Viruses

Characteristics of Viruses

• Minuscule, acellular, infectious agents having either DNA or RNA

• Obligate intracellular parasites

• Cause infections of humans, animals, plants, and bacteria

• Non-living

Are viruses living?

No, because…

• 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, or organelles (not even ribosomes)

Viron

individual viral particle

Capsomeres

the protein subunits that make up each capsid.

capsid

Composed of a protein coat

nucleic acid

The capsid surrounds

Intracellular state of Viruses

• Capsid is removed

• Virus exists as nucleic acid in host cell cytosol

host range

Most viruses infect only particular host’s cells

Host Range of Viruses

• May be so specific they only infect particular kind of cell in a particular host

• Generalists infect many kinds of cells or many different hosts

• All types of organisms are susceptible to some virus

• Generally, to be susceptible, the organism must have a certain type of receptor for the virus to interact with

virion shape

Viruses can be classified by

• Helical

• Polyhedral or Icosahedral

• Complex: generally any structure that doesn’t fall within the other two categories.

Three basic types of viral shapes

Bacteriophages

viruses that only attack bacteria.

Bacteriophage structure

• Viral genome is located within a polyhedral capsid

• The capsid attaches to the sheath, which is a helical structure

• At the end of the sheath are the tail fibers= Help the virus attach to the host cell surface

enveloped or naked (nonenveloped)

Animal viruses can be…

envelope

a small portion of the host cell phospholipid membrane and is obtained from the process of budding from the host cell.

lyse (burst) the cell and don’t obtain an envelope

Naked viruses often…

Yes

Do animal viruses- both enveloped and naked, have spikes for attachment?

• can be DNA or RNA but never both

• Can be dsDNA, ssDNA, dsRNA, or ssRNA

• May be segmented (several parts) or nonsegmented (one part)

• Much smaller than genomes of cells

Viral Genomes

Phages

Lysozyme to help break into host cytoplasm

RNA replicases

RNA-dependent RNA polymerases

Used to replicate RNA genomes in RNA viruses in animals

reverse transcriptase

Used in animal viruses as an enzynme that makes a complementary DNA molecule from viral RNA genome template

Viral species

a group of viruses sharing the same genetic information and ecological niche (host)

end in -virus

Naming Viruses: Genus name

end in -viridae

Virus Naming: Family name

T4, a Complex Bacteriophage

This bacteriophage specifically infects E. coli, a gram negative bacterium

Can be Lytic or Lysogenic

Dependent on hosts’ organelles and enzymes to produce new virions

Lytic replication

◦ Viral replication usually results in death and lysis (burst) of host cell

1. Attachment

2. Entry or Penetration

3. Synthesis

4. Assembly or Maturation

5. Release

◦ Five stages of lytic replication cycle

Burst time

the entire replication process from the time of attachment until release

Burst size

how many new virions are released from each bacterial cell.

100-200 virons

What is the burst size for Bacteriophage T4?

Lysogenic Replication of Bacteriophages

Modified replication cycle

Infected host cells grow and reproduce normally for generations before they lyse

Prophages

inactive temperate phages that are integrated into the host genome

Lysogen

name for a bacterial cell that has a prophage

Lysogenic conversion

Results when the phage carries genes that alter phenotype of a bacterium

Often makes bacterium more pathogenic

Steps of Lysogenic Replication

1.Attachment

2.Penetration or Entry

3.(Uncoating)

4.Synthesis

5.Assembly

6.Release

Steps in Animal Virus Replication

• Direct penetration

• Membrane fusion

• Endocytosis

Types of Animal Virus Entry

Direct penetration

Naked viruses inject their genomes into animal host

Membrane fusion

viral envelope merges with the cell membrane and capsid is released into cell cytosol. Uncoating of capsid to reveal viral genome follows.

Endocytosis

can occur for naked or enveloped virus. After attachment, entire particle is engulfed and internalized

retroviruses

RNA viruses that go against the central dogma by converting RNA into DNA using an enzyme called reverse transcriptase. The DNA can then be incorporated into the host genome and become a permanent part of the host!

provirus

integrated genome of a retrovirus

integrase

inserts the provirus