Unit 9 Viruses Pt 1

Where did the word Virus come from

Came from a Latin word meaning POISON

What was said about the first virus

1. First described as infectious fluid

2. Wendell Stanly isolated TOBACCO MOSAIC VIRUS (TMV) the first discovered virus

What’s the common saying about virus

All living things can be infected by viruses, but each virus is very specific for what they infect.

One virus per organism.

What do virus and bacteriophages infect

1. Virus infect animals/plants

2. Bacteriophages are viruses that infect bacteria

Virus characteristics

1. small

2. acellular (no plasma membrane)

3. contains RNA/DNA surrounded by a protein coat

4. no inherent metabolism, but can greatly affect the metabolism of the host cell

5. unable to replicate on their own

Viruses are in their own category, not quite living not quite non living

What are viruses classified as

Obligate (must) intracellular (inside cell) parasite (replicate in cell)

Characteristics of virus sizes.

1. viruses have various sizes in NM

2. Use TEM to size viruses

3. Small viruses usually contain RNA

4. Bigger usually contain DNA

What is the structure of a virus called

1. A virion

2. It is a complete, fully developed, infectious viral particle that is a vehicle of transmission from one host cell to another

3. Its the outside of the cell

What does a virion contain (information wise)

1. nucleic acids

2. Either DNA or RNA (never both)

3. Either single stranded or double stranded

4. Can be linear or circular

5. single molecules or multiple pieces/segments

What are the different types of genomes that a virus has

1. dsDNA

2. ssDNA

3. dsRNA

4. ssRNA

ds = double strand

ss = single strand

What does a virion contain (protection wise)

1. capsids

2. Its a protein coat surrounding the nucleic acid

3. Made of capsomeres (protein subunits)

4. arrangements vary and usually characteristic for their type

What are naked viruses (nonenveloped)

1. They are virions with just a capsid and nucleic acid

What are enveloped viruses

1. They contain a protein and lipid layer surrounding the capsids

What do enveloped viruses contain

1. They are sometimes formed from viruses leaving the host cell

2. Some contain SPIKES which are carbohydrate protein complexes that are used to attach to host cells

What do enveloped viruses mostly infect

1. They infect ANIMAL cells because they lack a CELL WALL

2. This allows them to leave the cell easier by budding.

3. So naked viruses are more exclusive to organisms with cell walls

Do virions have enzymes

1. First assumed that they did not have any

2. Now there are known virions with enzymes

What do virion enzymes do

1. Some are associated with the envelope or capsid but most are within the capsid

2. Those within the capsid are usually involved in nucleic acid synthesis

3. Ex. neuraminidase, reverse transcriptase

How does the general virus life cycle go

1. A virion goes to a cell.

2. It inserts the DNA from the virion

3. The host cell replicates the viral DNA

4. The cell recreates more virions

5. The cell lysis and releases new virions.

What are the stages in the virus growth curve

1. Latent period

2. Eclipse

3. Maturation

What happens in the latent period

No infectious particles OUTSIDE the cell.

It initially only has the particles in the cell.

This includes the eclipse phase and the first part of maturation phase

What happens in the eclipse phase

After injection of the viral DNA.

Virus begins the process of replication, nucleic acid is naked within the cells.

What happens in the maturation phase

New virus particles get assembled from replicated nucleic acids and new capsid structures.

What happens after maturation.

The particles begin to be released from the cells

Most phages have proteins that act as a molecular clock to time the lysis of the cell with the completion of new particle assembly.

What is the one step growth curve structure

Its a single large event which produces the structure.

Its the ASSEMBLY AND RELEASE

How can you count or observe viruses.

1. They are very small, but some can be counted using a microscope (fluorescence microscopy, TEM)

2. Attaches to and fluoresces the nucleic acids in the structures.

How can you observe viruses from their detrimental effects on hosts.

1. Viruses are very small

2. You can spot them indirectly by observing the damage (growth inhibiton) or death (lysis) they cause to host cells

What is Plaque when observing viruses.

