Chapter 6- Viruses

Capsid

• Protein shell surounding the viral

• Protects nucleic acid

• Helps the virus attach & enter host cells

• Made of repeating protien units (capsomeres)

Nucleocapsid

• Capsid + Nucleic acid

• In many viruses, the RNA genome together with its associated protiens & capsid forms the _____

Enveloped

These virsuses have an additional lipid membrane surrounding the nucleocapsid

Ex: Influenza virus, HIV, coronavirus, herpesvirus

Naked (non-enveloped)

These viruses lack a lipid envelope

Ex: Adenovirus, poliovirus, norovirus

Helical

Capsid protiens are arranged in a spiral pattern around the nucleic acid

Icosahedral

Capsid has this shape consisting of 20 triangular faces & 12 vertices

Complex

These viruses do NOT fit neatly into either helical or icosahedral symmetry

Ex: Poxviruses

dsDNA

Herpesvirus and Adenovirus are examples of _____

ssDNA

Parovirus is an example of _____

dsRNA

Rotavirus is an example of ____

+ssRNA

Poliovirus and Coronavirus are examples of _____

-ssRNA

Influenza virus and Rabies are examples of _____

Positive-Sense (+) RNA

• Viral RNA can function directly as mRNA

• The host ribosome can immediatly use it to make viral proteins

• Ex: _RNA enters the cell → ribosome reads it → viral protiens are produced

Negative-Sense (-) RNA

• Viral RNA CANNOT be directly translated by the ribosome

• It must first be copied into the _RNA

• The virus must bring an RNA-dependent RNA polymerase with it or produce one

• Ex: _RNA → _RNA → protein

DNA Viruses

DNA → RNA → Protein

+RNA Viruses

+RNA → Protein

-RNA Viruses

-RNA → +RNA → Protein

dsRNA Viruses

dsRNA → +RNA → Protein

Retroviruses

+RNA → DNA → Viral DNA → mRNA → Protein

1st stage of Lytic Cycle

The phage attaches to the surface of the host

2nd stage of the Lytic Cycle

The viral DNA enters the host cell

3rd stage of the Lytic Cycle

Phage DNA replicates & phage proteins are made

4th stage of the Lytic Cycle

New phage particles are assembled

5th stage of the Lytic Cycle

The cell lyses, releasing the newly made phages

1st stage of the Lysogenic Cycle

The phage infects a cell

2nd stage of the Lysogenic Cycle

The phage DNA becomes infcorportated into the host genome

3rd stage of the Lysogenic Cycle

The cell divides & prophage DNA is passed on to daughter cells

4th stage of the Lysogenic Cycle

Under stressful conditions, the prophage DNA is excised from the baterial chromosome & enters the lytic cycle

5th stage of the Lysogenic Cycle

Phage DNA replicates & phage proteins are made

6th stage of the Lysogenic Cycle

New phage particles are assembled

7th stage of the Lysogenic Cycle

The cell lysis, releasing the newly made phages

Temperate phages

Replicate using both lytic and lysogenic cycles

Lysogenic Conversion

Occurs at the end of lysogenic cycle

1st stage of the Lysogenic Conversion

The phage infects a cell

2nd stage of the Lysogenic Conversion

The phage DNA becomes incorporated into the host genome

3rd stage of the Lysogenic Conversion

The phage is excised from the bacterial chromosome along with a short piece of bacterial DNA. The DNA is then packaged into newly formed capsids.

4th stage of the Lysogenic Conversion

Phage containing both viral and bacterial DNA infect a new host cell.

5th stage of the Lysogenic Conversion

The phage DNA, along with the attached bacterial DNA, are incorportated into the new cell

1st stage of Viruses with Animal Hosts

Influenza virus becomes attached to a target epithelial cell

2nd stage of Viruses with Animal Hosts

The cell engulfs the virus by endocytosis

3rd stage of Viruses with Animal Hosts

Viral contents are released

4th stage of Viruses with Animal Hosts

Viral RNA enters the nucleus, where it is replicated by the viral RNA polymerase

5th stage of Viruses with Animal Hosts

New phage particles are assembled

6th stage of Viruses with Animal Hosts

New viral particles are made and released into the extracellular fluid. The cell, which is not killed in the process, continues to make new virus.

Bacteriophages

• Tail fibers attach to cell wall protiens

• Viral DNA injected into host cell

• Not required

• In cytoplasm

• Lysogeny

• Host cell lysed

Animal Viruses

• Attachment sites are plasma membrane proteins and glycoproteins

• Capsid enters by endocytosis or fusion

• Enzymatic removal of capsid proteins

• In nucleus (DNA viruses) or cytoplasm (RNA viruses)

• Latency; slow viral infections; cancer

• Enveloped viruses bud out; nonenveloped viruses rupture plasma membrane

Latent Infections

Maintain viral genome in the host cell without replacing (latency)

• Herpes simplex virus

• Herpes zoster virus

• Epstein-Barr virus

Chronic Infections

Shed the virus continuously for a long period of time

• Hepatitis C Virus

• HIV

Cultivating Virus

Obligate intracellular parasites

• In Vitro

• In Vivo

In Vitro

Virus is cultivated outside the organism in a controlled laboratory environment

Ex: Cells/tissue cultures

In Vivo

Virus is cultivated inside a living organism

Ex: Laboratory animals, embryonated chicked eggs

Distinct observable cell abnormalities due to viral infection

• Changes in size & shape

• Cytoplasmic inclusion bodies

• Cells fuse to form multinucleated cells

• Cell lysis

• Alter DNA (mutations)

• Transform cells into cancerous cells

Viroids

An infectious RNA particle, smaller than a virus, lacking a capsid that causes various plant diseases

Virusoid (Satellite nucleic acids/Viruses)

• Small ssRNA molecule lacking a capsid

• Lacks genes required for replication; requires a helper (satellite) virus to replicate

• Causes various plant diseases

Prions

• Misfolded proteins; no nucleic acid

• Extremely resistant to usual sterilizaiton techniques

• Cause transmissible spongiform encephalopathies

  • Fatal neurodegnerative diseases

Prions Disease

• Scrapie in sheep & goats

• Bovine spongiform encephalopathies (BSE), aka mad cow disease

• Wasting disease in elderly, moose & deer

• Creutzfeldt- Jakob Syndrom (CJS) & Kuru in humans