Viral Lifecycle - Entry and Exit; Viral Lifecycle - Genome Replication and Protein Expression Flashcards
Place the steps of the viral life cycle in order:
Entry
Synthesis of virus components
Genome replication & Transcription
Maturation
Assembly
Adsorption (attachment)
Uncoating
Release
quiz
- Adsorption (attachment)
- Entry
- Uncoating
- Genome replication and transcription
- Synthesis of virus components
- Assembly
- Release
- Maturation
The main determinant of viral tropism is:
-The presence of a specific receptor on a target cell
-The viral envelope
-The host species
-Antibodies
-The viral genome
quiz
- presence of specific receptor on target cell
Viral tropism describes the ability of a virus to attach and enter a host cell. This is determined by the presence of a specific receptor on the cell surface. This receptor is usually a glycoprotein, but can also be a phospholipid or carbohydrate. The receptor interacts/binds to a component of the virus e.g. Capsid protein, or envelope glycoprotein.
Human Immunodeficiency virus (HIV) uses which of the following receptors for entry into Macrophages?
-Sialic acid
-CD4 and CXCR4
-Heparin sulphate
-CD4 and CCR5
quiz
- CD4 and CCR5
Influenza virus uses Sialic acid, and Herpesviruses use Heparin sulphate to attach to target cells. HIV uses CD4 and CXCR4 to enter T-cells, and CD4 and CCR5 to enter macrophages
The following statements about viral entry are TRUE or FALSE?
-Some enveloped viruses fuse at the plasma membrane
-Some enveloped viruses fuse at the nuclear membrane
-Some viruses enter cells by endocytosis
-Viruses directly inject their genomes through the plasma membrane
Quiz
- True
- False
- True
- False
Some enveloped viruses directly fuse at the plasma membrane (e.g. HIV), and some are first taken up into an endosome by endocytosis before fusion can occur (e.g. Influenza virus). Viruses infecting eukaryotes do not directly inject their genomes – Bacteriophage use this mechanism to insert their genomes into bacterial cells.
Fusion of the Influenza virus envelope requires:
(Please select all that are correct)
-An increase in the pH of the endosome
-A decrease in the pH of the endosome
-A conformational change in the viral Haemagluttinin protein
-A conformational change in the viral Neuraminidase protein
-Replication of the viral genome
Quiz
- Decrease of pH
- change in Haemagluttinin protein
Following endocytosis of the Influenza virus, the pH of the endosome is lowered by pumping hydrogen ions across the membrane. This is a normal process of endosomes. The acidic pH causes a conformational change in the Influenza haemagluttinin protein which brings the viral envelope into closer proximity with the endosomal membrane, allowing fusion to occur and release of the viral genome into the cytoplasm.
Which of the following statements are TRUE or FALSE?
-HIV buds at the plasma membrane
-Influenza virus assembles in the nucleus
-Herpesviruses bud from the nuclear membrane
-Poliovirus assembles it’s capsid around the genome
-Coronaviruses bud into the Golgi
Quiz
- True
- False
- True
- False
- True
HIV and Influenza virus assemble and bud at the plasma membrane. Influenza has a mechanism to correctly insert eight separate genomic segments into the budding virion. Herpesviruses replicate and assemble in the nucleus and then initially bud out from the nuclear membrane before using the cell secretory endosomes for exit from the cell. Coronaviruses bud into the Golgi before being secreted from the cell. Polioviruses insert their genomes into pre-assembled capsids.
Which of the following statements about HIV maturation is TRUE or FALSE?
-Maturation occurs after virus budding
-Maturation is catalysed by the viral protease enzyme
-Maturation is catalysed by the viral reverse transcriptase
-Immature HIV virions are infectious
-The HIV protease is a target for antiviral drugs
Quiz
- True
- True
- False
- False
- True
HIV buds from the plasma membrane in an immature form which is non-infectious. The viral protease catalyses changes to the nucleocapsid structure resulting in rearrangements of protein components to produce a mature form of HIV which can infect target cells. The HIV protease is a good target for anti-HIV drugs which results in non-infectious particles being released from cells.
Explain the viral lifecycle
state the steps
- Adsorption (Attachement): random collision btw viral surface protein and host cell receptor
- Entry: virus must find receptors on host cell (ie glycoproteins)
- can either 1) endocytosis or 2) fuse with plasma membrane - Uncoating: eclipse period (viron cant be detected). 3 places where uncoating happens:
- 1) at plasma membrane
- 2) within endosomes
- 3) nuclear membrane - Genome replication and transcription
- Synthesis of virus components
- Assembly
- Release (and maturation)
How does HIV enter host cells?
SU protein on HIV attaches to CD4 on target cell (T cells /macrophages) as well as a chemokine receptor (CXCR4 or CCR5)
CCR5 expressed on macrophages, CXCR4 expressed on T- cells
How does Influenza enter host cells?
Virus binds sialic acid on host cell by virus HA, then the low pH of the endosome triggers change in HA which results in viral envelope fusing with endosomal membrane
HA= haemogluttin which is a glycoprotein
How does Polio enter host cells?
Polio is non- enveloped
Virus binds to host surface, then in endosome the structure changes to make a pore connecting virus and endosome membrane and RNA released into cytoplasm
How does Adenovirus enter host cells?
Attaches to host surface via host integrin and fiber receptor, then in endosome it breaks down its surface molecules and ruptures endosome, then attaches to nuclear pore delivers DNA into nucleus
Gets to the nucleus via microtubules
Where does viral assembly occur?
- Nucleus
- Cytoplasm
- Plasma membrane
Plasma membrane is where most enveloped viruses assemble
How does release differ between enveloped and non enveloped viruses?
enveloped: budding through plasma membrane
non enveloped: sudden rupture of cell
Release can occur at the same time as assembly
Which part of HIV release has been targetted by anti-viral drugs?
The final maturation step for HIV, where protease chops up proteins allowing them to rearrange themselves to become mature
