True or False:
Viruses lack metabolic enzymes and equipment for making proteins, like ribosomes. They are obligate intracellular parasites; in other words, they can reproduce only within a host cell.
each type of virus can infect cells of only a limited variety of hosts, called the __ of the virus. This host specificity results from the evolution of recognition systems by the virus.
Viruses identify their host cells by a __ fit between viral surface proteins and specific receptor molecules on the outside of cells. SOme have __ (West Nile Virsu) while others have very __ (measles virus).
broad host range
narrow host range
A viral infection begins when __?
- The mechanism of __ depends on the type of virus and the type of host cell.
- Once insude, the proteins it encodes can cooamdeer the host, reprogramming the cell to copy the viral nucleic acid and maufacture viral proteins.
a virus binds to a host cell and the viral genome makes its way inside
The host provides the nucleotides for making viral nucleic acids, as well as enzymes, ribosomes, tRNAs, amino acids, ATP, and other components needed for making the viral proteins. Most DNA viruses use the __ of the host cell to synthesize new genomes along the templates provided by the viral DNA.
In contrast, to replicate their genomes, __ use virally encoded __ that can use __ as a template.
After the viral nucleic acid molecules and __ are produced, they spontaneously self-assemble into new viruses. In fact, researchers can separate the RNA and __ of TMV and then reassemble complete viruses simply by mixing the components together under the right conditions.
The simplest type of viral reproductive cycle ends with the exit of hundreds or thousands of viruses from the infected host cell, a process that often damages or destroys the cell. Such cell damage and death, as well as the body's responsess to this destruction cause many of the symptoms associated with viral infects. The viral __ that exit a cell have the potential to infect additional cells, spreading the viral infection.
__ are the best understood of all viruses, although some are also among the most complex. research on phages led to the discovery that some double-stranded DNA viruses can reproduce by two alternative methods: __ and __.
A phage reproductive cycle that culminates in death of the host cell is known as a __. The term refeers to the last stage of infection, during which the bacterium __ (breaks open) and releases the phages that were produced within the cell.
Each of these phages cna then infect a healthy cell, and a few sucessive __ can destroy an entire bacterial population in just a few hours. A phage htat reproduces only by a __ is a __.
lytic cycle x2
True or False:
Phage treatments have been used medically in some countries to help control bacterial infections in patients.
Bacteria are not defenseless.
1st: __ favors bacterial mutants with reeptors that are no longer recognized by a particular type of phage
2nd: When phage DNA successfully enters a bacterium, the DNA is often identified as foreign and cut up by cellular enzymes called __, which are so named because their activity restricts the ability of the phage to infect the bacterium.
THe bacterial cell's own DNA is methylated in a way that prevents attack by its own restriction enzymes. But just as __ favors bacteria with mutant receptors or effective restriction enzymes, it also favors __ mutants that can bind the altered receptors or are resistant to particular restriction enzmyes. Thus, the parasite-host relationship is in constant evolutionary flux.
There is yet a third important reason bacteria have been spared from extinction as a result of phage activity. Instead of lysing their host cells, many phages coexist with them in a state called __.
In contrat to the __, which kills the hot cell, the __ allow replication of the phage genome without destroying the host. Phages capable of using both modes of reproducing within a bacterium are called __.
A __ called lambda is widely used in biological research.
Infection of an E. coli cell by phage lamba begins when the phage binds to the surface of the celland injects its linear DNA genome. Within the host, the lambda DNA molecule forms a circle.
What happens next depends on the reproductive mode: __ or __. During a __, the viral genes immediately turn the host cell into a lambda-producing factory, and the cell soon lyses and releases its viral products.
During a __, the lambda DNA molecule is incorporated into a specific site on the E. coli chromosome by viral proteins thta break both circular DNA molecules and join them to each other.
When integrated into the bacterial chromosome in this way, the viral DNA is known as a __. One __ gene codes for a protein that prevents transcription of most of the other prophage genes.
Thus, the phage genome is mosstly silent within th ebacterium. Every time the E. coli cell prepares to divide, it replicates the phage DNA along with its own and passes the copies on to daughter cells. A single infected cell can quickly give rise to a large population of bacteria carryin the virus in __ form. This mechanism enables viruses to propagate without killing the host cells on which they depend.
The term __ implies that prophages are capable of generating active phages that lyse their host ells. This occurs when the lambda genome is induced to exit the bacterial chromosome and initiate a __.
An environmental signal, suh as a certain chem or high-energy radiatio, usually triggers the switchover from the __ to the __ mode.
In addition to the gene for the __, a few other prophage genees may be expressed during __. Expression of these genes may alter the host's phenotype, a phenomenan that can have important medical significance.
True or False:
Like all viruses, those that cause iillness in humans and other animsals can reproduce only inside host cells. Many variations on the basic scheme of viral infection and reproduction are represented among the animal viruses.
One key variable is the nature of the viral genome: Is it composed of DNA or RNA? Is it double or signle-stranded?
