They Use the Host Cell Machinery to Make and Release Large Quantities of Viruses

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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

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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

Examples:

Tobacco mosaic Virus – helical

Adenovirus – Polyhedral

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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

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 Maturation

 Release

Lysogenic Cycle Stages

 Penetration

 Phage DNA combines with host cell DNA

 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

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 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

Vertical gene transfer – plants and animals transfer genes to their offspring

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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

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 Transferred to new host cell

 Recombination

 Specialized transduction – only particular genes are transferred via transduction

Transformation

 Genes are transferred as naked DNA from one bacteria to another and recombination occurs

 Frederick Griffith, 1928