Chapter 19 - Viruses Flashcards
What is a virus?
A very small infectious particle consisting of little more than genes (a nucleic acid) packaged in a protein coat (in some cases, a membraneous envelope)
- Cannot reproduce or carry out metabolism outside of a host cell
- Not alive, but exist in a shady area between life-forms and chemicals
What do viral genomes consist of?
- Double or single stranded DNA (DNA virus)
- Double or single stranded RNA (RNA virus)
- The genome is usually organized as a single linear or circular molecule of nucleic acid, although the genomes of some viruses consist of multiple molecules of nucleic acid
- Have between 3 and several thousand genes in their genomes
What is a capsid?
The protein shell enclosing the viral genome; capsids are built from protein subunits called capsomeres
Depending on the type of virus, the capsid may be rod-shaped, polyhedral, or more complex in shape
*A capsid can have a variety of structures
What are viral envelopes?
Derived from the membranes of the host cell; contain host cell phospholipids and membrane viral origin (they also contain proteins and glycoproteins of viral origin)
What are bacteriaphages (phages)?
Viruses that infect bacteria; have the most complex capsids among viruses
Phages have an elogated capsid head that encloses their DNA; a protein tail piece attaches the phage to the host and injects the phage DNA inside
What is the host range?
Each virus has a host range, a limited number of host cells in can infect
What are the general features of a viral replicative cycle?
- Once a viral genome has entered a cell, the cell begins to manufacture viral proteins
- The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules
- Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses
What is the lytic cycle?
A phage replicative cycle that culminates in death of the host cell; refers to the last stage of infection, during which the bacterium lyses (breaks open) and releases the phages that were produced within the cell
What is a virulent phage?
A phage that replicates only by a lytic cycle
What are restriction enzymes?
Cellular enzymes that cut up foreign DNA; bacteria defense against phages, restriction enzymes recognize and cut up certain phage DNA
Restriction enzymes restrict the ability of the phage to replicate within the bacterium
What is the lysogenic cycle?
Allows replication of the phage genome without destroying the host; the viral DNA molecule is incorporated into the host cell’s chromosome
What are temperate phages?
Phages capable of using both modes of replicating within a bacterium
What is a prophage?
When integrated into the bacterial chromosome, the integrated viral DNA is known as a prophage
Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
What are the steps of the lytic cycle?
- Attachment - phage binds to specific surface proteins that act as receptors
- Entry of phage DNA and degradation of host DNA - the phage DNA is injected into the cell
- Synthesis of viral genomes and proteins - the phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using components within the cell
- Self assembly - proteins self assemble to form phage heads, tails, and tail fibers
- Release - the phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter; the cell swells and finally bursts, releasing 100 to 200 phage particles
What are the steps of the lysogenic cycle?
- Phage DNA integrates into the bacterial chromosome, becoming a prophage
- The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells
- Daughter cell with prophage - many cell division produce a large population of bacteria infected wit the prophage
*Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle (An environmental signal - a certain chemical or high energy radiation - can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode)
What are the two key variables used to classify viruses that infect animals?
- An RNA or DNA genome
2. A single stranded or double stranded genome
Few bacteriophages have an envelope or an RNA genome, what type of viruses have both?
Many animal viruses have both
What are viral envelopes?
- Many viruses that infect animals have a membranous envelope
- Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell
- Some viral envelopes are derived from the host cell’s plasma membrane as the viral capsids exit
What are the steps of the replicative cycle of an enveloped RNA virus?
- Glycoproteins on the viral envelope bind to specific receptor molecules on the host cell, promoting viral uptake by the cell
- The capsid and viral genome enter the cell; digestion of the capsid by cellular enzymes releases the viral genome
- The viral genome functions as a template for synthesis of complementary RNA strands by a viral RNA polymerase
- New copies of viral genome RNA are made using the complementary RNA strands as templates
- cRNA strands also function as mRNA, which is translated into both capsid proteins (in the cytosol) and glycoproteins for the viral envelope (in the ER and Golgi apparatus)
- Vesicles transport envelope glycoproteins to the plasma membrane
- A capsid assembles around each viral genome molecule
- Each new virus buds from the cell, its envelope studded with viral glycoproteins embedded in membrane derived from the host cell
What do all viruses that use an RNA genome as a template for mRNA transcription require?
RNA —> RNA synthesis;
These viruses use a viral enzyme capable of carrying out this process; there are no such enzymes in most cells
The broadest variety of RNA genomes is found in viruses that infect animals
What are retroviruses?
The RNA animal viruses with the most complicated replicative cycle (class VI)
Retroviruses use reverse transcriptase to copy their RNA genome into DNA
*Retro means backwards
What is reverse transcriptase?
An enzyme which transcribes an RNA template into DNA, providing an RNA —> DNA information flow, the opposite of the usual direction
What is the retrovirus that causes AIDS (acquired immunodeficiency syndrome)?
HIV (human immunodeficiency virus);
HIV and other retroviruses are enveloped viruses that contain two identical molecules of single stranded RNA and two molecules of reverse transcriptase
After HIV enters a host cell, its reverse transcriptase molecules are released into the cytoplasm, where they catalyze synthesis of viral DNA; the newly made viral DNA then enters the cell’ nucleus and integrates into the DNA of a chromosome; the integrated viral DNA (provirus) never leaves the host’s genome, remaining a permanent resident of the cell
What is a provirus?
The integrated viral DNA which never leaves the host’s genome, remaining a permanent resident of the cell
- Unlike a prophage, a provirus remains a permanent resident of the host cell
- RNA polymerase transcribes the proviral DNA into RNA molecules
- The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell
