1. The Genetics of Viruses
Viral Genetics
Microbiology:
The Genetics of Viruses

2. Viruses &Bacteria: simplest biological systems
smaller/ simpler than bacteria
Contain nucleic acid and protein ONLY
Not considered to be “living” b/c they need a host and NOT make out of a cell
Bacteria:
larger than viruses
Contain ALL marcomolecules
considered living

3. Virus
Bacterium
Animal
cell
Animal cell nucleus
0.25 µm

4. Bacteria with Bacteriophages infecting the cell:

5. What is a virus? Has a genome (RNA or DNA) but…
can reproduce only within a host cell
Scientists detected viruses indirectly long before they could see them
The story of how viruses were discovered begins in the late 1800s:
Tobacco mosaic disease stunts growth of tobacco plants  gives their leaves a mosaic coloration
In late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease
In 1935, Wendell Stanley confirmed this hypothesis

6. Tobacco Mosaic Disease

7. Viruses are not (living) cells!
Viral structure
Viruses are not (living) cells!
Virus in Latin means “poison”
Made up of infectious particles consisting of a nucleic acid (RNA or DNA) in a protein coat (and sometimes another membrane)
Viruses have a Capsid
protein shell encloses viral genome (nucleic acid)
A capsid can have various structures
Viral genomes may consist of
Double- or single-stranded DNA (dsDNA, ssDNA)
Double- or single-stranded RNA (dsRNA, ssRNA)

8. Viral Structure Images

9. Depending on viral type of nucleic acid…
A virus is called a
DNA virus or
an RNA virus
The smallest viruses have only 4 genes, while others have 100’s

10. HOST CELL

11. Some viruses have membranous envelopes  help them infect hosts
Surround capsids
Viral envelopes (made from host cell’s membrane) contains a combo of viral and host cell molecules
HELPS infect other target host cells

12. Capsid
Capsid and viral genome
enter cell
RNA
HOST CELL
Envelope (with
glycoproteins)
Viral genome (RNA)
Template
mRNA ER
Capsid
proteins
Glyco-
proteins
Copy of
genome (RNA)
New virus

13. General Features of Viral Reproductive Cycles
Viruses are intracellular parasites
they can reproduce only within a host cell
Each virus has a host range, a limited number of host cells that it can infect
Viruses uses HOST molecules to make more (“offspring”) viruses

14. Virus uses HOST: DNA Polymerase RNA Polymerase Ribosomes tRNA’s
Entry into cell and
uncoating of DNA
VIRUS
DNA
Capsid
Replication
Transcription
HOST CELL
Virus uses HOST:
DNA Polymerase
RNA Polymerase
Ribosomes
tRNA’s
Amino acids
Viral DNA
mRNA
Viral DNA
Capsid
proteins
Self-assembly of
new virus particles
and their exit from cell

15. Reproductive Cycles of Phages
Phages = best understood of all viruses
lytic cycle vs. lysogenic cycle
Kill infected bacterial host cells
BACTERIRAL DEFENCES INCLUDE:
Some bacteria have mutant receptor sites that phage can no longer bind to
2. Some bacteria also produce restriction endonucleases

16. Viral reproduction: Lytic Cycle
A phage reproductive cycle  ends in DEATH of host cell
Produces new phages and digests the host’s cell wall, releasing the progeny (“offspring”) viruses
A phage that reproduces only by the lytic cycle is called a virulent (strong) phage
EXAMPLES:
Flu, common cold, Rabies

17. A bigger perspective of the Lytic Cycle:
Attachment
Entry of phage DNA
and degradation of
host DNA
Phage assembly
Release
Head
Tails
Tail fibers
Assembly
Synthesis of viral
genomes and proteins

18. Viral reproduction: Lysogenic Cycle
Think “S” for “Silent”
Replicates viral genome (DNA) without destroying host cell
Viral DNA becomes incorporated into host cell’s DNA (called prophage)
Every time host divides, phage DNA is copied/passed to daughter cells
Virus spread without killing host cells
Occasionally, virus switches to lytic cycle
Temperate virus
Viruses capable of using the lytic and lysogenic cycles.
EXAMPLES: HIV and Herpes

19. A bigger perspective of the Lysogenic Cycle:
Phage
DNA
Daughter cell
with prophage
The phage attaches to a
host cell and injects its DNA.
Many cell divisions
produce a large
population of
bacteria infected with
the prophage.
Phage
Phage DNA
circularizes
Occasionally, a prophage
exits the bacterial chromosome,
initiating a lytic cycle.
Bacterial
chromosome
Lytic cycle
Lysogenic cycle
Certain factors
determine whether
Prophage
The bacterium reproduces
normally, copying the prophage
and transmitting it to daughter cells.
The cell lyses, releasing phages.
Lysogenic cycle
is entered
Lytic cycle
is induced or
Phage DNA integrates into the
bacterial chromosomes, becoming a
prophage.
New phage DNA and proteins are
synthesized and assembled into phages.

20. RNA viruses = Retroviruses
Retroviruses: transcribe DNA from an RNA template (RNA DNA)
These viruses use Reverse transcriptase (catalyzing enzyme)
Ex: HIV  AIDS

21. Retrovirus (HIV) Viral envelope Glycoprotein Capsid RNA (two identical
strands)
Reverse
transcriptase

22. LE 18-10 HOST CELL Reverse transcription Viral RNA RNA-DNA hybrid
Membrane of
white blood cell
HIV
HOST CELL
Reverse
transcription
Viral RNA
RNA-DNA
hybrid
0.25 µm
HIV entering a cell
DNA
NUCLEUS
Provirus
Chromosomal
DNA
RNA genome
for the
next viral
generation
mRNA
New HIV leaving a cell

23. Retrovirus Animations

24. Vaccines Vaccines Harmless derivatives (ex: viral proteins) of viruses
Vaccines
Harmless derivatives (ex: viral proteins) of viruses
stimulates immune system to create antibodies against actual pathogen
Vaccination has eradicated smallpox
Effective vaccines are available against polio, measles, rubella, mumps, hepatitis B, and more

25. Medical Technology cannot…
Treat a viral infection that has already occurred (vaccines do not ‘kill’ viruses)
Antibiotics do not treat viral infections
Antiviral drugs resemble nucleoides and interfere with the viral nucleic acid synthesis
Valtrex stops herpes virus reproduction by inhibiting viral DNA polymerase that synthesizes viral DNA
Azidothymidine (AZT) curbs HIV reproduction by interfering with DNA synthesis by reverse transcriptase
Currently the most effective treatment is a ‘cocktail’ multidrug treatment method

27. Small Pox

28. Polio
Polio

29. Herpes Simplex

30. Hepatitis
                          

31. Varicella Zoster (chicken pox)

32. Mumps

33. Measles - Rubeola