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Questions and Answers
What is the function of ribosomes in the context of viral gene expression?
What type of viruses directly translate their RNA in the cytoplasm?
Which enzyme is responsible for replicating the genomes of (+)ssRNA viruses?
What is a common tactic employed by viruses to defeat host defenses?
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What is a significant target for antiviral drugs within the replication cycle of (+)ssRNA viruses?
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Why do (+)ssRNA viruses not use host RNA-dependent RNA polymerase for replication?
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What happens to polyproteins generated during the translation of (+)ssRNA viruses?
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What components are encoded in viral genomes that facilitate their life cycle?
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What happens immediately after a bacteriophage's DNA is delivered into a host cell?
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What role do restriction endonucleases play in bacterial defense?
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How does the host cell protect its own DNA from being cut by restriction endonucleases?
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What is a key characteristic of CRISPR in prokaryotes?
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What are the components encoded by the cas genes in a CRISPR system?
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During the bacteriophage lifecycle, when does the lysis of the host cell occur?
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What is the sequence of events for CRISPR when a new viral DNA enters a bacterial cell?
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What guides the Cas9 enzyme to cut homologous incoming viral dsDNA?
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What occurs after the transcription of the CRISPR array?
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Who were the recipients of the 2020 Nobel Prize in Chemistry for their contributions to CRISPR technology?
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Which aspect has sparked a hot ethical debate regarding CRISPR technology?
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What defense mechanisms do bacteria employ against viruses?
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How do eukaryotic cells primarily combat viral infections?
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What denotes molecular patterns recognized as foreign by eukaryotes?
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What do viruses depend on from their host to replicate?
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What is one way that viruses adapt their activities to the host cell?
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Study Notes
Bacteriophage Infection
- Bacteriophage replication occurs quickly in a host cell, often within 30 minutes.
- Host gene expression is halted immediately following penetration.
- Host DNA and RNA are degraded and protein synthesis is blocked.
- Bacteriophage DNA is delivered to the host cell after penetration.
- This interruption of host function allows the virus to control the cell's resources.
- Enzyme synthesis begins after 5 minutes.
- DNA replication begins after 10 minutes.
- Viral particle formation starts after 12 minutes.
- Cell lysis and release of viral particles occurs after 30 minutes.
Bacterial Antiviral Defenses
- Bacteria have two main defense mechanisms against viral infection, restriction endonucleases and CRISPR.
Restriction Endonucleases
- Cleave dsDNA at specific nucleotide sequences known as restriction sites.
- Provide a defense mechanism against invading viruses by specifically cutting up foreign viral DNA.
- Host DNA is methylated by a host cell enzyme, preventing cleavage by host restriction endonucleases.
- Restriction endonucleases are used as molecular scissors in gene cloning.
CRISPR
- CRISPR stands for "Clustered Regularly Interspaced Short Palindromic Repeats."
- Viral DNA sequences are integrated into the bacterial genome as spacers, separated by non-viral repetitive sequences.
- CRISPR arrays are located near cas genes, which encode DNA cutting enzymes.
- Bacteria use CRISPR to develop “immune memory” against previously encountered viruses.
CRISPR Steps
- Foreign viral DNA is cleaved by Cas9, specific to each virus.
- Cleaved viral DNA is inserted as a novel spacer in the CRISPR array.
- The CRISPR array is transcribed, creating RNAs that target homologous incoming viral DNA.
- This RNA then guides Cas9 cleavage of incoming viral DNA.
CRISPR as a Tool
- CRISPR is widely used as a molecular biology tool to edit genomes.
- It is utilized for editing cell lines, model organisms, and other areas of research.
Ethical Considerations
- Editing of the human germline using CRISPR is a hot ethical debate.
- There are significant ethical questions and concerns surrounding the use of CRISPR in human germline editing.
Eukaryotic Antiviral Defenses
- Eukaryotic cells do not possess restriction enzymes or CRISPR arrays.
- They have an immune system triggered by pattern recognition receptors (PRRs).
- PRRs recognize molecular patterns that are foreign, such as viral dsRNA and 5’-triphosphate RNA.
- These viral targets are called "pathogen-associated molecular patterns" (PAMPs).
Viral Genomes
- All viral genomes are either DNA or RNA.
- This is known as the Baltimore classification system, which categorizes viruses into 7 distinct types.
- Viral genomes are diverse in their composition and structure.
- They employ a variety of methods to encode and decode information in nucleic acid.
Viral mRNA
- Viral genomes must produce mRNA that can be decoded by host ribosomes.
- This allows the virus to utilize the host's translation machinery for viral protein synthesis.
Information Encoded in Viral Genomes
- Viral genomes carry information for replicating the viral genome.
- They encode instructions for assembling and packaging the genome into viral particles.
- Viral genomes regulate and synchronize the replication cycle.
- They contain information to counteract host defenses and spread to other cells and hosts.
(+)ssRNA Virus Replication
- (+)ssRNA viruses are directly translated by host ribosomes in the cytoplasm.
- Translation generates polyproteins that are cleaved by viral or host proteases to produce the full set of viral proteins.
- Examples include poliovirus, Zika virus, and HCV.
Polyprotein Processing
- Polyprotein processing is co-translational.
- This means processing occurs simultaneously with protein synthesis by ribosomes.
Drug Targets
- Viral proteases are excellent drug targets because they are essential for viral protein production, but distinct from host proteases.
- RDRPs (RNA-Dependent RNA Polymerases) are also valuable drug targets because they are unique to viruses and essential for replicating their RNA genomes.
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Description
Test your knowledge on the mechanisms of bacteriophage infection and bacterial antiviral defenses. This quiz covers topics such as the stages of bacteriophage replication and the role of restriction endonucleases and CRISPR in bacterial immunity against viruses. Challenge yourself to understand how these processes interact!
