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)