What is a virus?
- Miniscule, acellular, infectious agent having either DNA or RNA
- Causes many infections of humans, animals, plants, and bacteria
- Causes most of the diseases that plague the industrialized world
e.g. common cold, influenza, herpes, SARS, AIDS
List the characteristics of Viruses.
- Cannot carry out any metabolic pathway
- Neither grow nor respond to the environment
- Cannot reproduce independently
- Recruit the cell’s metabolic pathways to increase their numbers
- No cytoplasmic membrane, cytosol, organelles
- Have extracellular and intracellular state
What are the characteristics of an extracellular state?
- Called virion
- Protein coat (capsid) surrounding nucleic acid
- Nucleic acid and capsid also called nucleocapsid
- Some have phospholipid envelope
- Outermost layer provides protection and recognition sites for host cells
What are the characteristics of an Intracellular state?
- Capsid removed
- Virus exists as nucleic acid
What is the genetic material of viruses?
- Show more variety in nature of their genomes than do cells
- Primary way scientists categorize and classify viruses
- May be DNA or RNA, but never both
- Can be dsDNA, ssDNA, dsRNA, ssRNA
- May be linear and segmented or single and circular
- Much smaller than genomes of cells
What types of hosts does a virus infect?
- Most viruses infect only particular host’s cells
- Due to affinity of viral surface proteins for complementary proteins on host cell surface
- May be so specific they only infect particular kind of cell in a particular host e.g. HIV
- Generalists – infect many kinds of cells in many different hosts including bacteria, archaea, protozoa, fungi, plants, animals and humans.
E.g. Influenza virus, West Nile virus
What are the sizes of viruses compared to other organisms?
Red Blood cell: 10,000nm in diameter
Bacterial ribosomes: 25nm
Poliovius: 30nm
Bacteriophage T4: (50nm x 225nm)
Smallpox virus : (200nm x 300nm)
Tabacco mosaic virus: ( 15nm x 300nm)
They are really small!
What is the history of viruses?
Ivanowski first demonstrated in 1892 that viruses are acellular
Conducted an experiment to prove acellularity
Stanley in 1935 isolated and characterized tobacco mosaic virus
What are the characteristics of Capsid Morphology?
- Capsids:Protein coats that provide protection for viral nucleic acid and means of attachment to host’s cells
- Composed of proteinaceous subunits called capsomeres
- Capsomere may be made of single or multiple types of proteins
Name the shapes of virions.
- Helical (TMV)
- Polyhedral (common cold)
- Complex (smallpox virus and rabies virus)
What are enveloped viruses?
- A virus with an outer envelope surrounding the capsid is an enveloped virion; virion without an envelope is called a non-enveloped or naked virion.
- Matrix proteins fill the region between capsid and envelope.
How are viral envelopes acquired by a virus?
- Acquired from host cell during viral replication or release
- Envelope is portion of membrane system of host
- Composed of phospholipid bilayer and proteins
- Some proteins are virally coded glycoproteins (spikes)
- Envelope’s proteins and glycoproteins often play role in host recognition
What are the classification of viruses?
Classified based on the type of nucleic acid, presence of envelope, shape and size
What are the characteristics of Viral Replication?
Dependent on hosts’ organelles and enzymes to produce new virions
Lytic replication
Replication cycle usually results in death and lysis of host cell
Stages of lytic replication cycle
Attachment
Entry
Synthesis
Assembly
Release
Describe the lytic cycle in a bacteriophage
What is burst time?
Period of time required to complete the entire process of synthesis, assembly and release
What is Burst size?
The number of new virions released per lysed host cell
What is lysogeny and describe the lysogenic replication cycle.
Lysogeny
- Modified replication cycle
- Infected host cells grow and reproduce normally for generations before they lyse
- Temperate phages or lysogenic phages
- Prophages – inactive phages
- Lysogenic conversion results when phages carry genes that alter phenotype of a bacterium
What are some of the characteristics of animal viruses?
- Same basic replication pathway as bacteriophages
- Differences result from
- Presence of envelope around some viruses
- Eukaryotic nature of animal cells
- Lack of cell wall in animal cells
How do animal viruses attach to the host cell?
- Attachment of animal viruses
- Chemical attraction
- Animal viruses do not have tails or tail fibers
- Have glycoprotein spikes or other attachment molecules that mediate attachment
What are the three mechanisms of entry of animal viruses?
- Direct penetration
- Membrane Fusion
- Endocytosis
How are animal viruses synthesized?
- Synthesis of animal viruses
- Each type of animal virus requires different strategy depending on its nucleic acid
- DNA viruses often enter the nucleus, except poxviruses
- RNA viruses often replicate in the cytoplasm
Must consider:
- How mRNA is synthesized
- What serves as template for nucleic acid replication
How does the replication of dsDNA viruses occur?
Similar to normal replication of cellular DNA and translation of proteins.
dsDNA -mRNA - viral proteins
How does the replication of ssDNA viruses occur?
A new strand of complementary DNA is made which then binds to the ssDNA of the virus to form a dsDNA molecule. Replication then proceeds like dsDNA virus.
ssDNA - dsDNA - mRNA - viral proteins
