VIRUSES

Characteristics of Viruses• Tiny, non-livingnon-living particles

• Do not respire, move or grow

• Cannot reproduce on their own– Must reproduce inside of a host

cell

• Typical infection = 7-10 days

• Not given Latin names: – named for disease caused– organ infected– region detected ***Draw Fig. 19-13 page 483

Virus Structure• All viruses = 2 basic parts: – nucleic acid– protein coat (capsid)

• Capsid: protein coat containing inner core of: DNA or RNA

• Protein coat gives viruses diff. shapes:– Polyhedral– Cylindrical– round, etc.

• May have tail fibers and projections to aid in attachment

• Some have extra outer layer called: Envelope

Viral Recognition & Attachment

• Virus must recognize specific receptor site on host cell

• Lands on host and “locks in”

• Attachment is specific: – virus can enter and reproduce

in only a few types of cells

• Ex: Tobacco Mosaic Virus cannot affect human cells even though we are exposed to it daily

                  

Viral Reproduction.• Viruses must reproduce inside of a host cell• They are classified as Intracellular Parasites• The method of entry into the host cell depends on the

virus• There are two main types of viral reproduction:

– Lytic cycle– Lysogenic

cycle

Steps of the Lytic Cycle

1. ATTACHMENT: Bacteriophage attaches to the cell surface of a bacterium

2. PENETRATION:DNA enters bacterial cell

3. REPLICATION:Bacteriophage is replicated & proteins are synthesized

4. ASSEMBLY:“Phage” components are assembled into mature viruses

5. RELEASE/LYSES:Bacterial cell lyses & releases many “phages” that can now infect other cells

Lysogenic Cycle• Viruses attack cells

– Do not kill cell immediately• Viral DNA is mixed with host DNA • Does not immediately begin to reproduce

– Host cellmay function normally while virus staysdormant

• Example: Herpes (cold sores, fever blisters)

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RetrovirusesRetroviruses

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Characteristics of Characteristics of RetrovirusesRetroviruses

•Contain RNA, not DNA•Family Retroviridae•Contain enzyme called Reverse Transcriptase•When a retrovirus infects a cell, it injects its RNA and reverse transcriptase enzyme into the cytoplasm of that cell

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RetrovirusesRetroviruses

•The enzyme reverse transcriptase (or RTase), which causes synthesis of a complementary DNA molecule (cDNA) using virus RNA as a template

RTasecopyright cmassengale

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Viroids & PrionsViroids & Prions

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ViroidsViroids•Small, circular RNA molecules without a protein coat•Infect plants•Potato famine in Ireland•Resemble introns cut out of eukaryotic copyright cmassengale

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PrionsPrions•Prions are “infectious proteins”• They are normal body proteins that get converted into an alternate configuration by contact with other prion proteins• They have no DNA or RNA•The main protein involved in human and mammalian prion diseases is called “PrP”copyright cmassengale

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Prion Prion DiseasesDiseases

•Prions form Prions form insoluble insoluble deposits in the braindeposits in the brain•Causes neurons to Causes neurons to rapidly degeneration.rapidly degeneration.•Mad cow diseaseMad cow disease (bovine spongiform (bovine spongiform encephalitis: BSE) is an encephalitis: BSE) is an exampleexample•People in New Guinea People in New Guinea used to suffer from used to suffer from kurukuru, which they got , which they got from eating the brains from eating the brains of their enemiesof their enemiescopyright cmassengale