Lecture 17: Viruses Flashcards
what is a virus (simple term)
small self replicating organisms
- no independant metabolism, they are obligated to invade cells and hijack cell machinery
- small segment of nucleic acid encased in a simple protein shell
explain how virus are categorized (virus taxonomy)
-based on the form of nucleic acid present in the virion particles + pathway for expression of genetic material
what is the difference between pos sense rna and negative sense rna
- positive sense rna is similar to mrna and can immediately be translated by the host cell. (purified positive sense rna can directly cause infection although it may be less infectious than the whole virus particle.
- negative sense viral rna is complementary to the mrna and thus must be converted to pos sense RNA by RNA polymerase before translation.
ambisense rna?
Ambisense means that the virus resembles a negative sense RNA virus, however they can also translate genes from the positive strand
what is the difference between enveloped and non enveloped viruses
env: lipid bilayer membranes, impermeable barrier between their genomes and outside environment
non env: no membranes, no barrier and tightly packed into protein shell to exclude nucleases
Summarize the basics of virus entry and uncoating
- The virus binds to the surface of receptive host cells.
- Entry is initiated by:
- Inducing conformational changes in the virus that lead to association with other receptors, membrane fusion, and penetration
- Transmit signals through the membrane prepares the cell for invasion
- Induction of the endocytic pathway
- Viruses can only infect cells to which they can bind
- The virus triggers the penetration process and enters the cytosol
- DNA viruses are transported into the nucleus for genome un-coating or replication
- Several viruses have developed mechanisms to allow them to deliver their DNA to the nucleus
- RNA viruses may be translated in the cytosol
entry of the virus is iniated by which 3 ways
- inducing conformational changes
- transmitting signals through the membtane to prepare the cell for invasion
- induction of the endocytic pathway.
what are the main virus replication strategies?
RNA viruses must replicate their genomes one of two unique ways:
1- RNA-dependent RNA synthesis (RNA replication)
2- RNA-dependent DNA synthesis (reverse transcription) followed by DNA replication and transcription i.e. retroviruses
They need enzymes to make mrna and they can either get them from the host or encode their own enzymes.
all viruses must express their genes as ______ early in infection to make essential _____
- functional mRNA
- essential viral proteins
RNA viruses evolve at a much _____ rate than their host? what is special about this
- faster rate
- this means that the virus can escape from host produced neutralizing antibodies.
what is the purpose of macromolecules in virus assembly
they are transported to a point in the host cell where they can be assembled into a viral particle.
explain the process of virus assembly
- proteins which must be transported to the plasma membrane must travel through the secretory pathway of the cell.
- they associate with the ER –> translocate across via a proteinaceous pore
- transported to Golgi complex + on the plasma membrane unless they contain amino acid motifs that localize them to another location
what is the difference between assembly of enveloped and non env viruses
- non enveloped virus can assemble in the cytoplasm or the nucleus
- enveloped virus must acquire a lipid bilayer from a host cell membrane during assembly
explain the process of assembly for non envelopped virus
Non-Enveloped Viruses:
Macromolecules assemble into empty capsids
Viral DNA is inserted into the structure via a packaging sequence at one end of the genome
Precursor core proteins are also packaged into the empty capsid along with DNA
Proteolytic cleavage of the precursor proteins by proteinase yields the mature (infectious) virion
what are the 3 pathways for assembly of envelopped viruses
There are 3 pathways of enveloped virus assembly (labeled in previous figure)
- Proteins are transported to plasma membrane and assembly of the capsid and envelopment occur simultaneously
- Capsid assembly occurs in the cytoplasm and then the virus buds from the plasma membrane and gets its envelope
- Capsid assembly occurs in the cytoplasm and gets its envelope from the ER and is exported via exocytosis
