Viruses

Packet #24

Introduction A virus, or virion, is a tiny particle consisting

of DNA or RNA surrounded by a protein coat called a capsid.

Viruses cannot metabolize on their own and biologists consider them as non-living particles.

“Evolutionary” Connection Viruses may be bits of nucleic acid that

“originally” escaped from animal, plant or bacterial cells.

Some biologists, that believe in the theory of evolution, hypothesize that viruses evolved before the three domains diverged. Thought is that it is unlikely that viruses, that

infect Archaea and Eubacteria, evolved twice.

Bacteriophages Viruses that infect

bacteria. Viral reproductive

cycle can be lytic or lysogenic.

More than 2000 phages have been identified. Phycoviruses infect

algae and some studies suggest that phages inhibit or control the growth rate of algal blooms.

Lytic Reproductive Cycle The virus lyses/destroys the host cell. When the virus infects a susceptible host cell,

it forces the host to use its metabolic machinery to replicate viral particles.

Lytic Reproductive Cycle Attachment (or absorption)

Virus attaches to receptors on the host cell wall.

Penetration Nucleic acid of the virus,

phage DNA, moves through the plasma membrane and into the cytoplasm of the host cell.

Capsid of a phage remains on the outside

Replication and Synthesis Viral genome contains all

information needed to produce new viruses.

Once inside host, virus degrades the host cell nucleic acid

Phage DNA is replicated Phage proteins are

synthesized.

Lytic Reproductive Cycle Assembly

Newly synthesized viral components are assembled into new viruses.

Release Assembled viruses are

released from the cell. Lytic enzymes produced

late in the replication process destroys the host cell membrane. Animal viruses normally

are released slowly and bud off the plasma membrane.

Phages released are used to infect other cells.

Lysogenic Cycle Temperate viruses do

not always destroy their hosts.

In the lysogenic cycle, the viral genome usually becomes integrated into the host bacterial DNA and is then referred to as a prophage.

Bacterial cells, that carry prophages, are lysogenic cells

Lysogenic Cycle Attachment

The phage attaches to the cell surface of the bacterium.

Penetration Phage DNA enters the

bacterial cell Integration

Phage DNA integrates into bacterial DNA Bacterial cell is now called

a prophage Replication

The integrated prophage replicates when bacterial DNA replicates. The newly replicated cells

may exhibit new properties

Viruses Infecting Animal Cells Viruses enter animal cells via membrane fusion or

by endocytosis. Membrane fusion

Viral capsid and nucleic acid are both released into the animal cell while the viral envelope remains fused with the plasma membrane

Endocytosis Plasma membrane of animal cell invaginates to form a

membrane bound vesicle that contains the virus Including the viral envelope.

Viral nucleic acid is replicated within the host cell Proteins are synthesized New viruses are assembled and released from the

cell

Classes of Animal Viruses Viruses may contain

DNA or RNA Double stranded DNA

(dsDNA) Single stranded DNA

(ssDNA) Double stranded RNA

(dsRNA) Single stranded RNA

(ssRNA) Three classes

Retrovirus RNA viruses that have a

DNA polymerase named reverse transcriptase.

Enzyme is used to transcribe the RNA genome into a DNA intermediate

DNA becomes integrated into the host DNA

Viral RNA is synthesized by the host RNA polymerases HIV (Human

Immunodeficiency Virus) behaves in this fashion.

Viruses Infecting Plants Plant viruses can be spread by insect vectors Viruses spread through plants via the

plasmodesmata The genome of most plant viruses contain RNA Symptoms

Reduced plant size Spots Streaks Mottled patterns in leaves, flowers or fruits

Infected crops produce lower yields Cures are not known for most viral diseases of

plants Infected plants are normally burned Scientists today are trying to develop virus-

resistant strains of important crops

Viroids & Prions Smaller than viruses. Viroid

Consists of a short strand of RNA with no protein coat

Prion Consists only of

protein. Cause transmissible

spongiform encephalopathies (TSEs)