1. Viruses
Packet #47
Chapter #18

2. 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.

3. “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.

4. 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.

5. Lytic Reproductive Cycle

6. 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.

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

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

9. 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.

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

11. 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.

12. Lysogenic Cycle

13. 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.

14. 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.

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

16. Viruses & Animal Cells

17. 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

18. 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

19. 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.

20. Viruses & Plants (Plant Cells)

21. 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

22. 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

23. Viroids & Prions

24. 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)

25. Review

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