1. Viruses are mixed with a continuous layer of hosts cells (Lawn) on a medium like agar

2. The virus causes lysis or growth inhibition

3. This leaves a small visible clear spot called a Plaque

What causes the Plaque in observing viruses

1. An original infection of one cell is produced by one virus

2. The viruses produced keep infecting neighboring cells

What is the unit for the concentration of active viruses

plaque-forming units (PFU)

What is lytic replication

The lytic cycle is once the virus particles are replicated, the host cell dies (lysed) to release viruses.

What is burst size

The number of viruses released per cell

300-500 typically is the burst size

Do viruses have motility?

They don’t have motility, they just float

What are the stages of the lytic cycle

1. Attachment

2. Penetration

3. Synthesis

4. Assembly

5. Release

What happens in ATTACHMENT stage of lytic cycle

1. Virus/host is specificity is partially controlled by virus attachment to cell surface receptors.

2. Receptors can be proteins, glycoproteins, lipids, lipoproteins etc.

3. A cell with the appropriate receptor for virus entry is SUSCEPTIBLE

4. After entry, only PERMISSIVE cells replicate virus.

Permissive means if the cell can replicate the virus.

What happens in PENETRATION stage of lytic cycle

1. Some virus only inject nucleic acid (Virus > nucleic acid > new viruses)

2. Some also inject proteins (Virus > mRNA > proteins (capsid) > new viruses)

3. Others enter the cell completely and the capsid breaks down to expose the nucleic acid.

What happens in SYNTHESIS stage of lytic cycle

1. Viral nucleic acids and proteins are made (via transcription and translation) by HOST CELL MACHINARY (DNA/RNA polymerase)

2. The host cell uses the viral nucleic acids to make new viruses in the order below.

3. Early = replication proteins (to replicate nucleic acids. Or copying the viral genome)

4. Middle and late = structural proteins (the capsid etc.)

What happens in ASSEMBLY stage of lytic cycle

1. Viral pieces are then assembled to create complete virions

2. Its self assembled and doesn’t really use host.

What happens in RELEASE stage of lytic cycle

1. Viruses escape from the host cell

2. This can occur through cell lysis

3. Once the cell has died, the organic material in the cell is released too.

What are the two pathways that a virus can go once entering a cell

Lysogenic or Lytic pathway

The definition of Lysogeny

1. Phage DNA is incorporated into host cell DNA

What is the lysogenic pathway

1. Replication genes are not expressed and the DNA integrates into the cells genome.

2. The viral DNA is INTEGRATED into host DNA (becomes a PROPHAGE) and the viral DNA is replicated with the host DNA at cell division

3. There are various condition that can INDUCE the prophage to enter the lytic pathway.

What are the stages for replication of ANIMAL viruses

1. Attachment: Virion attaches to host cell

2. Entry*: Enter the cell

3. UNCOATING*: DNA is uncoated

4. Biosynthesis: Viral DNA is replicated, and some viral proteins are made

5. Maturation: Virions mature

6. RELEASE*: Virions are released

How does animal viruses enter

1. Endocytosis (For naked/nonenveloped viruses)

2. Fusion (for enveloped viruses)

How does Endocytosis work for entering animal cells (Naked/nonenveloped)

1. Virus is taken in through a vesicle, as if the cell is eating it

2. The virus then escapes from the vesicle leaving a free capsid (The capsid is free in the cell cytoplasm)

How does Fusion work for entering the animal cells (enveloped)

1. The envelope merges with the plasma membrane of the host cell

2. Leaving a free capsid in the cytoplasm

What is UNCOATING in animal viruses

1. The separation of the viral nucleic acid from the protein capsid

2. The capsid is either digested or released into the cytoplasm.

3. This exposed the viral nucleic acid

How does the RELEASE work in animal viruses

1. Viruses escape from the host cell

2. This can occur through budding

3. Budding (enveloped viruses) use the host membrane to reform envelope, may or may not cause cell death. This happens one virus at a time

For an example explain how Influenza gets replicated.