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All eukaryotes act as __ for viruses of one kind or another. Indeed, most eukaryotes are __ by a broad range of virus types. In many respects, eukaryotic viruses resemble __, but the fact that their hosts are eukaryotes rather than bacteria forces some differences upon them. Their genes have to __ and so must __. They therefore need the __ required to __, and they may contain __.
- susceptible to infection
- be expressed within eukaryotic cells
- resemble eukaryotic genes
- complex upstream sequences
- activate transcription by RNA polymerase II,
The capsids of eukaryotic viruses are either __ or __—the __ is unique to bacteriophages
- icosahedral or filamentous
- head-and-tail structure
A second distinct feature of eukaryotic viruses, especially those with animal hosts, is that __. This membrane is derived from the host when the new virus particle __, and may subsequently be modified by __
- the capsid may be surrounded by a lipid membrane, forming an additional component to the virus structure
- leaves the cell
- insertion of virus-specific proteins
Virus genomes have a lot of structures: __ Plant viruses usually have __ genomes
- DNA, RNA< ss, ds, linear, circular, nonsegmented, segmented
Most euk viruses follow only the __, but few of these viruses do what?
- lytic infection cycle
- take over the host cell’s genetic machinery in the way that a lytic bacteriophage takes over a host bacterium
Many viruses __. The host-cell functions cease only toward the end of the infection cycle, when __
coexist with their host cells for long periods
the virus progeny that have been stored in the cell are released
Viral retroelements do what?
integrate into the host-cell DNA
Many eukaryotic viruses can do what, but this doesn't mean that there aren’t euk equivalents to __
Some viruses can __.
set up long-term infections without integrating into the host-cell DNA
integrate into the genomes of the hosts
Viral retroelements are an example. What are the two?
Retroviruses= capsids contain the RNA version of the genome
Pararetroviruses= encapsidated genome is made of DNA
The ability of viral retroelements to convert RNA into DNA requires an enzyme called __, which is capable of making a DNA copy of an RNA template.
After entry into a cell, a retroviral genome is __ .
What then happens?
copied into the double-stranded DNA by a few molecules of reverse transcriptase that the virus carries in its capsid
The double-stranded version then integrates into the host DNA
Unlike lambda, the retroviral genome has __
no sequence similarity with its insertion site in the host DNA and appears to insert at more or less random positions
__ is a prerequisite for expression of the retrovirus genes. There are three of these, called __(3)__. Each codes for a polyprotein that is __, after translation, into __, which include the __ and the __
- Integration of the viral genome
- gag, pol, and env
- two or more functional gene products
- virus coat proteins (from env) and the reverse transcriptase (from pol)
The protein products do what to what?
combine with full-length RNA transcripts of the retroviral genome
produce new virus particles
Several retroviruses can __.
Cell transformation involves __.
induce cell transformation, possibly leading to cancer
changes in cell morphology and physiology
In cell cultures, transformation results in a __, so that transformed cells do what? In whole animals, __ is thought to underlie the development of tumors
- loss of control over growth
- grow as a disorganized mass, rather than as a monolayer
- cell transformation
There are two distinct ways in which retroviruses cause cell transformation. What is the first?
With some retroviruses, cell transformation is a natural consequence of infection, although it may be induced only after a long latent period during which the integrated form of the virus lies quiescent within the host genome
There are two distinct ways in which retroviruses cause cell transformation. What is the second?
Other retroviruses cause cell transformation because of abnormalities in their genome sturctures. These acute transforming viruses carry cellular genes that they have captured from previous cells that they have infected
In Rous sarcoma virus, which is a transforming retrovirus, this cellular gene ( the one that is captured from previous cells) is __.
In others the cellular gene __. In the latter case, the retrovirus may be defective (unable to replicate and produce new viruses). Why?
- present side by side with the standard retroviral genes
- replaces part of the retroviral gene complement
- it has lost genes coding for vital replication enzymes and/or capsid proteins
These defective retroviruses are not always inactive, however, as they can __
make use of proteins provided by other retroviruses in the same cell
The ability of an acute transforming virus to cause cell transformation lies in the __. Often this captured gene is a __, a gene that codes for __
- nature of the cellular gene that has been captured
- v-onc gene (v= viral/ onc= oncogene)
- a protein nvolved in cell proliferation