Lecture 2 & 3 - Virology: Viral Replication Flashcards
What happens during the attachment step of viral replication?
Attachment: Glycoproteins on the surface of the virion bind to receptors on the host cell surface.
The presence or absence of specific receptors determines the tropism (cell type preference) of the virus.
Initial binding occurs through electrostatic forces.
Specific affinity must exist between the virus and the cell for attachment to happen.
Factors influencing attachment: Temperature, pH, and ions.
Example: For HIV, CD4 receptors on immune cells are essential for attachment.
What occurs during the penetration step of viral replication?
Penetration: The virus enters the host cell.
Fusion: Enveloped viruses fuse with the host cell membrane, allowing the viral nucleocapsid to enter the cytoplasm.
Endocytosis: The virus can also enter via receptor-mediated endocytosis, where the virus is engulfed into a vesicle called an endosome.
What happens during the uncoating step of viral replication?
Uncoating: The viral protein coat is removed to release the viral nucleic acid.
This is facilitated by lysosomes from the host cell.
Multiple protein coats (if present) are removed in a systematic manner.
Some viruses have their own uncoating enzymes.
The nucleic acid is then freed into the cytoplasm, marking the start of the eclipse phase (when the virus is no longer detectable but its components are being synthesized).
What happens during the synthesis step of dsDNA viral replication?
dsDNA Viral DNA is transcribed in the nucleus.
Early proteins are mostly non-structural (used for replication).
Late proteins are primarily structural (form the virus particles).
Assembly of the virus occurs in the nucleus.
Herpesviruses bud through the nuclear membrane and acquire an envelope.
Some viruses, like poxviruses, have large genomes and carry their own transcriptase.
Replication for such viruses occurs in the cytoplasm.
How does ssDNA viral replication occur?
Single-stranded DNA (ssDNA) is used as a template to synthesize double-stranded DNA.
Viral replication occurs in the nucleus.
Some ssDNA viruses may require the presence of larger DNA viruses for replication (a process called superinfection).
How do positive-sense (ssRNA) viruses replicate?
Viral RNA acts directly as mRNA.
Polyprotein is cleaved to produce both structural and non-structural proteins.
Assembly occurs in the cytoplasm.
Alternatively:
Part of the virion RNA acts as mRNA to produce a polymerase.
A full-length negative strand is produced, and then mRNAs are synthesized.
How do negative-sense RNA viruses replicate?
Viral transcriptase is used to produce viral mRNAs.
A full-length positive strand is transcribed as a template for viral RNA.
Assembly of the virus takes place by budding.
How do segmented double-stranded RNA viruses replicate?
Viral RNA transcriptase is used to produce mRNAs.
Plus-sense RNAs also serve as templates for replication.
Viral assembly occurs in virus-like entities (core-like particles) in the cytoplasm.
How do retroviruses replicate?
Viral RNA is plus-sense single-stranded RNA ((+)-sense ssRNA).
Reverse transcriptase enzyme synthesizes an RNA/DNA hybrid, which is then used to produce double-stranded DNA.
The resulting DNA is integrated into the cellular genome.
How do dsDNA viruses that replicate through an ssRNA intermediate work?
The single-stranded portion of the dsDNA is completed to form double-stranded DNA.
A full-length plus-strand RNA serves as a template for the production of a minus-strand DNA.
The minus-strand DNA then serves as a template for the synthesis of dsDNA.
Example: Hepatitis B virus.
How does viral assembly occur?
Helical capsids assemble around strands of nucleic acids.
Icosahedral capsids are often assembled as empty shells and then filled with nucleic acid.
Complex viruses are progressively assembled to form the complete virion.
DNA viruses mainly assemble in the nucleus.
RNA viruses mainly assemble in the cytoplasm.
Envelopes are acquired as viruses bud through host membranes.
How are virions released from host cells?
Naked viruses are usually released when the host cells are lysed.
Enveloped viruses, which bud through host membranes, may disrupt the host cell metabolism, potentially causing cell lysis.
Host membranes affected:
Nuclear membrane
Cell membrane
Golgi apparatus
Some enveloped viruses (e.g., retroviruses) cause minimal damage to the host cells and can be released without disrupting the host.
