Ch 6 Intro to Viruses Flashcards
What are Viruses?
- Acellular infectious agents smaller than cells
- Obligate intracellular parasites: can only multiple when it invades a host cell for genetic and metabolic material
What makes a virus so complicated?
- It causes disease in plants, animals, bacteria, protozoa, fungi, & algae
- There is no agreement as to how or when a virus originated
- Plays a role in the evolution of Bacteria, Archaea, and Eukarya
- Debate as to whether it is living or nonliving
- Does NOT classify as prokaryote or eukaryote
Viruses contain only the parts needed to ____________. Viruses bear no resemblance to cells because they _______________.
a. invade and control a host cell
b. Lack protein-synthesizing machinery
What structures make up a virus?
- Virus particle
- Covering -> Capsid & Envelope (some)
- Central Core -> DNA or RNA & Matrix protein enzymes (some)
Envelope
Def: Lipid Bilayer
Function:
- Exposed proteins on the outside of the envelope, called spikes used to attach themselves to the host
- Infect cells by fusion of the envelope with the host’s plasma membrane
Capsid
Def: Protein coat made up of capsomers
Function:
- Attach to host cells
- Protects Nucleic Acids
- Determines shape
The capsid together with the nucleic acid is the _________. Some viruses have an external covering called an ________; those lacking an envelope are __________.
a. nucleocapsid
b. envelope
c. naked
Two structural capsid types:
- Helical - continuous helix of capsomers forming a cylindrical nucleocapsid
- Icosahedral - 20-sided with 12 corners
Nucleic acids & their characteristics
- DNA: usually double-stranded, can be single, linear, or circular
- RNA: usually single-stranded, can be double-stranded, may be segmented into separate RNA pieces
Complex viruses
Def: asymmetrical structures
- lacks a typical capsid, may not be helical or icosahedral
- may have helical tail and attachment fibers
Give an example of an enveloped, naked, and complex virus
Envelope: mumps virus
Naked: plum poxvirus
Complex: poxviruse
Viral genome
- DNA or RNA
- Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses
ssRNA genomes ready for immediate translation are ______________.
ssRNA genomes that must be converted into proper form are _________________.
a. positive-sense RNA
b. negative-sense RNA
How are viruses classified?
Structure, Chemical composition, & Genetic make-up
How do animal viruses replicate?
Adsorption – virus attaches itself to the receptor of the host cell
Penetration – genome enters the host cell
Uncoating – the viral nucleic acid is released from the capsid
Synthesis – viral components are produced
Assembly – new viral particles are constructed
Release – assembled viruses are released to infect other cells
What makes the replication process of RNA and DNA viruses different?
RNA needs DNA to replicate, so it undergoes reverse transcription to create the DNA to replicate.
Traditional transcription: DNA = RNA = Protein
Reverse transcription: Protein = RNA = DNA
Penetration: Endocytosis
the entire virus is engulfed and enclosed in a vacuole or vesicle
Penetration: Fusion
envelope merges directly with membrane resulting in nucleocapsid’s entry into cytoplasm
DNA viruses generally are replicated and assembled in the __________.
nucleus
RNA viruses generally are replicated and assembled in the _________.
cytoplasm
Negative-sense RNA must be converted into ________________ because a positive-sense RNA contains the message for ____________.
a. positive-sense message
b. translation
What is the difference between translation and transcription?
How do bacteriophage viruses replicate?
Adsorption – virus sticks onto specific molecules on host cell
Penetration – genome enters host cell
Replication – viral components are produced
Assembly – viral components are assembled
Maturation – completion of viral formation
Lysis & Release – viruses leave the cell to infect other cells