The nature of the genome is the basis for the common classification of viruses. Single stranded RNA viruses are further classified into three classess __ accordingto how the RNA genome functions in a host cell.
Whereas few __have an envelope or RNA genome, many animal viruses have both. In fact, nearly all animal viruses with RNA genomes have an envelope, as do some with DNA genomes.
An animal virus equipped with an __- an outer membrane- uses it to enter the host cell. Protruding from the outer surface of this envelope are viral __ that bind to specific receptors moleucles on the surface of a host cell.
The protein parts of __ are made by ribosomes bound to the ER of the host cell; cellular enzymes in the ER and Golgi apparatus then add the sugars. The resulting viral __, embedded in hlost ell-derived membrane, are trasported to the cell surface.
In a process much like __, new viral __ are wrapped in membrane as they bud from the cell. In other words, teh viral envelope is deried from the host cell's plasma membrane, althouh some of the moelcules of this membrane are specified by viral genes. The enveloped viruses are now free to infect other cels. This reproductive cycle does not necessarily kill the hst cell, in contrast to the __ of phages.
True or False:
Some viruses have envelopes that are not derived from plasma membrane.
Although some phages and most plant viruses ar e__, the broades variety of RNA genomes is found among hte viruses that infect animals. Among the three types of single-stranded RNA genomes found in animal viruses, the genome of class __ viruses can directly serve as mRNA and thus can be translated into viral protein immediately after infection.
SOmetimes, when the RNA genome serves as a __ for mRNA synthesis, the RNA genome is transcribed into complementray RNA strands, which function both as __ and as templates for the synthesisi of additional copies of genomic RNA.
All viruses that require __ to make mRNA use a viral enzyme capable of carrying out this process; there are no such enzymes in most cells. The viral enzyme is packaged with the genome inside the viral capsid.
RNA --> RNA synthesis
THe RNA animal viruses with teh most complicated reproductive cycles are hte __ (class IV). The viruses are equippend with an enzyme called __, which transcribes an RNA template into DNA, providing an __ flow, the opposite of the usual direction.
o This unusual phenomenon is th ource of the name __.
RNA --> DNA info
Of particular medical importance is __, the retrovirus that causes __. __ and other retroviruses are enveloped viruses that contain two identical molecules of single-stranded RNA and two molecules of __.
After __ enters a host cell, its __ molecules are released into the cytoplasm, where they catalyze synthesis of viral DNA. The newly made viral DNA then enters the cell's nucleus and integrates into the DNA of a chromosme.
The integrated viral DNA, called __, never leaves the host's genome, remaining a permanent resident of the cell. (Recall that a __, in contrast, leaves the host's genome at the start of a __.) The host's __ transcribes the __ into RNA molecules which can function both as mRNA for the synthesis of viral proteins and as genomes for the new viruses htat will be assembled and released from the cell.
Viruses don't fit the def of living organisms. An isolated viirus is biologically __, unable to replicate its genes or regenerate its own supply of ATP. Yet it has a gnetic program written int he universal language of life.
True or False
Viruses have been foud that infect every form of life- not ust bacteria, animals, and plants but also archaea, fungi, and galgae and other protists. Because they depend on cells for their own propagation, it seems likely that viruses are not descendants of precellular forms of life but evolved after the first cells appeared, possibly multiple times.
Most molecular biologists favor the hypothesis that viruses originated from naked bits of cell __ that moed from one cell to anotehr, perhaps via tinured cell surfaces. The evolution of genes coding for capsid proteins may hae facilitated the infection of uninjured ells.
Candidatres for the original sources of viral genomes include plasmids and transposons. __ are small, circular DNA molecules found in bacteria and in unicell euk called __. __ exist apart from the cell's geome, can replicate independently of the genome, and are occasionally transferred betwen cells.
__ are DNA segments that can move from one location to another within a cell's genome. Thus, plasmids, transposons, and viruses all share an important feature: They are __.
mobile genetic elements
True or Flase:
Consisten with this visio of pieces of DNA shuttling from cell to ell is the observation that a viral genome can have more in common with the genome of its host than with the genomes of viruses that infect other hosts. Indeed, some viral genes are essentially identical to genes of hte host.
True or False:
On the other hand, recent sequencing of many viral genomes has shown that the genetic sequences of some viruses are quite similar to those of seemingly distantly related viruses.
__: the largest virus yet discovered
__ is a double-stranded DNA virus with an icosahedral apsid that is 400 nm in diater.
(Genome contains 1.2 mil bases and an est. 1000 genes)
Perhaps the most surprising aspect of __ is that some of the genes apear to code for products previously thought to be hallmarks of cell genomes. These products include proteins involved in __, DNA repair, protein folding, and polysaccharide synthesis. The researchers who described __ propose that it most likely evlved before ht efirst cells and then deeloped an exploitative relationship with them.
True or False: Other scientists disagree, mainitaing that the virus evolved more recently than cells and has simply been efficient at scavenging genes from its hosts.