1. VIRUSES

2. Characteristics of Viruses
Tiny, non-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 page 483

3. 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

4. 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
                  

5. 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

6. Steps of the Lytic Cycle
ATTACHMENT: Bacteriophage attaches to the cell surface of a bacterium
2. PENETRATION: DNA enters bacterial cell

7. 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

9. Lysogenic Cycle Viruses attack cells Do not kill cell immediately
Viral DNA is mixed with host DNA
Does not immediately begin to reproduce
Host cell may function normally while virus stays dormant
Example: Herpes (cold sores, fever blisters)

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Retroviruses
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11. Characteristics of Retroviruses
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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Retroviruses
The enzyme reverse transcriptase (or RTase), which causes synthesis of a complementary DNA molecule (cDNA) using virus RNA as a template
RTase
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Viroids & Prions
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Viroids
Small, circular RNA molecules without a protein coat
Infect plants
Potato famine in Ireland
Resemble introns cut out of eukaryotic
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Prions
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”
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Prion Diseases
Prions form insoluble deposits in the brain
Causes neurons to rapidly degeneration.
Mad cow disease (bovine spongiform encephalitis: BSE) is an example
People in New Guinea used to suffer from kuru, which they got from eating the brains of their enemies
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