Unit 3 Edit 6.09
Viruses
What is a virus?

Obligate

Intracellular

Parasite
They use the host cell machinery to make and release large quantities of
viruses.
Taxonomy of Viruses
Viral species are a group of viruses sharing the same type of
genetic information or ecological niche.
Grouped according to :

Type of nucleic acid – RNA or DNA but never both; double or
single stranded; linear or circular or in pieces.

Replication strategy
o Lytic cycle – multiplication phase that result in host
death and the release of more viruses
o

Lysogenic phase – multiplication phase where the virus
remains latent in the host cell DNA, host cell is not lysed
Morphology
o Protein coat – subunits called capsomeres, arrangement
of these subunits is indicative of viral type.
o
Envelope – some have, some don’t – envelope is added
when the virus pushes itself out through the host cell’s
plasma membrane.
o
Spikes – some have, some don’t – reliable way to ID,
stimulate host’s immune system. Mutate easily
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Unit 3 Edit 6.09
Examples:
Tobacco mosaic Virus – helical
Adenovirus – Polyhedral
Bacteriophage – Complex
Culturing Viruses is difficult
 Often can only be cultured with host cells
Culture Methods
 In living animals (host cells)
o Advantages
o
Problems
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
Embroynated eggs
o Advantages
o

Problems
Cell cultures
o Advantages
o
Problems
Bacterial Replication – most of this information known by studying
bacteriophages
Most viruses only contain a few genes (structural components),
the host provides enzymes for protein synthesis, replication, etc.
Two types: Lytic cycle and Lysogenic cycle
Lytic Cycle stages

Attachment

Penetration

Biosynthesis

Maturation

Release
Lysogenic Cycle Stages

Penetration

Phage DNA combines with host cell DNA
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
Spontaneous event can lead to excision of phage DNA and
initiation of lytic cycle
o
Results of lysogenic cycle
1. Immune to infection by another of same virus
2. Phage conversion = new properties
3. Specialized transduction – pieces of bacterial
DNA are packaged in virus coat and moved
Terminology
 Burst time

Burst size

Eclipse time
Animal Virus Replication is different

Penetration by endocytosis

Exit by budding

Receptor sites are inherited

Uncoating
Viruses and Cancer

Can go unrecognized because the viruses invade but do not
cause cancer immediately

Not contagious as most viral diseases

Virus can be activated to abnormal function by mutagenic
agents
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Unit 3 Edit 6.09
Vertical gene transfer – plants and animals transfer genes to their
offspring
Horizontal gene transfer – Bacteria can transfer genetic material to each
other as well as their offspring.

Conjugation

Transduction

Transformation
Conjugation

Plasmid = circular DNA

Requires cell to cell contact

Opposite mating type

Plasmid carries gene for sex pili
Transduction

Bacterial DNA broken down by viral enzyme

Pieces of bacterial DNA packaged in new viruses

Transferred to new host cell

Recombination

Specialized transduction – only particular genes are transferred
via transduction
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Unit 3 Edit 6.09
Transformation

Genes are transferred as naked DNA from one bacteria to
another and recombination occurs

Frederick Griffith, 1928
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