* 19 Flashcards
1
Q
capsid
A
- protein shell enclosing the viral genome
- may be rod-shaped, polyhedral, or more complex
- built from a large number of protein subunits called CAPSOMERES, but the number of diff kinds of proteins in a capsid is small
2
Q
rod-shaped viruses
A
- ex: tobacco mosaic virus
- has a rigid, rod-shaped capsid made from over a thousand molecules or a single type of capsomere arranged in a helix: “helical virus”
- RNA inside
3
Q
adenoviruses
A
- infect animals’ respiratory tracts
- have 252 identical capsomere molecules arranged in a polyhedral capsid w/ 20 triangular facets – an icosahedron “icosahedral viruses”
- surface studded w/ protruding glycoproteins
- DNA inside
4
Q
influenza virus
A
- has membranous envelope that surrounds the capsid
- these viral envelopes, derived from the membranes of the host cell, contain host cell phoshpolipids and membrane proteins
- glycoproteins (protruding) and proteins of viral origin
- spherical
- helical virus structure inside (RNA surrounded by capsid)
5
Q
bacteriophages
A
- most complex capsids
- first 7 studied infefcted E coli: named T1 … T7
- T-evens are very similar in structure: capsids have elongated icosahedral heads enclosing DNA. attached to the head is a protein tail piece (sheath + fiber) that attaches to bacterium and injects viral DNA
6
Q
host range
A
The limited number of species whose cells can be infected by a particular virus.
7
Q
viral entry
A
- DNA injection by tail fiber
- endocytosis
- fusion of viral envelope w/ plasma membrane
8
Q
2 phage types
A
- virulent: A phage that replicates only by a lytic cycle. (ex: T4)
- temperate: A phage that is capable of replicating by either a lytic or lysogenic cycle. (ex: lambda, which resembles T4, but its tail has only 1 short tail fiber)
9
Q
lytic cycle
A
- the T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of E coli cell
- the tail sheath contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. the cell’s DNA is hydrolyzed.
- phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using cellular components
- 3 separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. the phage genome is packaged inside the capsid as the head forms.
- the phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. the cell swells and lyses, releasing 100 - 200 phage particles.
10
Q
why haven’t phages exterminated all bacteria?
A
- bacterial mutants w/ receptors that are no longer recognized by a particular type of phage
- when DNA successfully enters a bacterium, the DNA often is idenfitied as foreign and cut up by cellular enzymes called RESTRICTION ENZYMES, which are so named b/c their activity restricts the ability of the phage to infect the bacterium (the bacterium’s own DNA is METHYLATED in a way that prevents attack by its own restriction enzymes)
- many phages coexist w/ bacteria in a state called lysogeny
11
Q
prophage
A
- during a lysogenic cycle, the lambda DNA molecule is incorporated into a specific site on the E coli chromosome by viral proteins that break both circular DNA molecules and join them to e/o.
- when integrated this way, the viral DNA is known as a prophage.
- one prophage gene codes for a protein that prevents transcription of most of the other prophage genes; thus, the phage genome is mostly silent w/in the bacterium.
- other prophage genes cause host cell to make toxins
12
Q
lysogenic cycle
A
- phage attaches to host cell, injects its DNA
- phage DNA circularizes
- certain factors determine whether lytic/lysogenic cycle is entered
- phage DNA integrates into bacterial chromosome, becoming a prophage
- bacterium reproduces normally, copying the prophage and transmitting it to daughter cells
13
Q
replicative cycle of enveloped RNA virus
A
(single stranded RNA)
- glycoproteins on the viral envelope bind to specific receptor molecules on host cell, promoting viral entry
- capsid + viral genome enter cell. digestion of capsid by cellular enzymes releases the viral genome.
- viral genome functions as template for synthesis of complementary RNA strands by a viral RNA polymerase
- these new copies are used as templates for more replication
- the complementary RNA strands also function as mRNAj, which is translated into both capsid proteins (in cytosol) and glycoproteins for the viral envelope (in ER and Golgi)
- vesicles transport envelope glycoproteins to plasma membrane
- a capsid assembles around each viral genome molecule
- new virus buds from cell
14
Q
herpesvirus
A
- temporarily cloaked in membrane derived fro the host’s nuclear envelope; they then shed this membrane in the cytoplasm and acquire a new envelope made from Golgi membrane
- dsDNA
- replicate w/in host cell nucleus, using both viral and cellular enzymes
- copies of viral DNA can remain behind as mini-chromosomes in the nuclei of certain nerve cells; there they remain latent until some sort of physical/emotional stress triggers a new round of active virus production
- acyclovir, which resembles nucleosides, impedes herpesvirus replication by inhibitng the viral polymerase that synthesizes viral DNA
15
Q
class IV viruses
A
- single stranded RNA; animal viruses
- the genome can directly serve as mRA and thus can be translated into viral protein immediately after infection
