ch 12 Flashcards
What is a virus, size of virus, what microscope would you need to see one?
a tiny, noncellular (acellular) particle with a genome (DNA or RNA) contained by a capsid (protein coat
Some are enveloped, some are naked
Different shapes include icosahedral, filamentous, bullet shaped, amorphous, and complex like the bacteriophage
Filamentous- Helical symmetry; a helical tube around the genome, which is wound helically within the tube.
Tailed Bacteriophages-In a tailed phage, the icosahedral protein package, called the “head” is attached to an elaborate delivery device. The head contains the nucleic acid that is “injected” into the host cell envelope and DNA is released into the host cytoplasm.
Amorphous viruses-Some viruses have no symmetrical form. The nucleic acids from these amorphous viruses are contained by a flexible “core wall” that also encloses a number of enzymes and accessory proteins, similar to a cell’s cytoplasm. The core is enclosed loosely by a viral envelope studded with spike proteins
Viroids
virus-like infectious agents in which an RNA genome is itself the entire infectious particle. There is no protective capsid.
Most viroids infect plants, including many kinds of fruits and vegetables. They can cause serious economic losses in the citrus industry.
The viroid typically consists of a circular, single-stranded RNA that doubles back on itself to form base pairs interrupted by short unpaired loops.
Prions—do know what diseases are caused by a prion from slide
Prions are infectious agents that do not have nucleic acid at all, rather, they are misfolded proteins arising out of a preexisting cell.
Prions cause Creutzfeldt-Jakob Disease (CJD) in humans as well as Mad Cow Disease in cattle. Scrapie is a prion disease of sheep, and kuru is found in a small group of people who consumed the brains of deceased relatives.
The prion form of the protein (PrPsc) acts by binding to normally folded protein (PrPc) of the same class and altering the conformation to that of a prion.
Antigenic drift
Rapid mutation and evolution of a virus leads to antigenic drift—a population of viruses whose mutant proteins are no longer recognized by host antibodies.
Antigenic drift generates new strains of virus that can cause serious disease.
Know how viruses are classified (DNA vs. RNA, ss vs. ds, + sense vs. – sense)
*** You do not have to know the details of how they replicate except know that HIV uses reverse transcriptase to make DNA from RNA and that enzyme is the target of antiviral drugs to treat AIDS
Viruses of the same genome (such as dsDNA) are more likely to share ancestry with each other than with viruses of a different type of genome (such as RNA).
If single-stranded, whether the strand encodes protein or requires synthesis of a complement that encodes proteins.
Know the steps of replication and order (adsorption (attachment), penetration (entry), synthesis and maturation (assembly), and release (exit)
host recognition and attachment-adsorption–Viruses must contact and adhere to a host cell that can support their type of replication.
genome entry-penetration–The viral genome must enter the host cell and gain access to the cell’s machinery for gene expression.
assembly of progeny virions-synthesis, maturation–Viral components must be expressed and assembled. Components usually self-assemble spontaneously.
exit and transmission-Release– Progeny virions must exit the host cell and reach new host cells and, if multicellular, new hosts to infect.
Know the difference between lytic and lysogenic cycles including the terms prophage and induction associated with lysogeny
Lysogeny occurs when the phages retain the host DNA by integrating it into the genome of the host.
Lysogenic phages may carry genes that give the host cell an advantage (example: Shiga toxin gene in E. coli O157:H7).
- -The integrated phage genome is called a prophage.
- -The prophage is now replicated along with the host cell DNA.
- -The prophage may revert back to the lytic cycle by directing its own excision from the host genome and initiating lysis of the host cell
