Chapter 6 Flashcards
(32 cards)
What are the different acellular agents?
- Viruses
- Viroids
- Satellites
- Prions
What are bacteriophages? Are there archae viruses? Eukaryotic viruses?
- Bacteriophages are viruses that infect bacteria
- There are few archaeal viruses
- Most are eukaryotic viruses (plants, fungi, animals, and protisits)
What is a virion?
- A complete virus particle
- Cannot reproduce independent of living cells nor carry out cell division
Describe the important characteristics of virions.
- Range in size from about 10 to 400 nm in diameter
- Constructed of a nucelocaspid that is composed of either DNA or RNA (nucleic acid) and a protein coat called the capsid
- Capsids protect viral genetic material and aids in its transfer between host cells
- Capsid proteins are called protomers
- There are three symmetry types of capsids including helical, icosahedral
- Some virions have envelopes and are called enveloped viruses, whereas those lacking an envelope are called noneveloped. These only contain a nucleocaspid
What are helical capsids?
- Shaped like hollow tubes with protein walls
- Tobacco mosaic virus is a well-studied example of helical capsid structure
- Protomers assemble into a helical tube enclosing an RNA genome. Some are rigid and some are more flexible
What are icosahedral capsids?
- A regular polyhedron with 20 equilateral triangular faces and 12 vertices
- These are the most efficient way to enclose a space
- Constructed from ring or knob shaped units called capsomers made of 5 to 6 protomers
What are some examples of virions that have capsids with complex symmetry?
- Some viruses do not fit into the category of having helical or icosaderal capsids
- Ex: Poxvirus: among the largest of animal viruses. Posses an complex internal structure with an ovoid to brick shaped exterior (vaccina virus)
- Ex: Virions of T2, T4, and T6 that infect E.coli ar said to have binary symmetry because they have head that resembles an icosahedron and a tail that is helical
Describe viral envelopes and enzymes.
- The nucleocapsids of many animal viruses, some plant viruses, and at least one bacterial virus are surrounded by an outer membranous layer called an evelope
- Animal virus envelopes usually arise from the plasma or nuclear membrane of host cell
- However, envelope proteins are coded by viral genome and project from surface as spikes (called peplomers)
- Spikes are important for in viral attachment to host cell and are used for the identification of viruses
What is the function of the spikes on the influenza virus?
- Some spikes consist of the enzyme neuraminidase, which functions in the release of mature virions from the host cell
- Other spikes are hemagglutinin proteins because they bind virions to red blood cells to clump together, a process called hemagglutination.
What is the difference between cellular genomes and virus genomes?
- Cellular genomes are always double-stranded (dsDNA)
- Viruses on the other hand employ all four possible nucleic nucleic acid types including both single stranded and double stranded RNA and DNA
What is the general cycle of virus multiplication?
- Attachment of virion to host cell
- Entry of viral nucelocapsid
- Synthesis of viral proteins and nucleic acids
- Self-assembly of virions
- Release of progeny virions
How do viruses attach to host cell?
- Attachment to a host cell is accomplished by specific interactions between molecules on the surface of the virion (ligands) and molecules on the surface of the host cell called receptors
- Plant viruses DO NOT have to attach
- Binding of an animal virus particle to its receptor often causes conformational changes in virion proteins that facilitate interaction with secondary receptors, entry into the host, and uncoating.
What does tropism mean?
- Tissue specific receptors for viruses
How do virions attach to and enter the virus cell since they do not have receptors like animal cells?
Damage of host cell is required for the virus particles to access and enter the host. This is often achieved by plant-eating insects that carry virions from one plant to another. The virions are then deposited into plant tissues when they devour it.
In what way do bacteriophages and eukaryotic viruses enter the cell?
- Many bacteriophages inject their nucleic acid into the cytoplasm of their host cell, leaving the capsid outside and attached to the cell
- In contrast to phages, the nucelocaspid of many viruses of eukaryotes enters the cytoplasm with the genome still enclosed. Once inside the cytoplasm, some shed their capsid proteins in a process called uncoating
What are the three ways in which viruses enter the cell?
- Fusion of the viral envelope with the host cell membrane
- Entry by endocytosis
- Injection of viral nucleic acid into the cytoplasm if the host cell, leaving the capsid outside
Describe entry of enveloped virus by fusion with plasma membrane
- Viral envelope spikes bind to receptors on surface of host cell
- Lipid bilayer of viral envelope fuses with host cell membrane
- Nucelocapsid is released into cytoplasm
Describe entry of enveloped virus by endocytosis.
- Viral envelope spikes bind to receptors on the cell’s surface
- Binding to the receptor triggers receptor-mediated endocytosis
- Increased acidity allows nucleocapsid to escape from the endosome ad enter the cytoplasm
Describe entry of non-enveloped virus by endocytosis.
- Capsid proteins bind to receptors on cell surface and trigger receptor-mediated endocytosis
- Nucleic acid is extruded from the endosmone into the cytoplasm
Why does the stage of the viral life cycle differ dramatically among viruses?
Because the genome of a virus dictates the events that occur.
How are double stranded DNA viruses synthesized?
- The synthesis stage can very similar to the typical flow of information in cells.
- The genetic information is stored in DNA and replicated by enzymes called DNA polymerase, recorded as mRNA (transcription) and decoded during protein synthesis (translation)
- These viruses can rely soley on their host cell’s biosynthetic machinery to replicate their genomes and synthesize their proteins
How do RNA viruses replicate?
- Cellular organisms lack the enzymes needed to replicate RNA or to synthesize mRNA from RNA genome.
- Therefore, RNA viruses must carry in their nucleocapsids the enzymes needed to complete the synthesis stage, or they must be synthesized during the infection process.
- Synthesis of viral proteins is highly regulated
- Some proteins are synthesized early (early proteins) by early genes. Other proteins including capsid proteins are synthesized later (later proteins) by later genes
What all is involved in assembly and virion release?
- Several kinds of late proteins are involved in the assembly of mature virions and in release
- Proteins and other factors synthesized by the host cell may be involved in assembling mature virions
What is the assembly process along with the subassemblies?
Once the baseplate is finished, the tail tube is built on it and the sheath is assembled around the tube. The page prohead (procaspid) is constructed with the aid of scaffolding proteins that are degraded or removed after assembly is completed. DNA is incorporated into the prohead by a complex of proteins sometimes called the “packasome”. The packasome consists of proteins called the portal protein, which is located a the base of the prohead, and a set of proteins called the terminase complex, which moves DNA into the prohead. The movement of DNA consumes enrgy in the form of ATP, which is supplied by the metabolic activity of the host bacterium. After the head is completed, it spontaneously combines the tail assembly.
