Viruses Packet #47 Chapter #18

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 According to the theory of evolution, viruses may be bits of nucleic acid that “originally” escaped from animal, plant or bacterial cells. Some biologists 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 Bacteriophages are viruses that infect bacteria and more than 2000 have been identified. There are two possible reproductive cycles for viruses: - Lytic Lysogenic. Phycoviruses infect algae and some studies suggest that phages inhibit or control the growth rate of algal blooms.

Lytic Reproductive Cycle

Introduction When the virus infects a susceptible host cell, it forces the host to use its metabolic machinery to replicate viral particles. Afterwards, the virus lyses/destroys the host cell.

Lytic Reproductive Cycle Attachment (or absorption) Virus attaches to receptors on the host cell wall.

Lytic Reproductive Cycle Penetration Virus injects its nucleic acid into the cytoplasm of the host cell. Capsid of a phage remains on the outside

Lytic Reproductive Cycle Replication and Synthesis Once inside host, virus degrades the host cell nucleic acid Phage DNA is replicated and phage proteins are synthesized.

Lytic Reproductive Cycle Assembly Newly synthesized viral components are assembled (genome and proteins) into new viruses.

Lytic Reproductive Cycle 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

Introduction Viruses, that replicate via the lysogenic cycle, are called temperate viruses. Composed of a sleeper stage and the lytic cycle. During the “sleeper” stage, the host does not die immediately and carries the viral genome. When the virus is ready to multiply and be released, it utilizes the lytic cycle to complete the reproductive process—destroying the host.

Lysogenic Cycle II During the “sleeper” stage and, more importantly, once the viral genome becomes integrated into the host, the bacteriophage is referred to as a prophage. These prophages are also known as lysogenic cells.

Lysogenic Cycle III Once conditions are right, the temperate virus reverts back to the lytic cycle, producing new viruses and killing the host cell.

Viruses & Animal Cells

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 & Plants (Plant Cells)

Viruses Infecting Plants Plant viruses can be spread by insect vectors Once viruses are in the plant, they spread via the plasmodesmata The genome of most plant viruses contain RNA

Viruses Infecting Plants 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

Viroids & Prions Smaller than viruses. Viroid Prion Consists of a short strand of RNA with no protein coat Prion Consists only of protein. Cause transmissible spongiform encephalopathies (TSEs)

Review

Review Viruses Bacteriophages Viruses & Animal Cells Viruses & Plant Cells (Plants) Viroids & Prions