Questions and Answers
Which type of virus has a replication cycle that starts with its RNA genome being directly used as mRNA?
Which type of virus relies on a viral RNA-dependent-RNA polymerase to transcribe its mRNA?
What is the key enzyme responsible for integrating retroviral DNA into the host cell's genome?
Which of the following viruses replicates entirely within the cytoplasm?
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Which viral group utilizes its own RNA-dependent-RNA polymerase for the initial steps of viral replication?
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What is the unique characteristic of retroviruses that gives them their name?
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Which viral group requires a host cell DNA-dependent-RNA polymerase for its replication?
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Which of these steps is NOT involved in the replication cycle of a retrovirus?
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Which of the following statements is FALSE regarding viral attachment and penetration?
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Which viral protein is involved in the attachment of influenza viruses to host cells?
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What is the primary role of the chemokine receptor CCR5 in HIV infection?
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The process of uncoating the viral genome is essential for viral replication. Which of the following statements accurately describes the process?
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How does the pH change during uncoating contribute to the release of the viral genome?
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Which of the following statements best describes the term 'cell tropism' in the context of viral infection?
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Which of the following statements is TRUE regarding the role of viral proteins in attachment and penetration?
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Which of the following best describes the significance of the viral genome being exposed during uncoating?
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What is the main function of virus-encoded polymerase?
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What is the process by which progeny virions are released from the cell?
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What is the result of recombination between two different viruses infecting the same cell?
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What is the main difference between recombination and reassortment?
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What is the process of genetic reactivation also known as?
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What is the result of complementation between two viruses?
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What is the process by which viruses with segmented genomes exchange genetic material?
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What is the reason for recurrent influenza epidemics?
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Study Notes
Viral Replication Cycle
- Stages of viral growth cycle:
- Attachment and penetration
- Uncoating of the viral genome
- Early viral mRNA synthesis
- Early viral protein synthesis
- Viral genome replication
- Late viral mRNA synthesis
- Late viral protein synthesis
- Progeny virion assembly
- Virion release from the cell
Attachment and Penetration
- Specific viruses use specific cellular receptors on the cell surface, determining cell tropism and disease pathogenesis
- Examples: HSV-1 attaches to fibroblast growth factor receptor, Influenza viruses use haemagglutinin (HA) protein to attach to sialic acid-containing oligosaccharides on the cell surface
- Viruses may use more than one cell receptor, such as HIV using CD4 receptor and chemokine receptor CCR5 as a co-receptor
- Most viruses use multiple receptors on the cell surface in a sequential binding process
- Viral particle penetration occurs through engulfment in a pinocytotic vesicle for enveloped viruses, or fusion of lipid envelope with cell membrane
Uncoating of the Viral Genome
- Before replication, the viral genome must be exposed by removal of associated viral proteins
- Mediated by merging of endocytosed viral particle with cellular lysosomes
- Resulting drop in pH dissociates viral genome from its binding protein
- Rupture of vesicle deposits inner core of virus into cytoplasm
Early Viral mRNA Synthesis
- In DNA viruses, replication occurs in the nucleus, using host cell DNA-dependent-RNA polymerase to synthesize mRNA (except poxviruses, which replicate in the cytoplasm)
- In RNA viruses, entire replicative cycle occurs in the cytoplasm, except retroviruses and influenza viruses, which have important replicative steps in the nucleus
- RNA viruses have different strategies for synthesizing mRNA:
- Single-stranded RNA viruses of positive sense use their RNA genome directly as mRNA
- Single-stranded RNA viruses of negative sense transcribe their mRNA using the negative strand as a template
- Double-stranded RNA viruses convert the -ve RNA strand into a complementary RNA to be used as mRNA
- Retroviruses have single-stranded RNA of positive sense, transcribed into double-stranded DNA by RNA-dependent DNA polymerase (reverse transcriptase)
Early Viral Protein Synthesis
- Viral mRNA is translated by host cell ribosomes into viral proteins, including early proteins (enzymes required for replication of viral genome) and late proteins (structural proteins of progeny viruses)
Viral Genome Replication
- Most viruses make virus-encoded polymerase that make many copies of the parental genome, becoming the genome of the progeny virions
Late Viral mRNA Synthesis and Protein Synthesis
- Occur after replication of the genome
- Result in the production of late proteins, including structural proteins of progeny viruses
Progeny Virion Assembly and Release
- Progeny virion assembly involves packaging the viral nucleic acid within the capsid proteins (late proteins)
- Virion release from the cell occurs through either rupture of the cell membrane and release of mature virus particles or release of viruses by budding through the outer cell membrane in enveloped viruses
Virus Genetics and Interactions Among Viruses
- Interactions include recombination and reassortment, genetic reactivation (marker rescue), complementation, phenotypic mixing, and interference
- Recombination: crossing over between the two chromosomes of different viruses infecting the same cell, resulting in progeny virus carrying traits not found in either parent
- Reassortment: exchange of genetic material between viruses with segmented genomes, resulting in higher frequency of gene exchange than recombination
- Genetic reactivation (marker rescue): between the genome of active virion and inactivated virus genome, rescuing certain markers of the inactivated parent and appearing in the progeny
- Complementation: one virus provides a gene product in which the second is defective, allowing the second virus to grow
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Description
This quiz covers the stages of viral growth cycle, including attachment, penetration, uncoating of the viral genome, and more. Learn about the different steps involved in viral replication.
