VIRUSES
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. 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 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 cell may function normally while virus stays dormant Example: Herpes (cold sores, fever blisters)
copyright cmassengale Retroviruses copyright cmassengale
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 copyright cmassengale
copyright cmassengale Retroviruses The enzyme reverse transcriptase (or RTase), which causes synthesis of a complementary DNA molecule (cDNA) using virus RNA as a template RTase copyright cmassengale
copyright cmassengale Viroids & Prions copyright cmassengale
copyright cmassengale Viroids Small, circular RNA molecules without a protein coat Infect plants Potato famine in Ireland Resemble introns cut out of eukaryotic copyright cmassengale
copyright cmassengale 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” copyright cmassengale
copyright cmassengale 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 copyright cmassengale