacteriophages, the lysis of bacterial cells make the culture appear clear (plaques).

Plaques

Spots of dead bacteria

Lytic cycle 5 main stages

1. Adsorption

2. Entry

3. Replication

4. Assembly

5. Lysis & Release

Adsorption

Attachment of the virus to the host cell

Bacteriophage attaches itself tail down to a specific host bacterial cell, because tail fibers of the phage are chemically attracted to specific receptor sites on the bacterial cell wall (lock and key).

Entry

Injection of the viral genetic material into the host cell.

Bacteriophage releases an enzyme that breaks down the bacterial cell wall. Injects DNA through hollow tail into host bacterium.

Replication

Alteration of the host cell’s genetic material, causing synthesis of new viral nucleic acids and proteins.

Bacteriophage’s DNA takes over control of the host bacterial cell’s activity. replication of new viral DNA and synthesis of viral proteins.

Assembly

Assembly of the viral nucleic acids and proteins into new, complete viruses.

Proteins (enzymes) coded for by the phage DNA put new viruses together. Results in a bacterial cell stuffed with many copies of new viruses.

Replication and Assembly …

Virus uses the host cell’s enzymes, nucleotides, ribosomes, etc

Lysis & Release

release of new viruses

Newly assembled bacteriophages release an enzyme that digests the bacterial cell wall from within (Lysis). The cell bursts (cell death too), releasing hundreds of new viruses.

The new viruses can …

infect other cells, and the lytic cycle can start again.

Reasons why bacteriophage haven’t completely wiped out bacteria

Natural selection favors bacterial mutants with receptor sites no longer recognized by a specific phage.

When phage DNA enters a bacterium, DNA is often recognized and cut up by cellular enzymes (restriction enzymes).

Instead of lysing and killing host cells, many phaged coexist within them (lysogenic cycle)

Lysogenic infection

Virus does not immediately reproduce and lyse (kill) the host cell. Viral genetic material is inserted into the DNA of the host cell, where it can remain for many generations (of cells) before becoming active.

Lysogenic cycle

“Latent infection”

The inactive cycle

Takes a long time for cellular damage to occur compared to virulent viruses (lytic)

The lysogenic cycle is similar to …

the initial steps in the lytic cycle. There is:

1. Adsorption - bacteriophage attaches to a specific host cell.

2. Entry - genetic material enters the bacterium.

Then during the lysogenic cycle

Viral DNA molecule combines with the DNA of the host bacterial cell (recombination)

Prophage

Host cell’s DNA and the viral DNA.

Only in bacteria.

Provirus

Not a prophage, has the same defintion.

Latent period

Prophage remaining inactive in the host cell for a long time

A virus may …

not stay in the prophage form indefinitely.

Certain factors,

such as sudden changes in temperature, availability of nutrients, and exposure to radiation & certain chemicals can activate the DNA of the prophage, removing itself from the DNA host bacterial cell.

Continues on a similiar to the lytic cycle with:

3. Replication of phage proteins and nucleic acids

4. Assembly into new viruses

5. Lysis & Release of new viruses, results in destruction of host bacterial cell.

Herpes viruses

remain latent until some sort of physical or emotional stress triggers a new round of active active. Infection of other cells causes cold sores.

Retrovirus

A group of animal viruses that contain double-stranded RNA as their genetic information.

Equipped with reverse transcriptase enzyme transcribes RNA template “back” into DNA (reverse transcription) and DNA recombinates with host’s chromosomes (provirus).

Retroviruses reproductive cycle

Lysogenic cycle (Lysogenic infection) that at some point the cell become lytic again.

Retroviruses are called that because …

they copy their genetic information backwards from RNA → DNA. prefix retro- means “backward”.

HIV

(human immunodeficiency virus) the retrovirus that causes AIDS (acquired immunodeficiency syndrome)

Virus are responsible for …

A number of diseases in humans, plant crop, livestock, and pets.

All viruses infect …

Living cells, and disease results when infection causes harm to the host.

Infections occur when …

There is a chance contact (exposure) with a host cell.

Pathogen

Disease causing viruses (are pathogenic)

Virulence

The ability of a virus to cause disease ( they are virulent)

Factors that determine the virulence of a virus.

1. Presence and activity of receptors on the surface of the cell, which enables the virus to attach.

2. Speed at which the virus multiplies once it invades a host cell (virulent / lytic fast VS latent / lysogenic infection slow)

3. Response of the host organism to the invading viruses.

Could viruses been the first form of life on Earth?

Probably not, since viruses require a living host for reproduction, must have been other living things before them.

3 different ways viruses can produce infections in humans:

1. Lytic infection

2. Lysogenic infection

3. Tumors

Lytic infection

Virulent infection

A virus that multiplies and then undergoes lysis and destroys the host cell rapidly (virulent)

Lysogenic infection

Latent infection

A virus that exists in a cell but remains inactive (latent)

Latent virus may become activated and the virulent lytic cycle follows.

Tumors

Produce masses of abnormal cells.

Warts - benign tumors which do not have the ability to spread from their site of origin

“Cancers” - malignant tumors are capable of being spread throughout the body.

Defenses against viral infections

Human body has several natural defenses against infections. An invading virus must successfully penetrate each line of defense for an infection to occur.

Non-specific defenses

Non-specific because they operate in the same way against all disease causing microorganisms (e.g. viruses, bacteria, slivers of wood, cancer cells, etc)

First line of defence

The skin and mucous membrane, provide a barrier against pathogens.

Skin

Viruses usually cannot penetrate the skin.

Mucous membranes

Cilia and mucous inhibit the entry of some pathogens. Sticky virus stuck and swept away by ciliary action. They’re mucous, saliva, gastric juices, etc

Second line of defence

Inflammatory response begins If large numbers of viruses do enter the body (infection)

Inflammation 4 characteristic symptoms (incomplete)

1. Redness

2. Heat

Redness

When blood vessels dilate and blood rushes to site of the wound (infection), to deliver white blood cells.

Heat

Due to dilated blood vessels (blood carries heat)

By itself is protective: rising temperatures can create an environment so hot that certain microorganisms are weakened or even killed