Viral Genome Replication and mRNA Synthesis

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Questions and Answers

Why must the repair process precede mRNA synthesis?

The repair process enhances mRNA stability.

The repair process is not necessary for mRNA synthesis.

Host RNA polymerase can only work with single-stranded RNA.

Host RNA polymerase can only transcribe fully double-stranded DNA. (correct)

What is produced from an RNA template by a virus-encoded enzyme?

Double-stranded RNA

Single-stranded DNA (correct)

Circular RNA

Protein

Which of the following viral families is recognized to contain ssDNA genomes?

Adenoviridae

Circoviridae (correct)

Poxviridae

Herpesviridae

What type of genome must be repaired before entering the replication cycle?

Gapped DNA Signup and view all the answers

What common feature is found in the structure of dsDNA viral genomes?

Inverted terminal repeats Signup and view all the answers

Which statement about the replication cycle of some viral genomes is true?

Some genomes can directly enter the replication cycle without repair. Signup and view all the answers

What characterizes the viral families with dsDNA genomes in terms of size?

Their genomes range from 5 kbp to 375 kbp. Signup and view all the answers

Which type of enzyme is responsible for synthesizing single-stranded DNA from RNA?

Reverse transcriptase Signup and view all the answers

Which of the following families include viruses that infect vertebrates?

Retroviridae Signup and view all the answers

What is the first step in the replication process of (+) strand RNA genomes?

Synthesis of a full-length (−) strand Signup and view all the answers

What do the (+) strand RNA genomes of viruses typically require for protein translation?

Host ribosomes Signup and view all the answers

In the case of retroviruses, what is significant about their (+) strand RNA genome?

It is converted to a dsDNA intermediate. Signup and view all the answers

Which of the following virus families does NOT contain (+) strand RNA viruses?

Herpesviridae Signup and view all the answers

During the genome replication process, what is produced after the (−) strand is copied?

Full-length (+) strand genome Signup and view all the answers

What unique feature distinguishes the genome of retroviruses from other (+) strand RNA viruses?

Conversion to a dsDNA intermediate during replication Signup and view all the answers

Which component is considered part of the structure of viral single-stranded (+) RNA genomes?

Untranslated regions (UTR) Signup and view all the answers

Which viruses were first found in aquatic vertebrates?

Hantaviruses Signup and view all the answers

What are the two largest known viral genomes?

Pandoravirus salinus and Pandoravirus dulcis Signup and view all the answers

What is the genome size of anelloviruses?

1,759 base ssDNA Signup and view all the answers

What is the significance of viral genome sequences?

They assist in identifying and tracking viral infections. Signup and view all the answers

Which of the following is NOT true regarding viroids?

They can encode proteins. Signup and view all the answers

What does the phylogenetic diversity of fish viruses indicate?

They have a larger phylogenetic diversity than mammalian viruses. Signup and view all the answers

Which of the following statements regarding viral genome studies is accurate?

Partial sequences are important during outbreaks. Signup and view all the answers

What is a characteristic of viroids compared to other viruses?

They are much smaller than traditional viruses. Signup and view all the answers

What does the genetic analysis of Pandoravirus salinus indicate about its genes?

Many genes show no similarity to any known proteins. Signup and view all the answers

What conclusion can be made regarding the evolutionary lineage of viruses?

Viruses may have multiple ancestral origins. Signup and view all the answers

Which of the following represents a significant challenge in understanding the origin of viral genomes?

Absence of preserved viral fossils. Signup and view all the answers

What type of genes are considered 'hallmark genes' in different viruses?

Genes responsible for viral replication. Signup and view all the answers

The discovery of ancient viral DNA in environmental samples provides evidence for what aspect of viral evolution?

Viral genomes can provide insight into ancient evolution. Signup and view all the answers

What is suggested about viral replication enzymes in relation to their origins?

They originated from precellular self-replicating elements. Signup and view all the answers

Why is the RNA-dependent RNA polymerase significant among viral proteins?

It is specifically found in RNA viruses. Signup and view all the answers

What process contributes to the diversity of capsid protein genes in viruses?

The capture of unconnected cellular genes. Signup and view all the answers

What is meant by the term 'wild type' in virology?

A laboratory-adapted virus from which mutants are derived. Signup and view all the answers

Which of the following best describes temperature-sensitive mutants?

Mutants that can replicate at specific temperatures. Signup and view all the answers

What characteristic is commonly sought after in viral mutants during screening?

Changes in plaque size or morphology. Signup and view all the answers

What is the difference between laboratory wild types and field isolates?

Field isolates are obtained from natural hosts. Signup and view all the answers

Which terms are commonly used by virologists to describe variations of a virus?

Strains, variants, mutants. Signup and view all the answers

What does the term 'lethal mutations' refer to?

Mutations that render the virus non-viable under all conditions. Signup and view all the answers

Which characteristic is NOT associated with silent mutations?

They may change the amino acid sequence of proteins. Signup and view all the answers

What does the term 'host range' refer to in virology?

The types of species that the virus can infect. Signup and view all the answers

Study Notes

Viral Genome Repair and Replication Strategies

Host RNA polymerase can only transcribe fully double-stranded DNA (dsDNA).
Gapped DNA genomes produced by some viruses require repair before mRNA synthesis.
Viral genomes can enter the replication cycle directly or first necessitate the synthesis of essential viral gene products.

Types of Viral Genomes

Single-Stranded DNA (ssDNA):
- Thirteen families of ssDNA viruses identified, including Anelloviridae and Circoviridae.
Double-Stranded DNA (dsDNA):
- Includes families such as Polyomaviridae, Adenoviridae (30–50 kbp), Herpesviridae (120–240 kbp), and Poxviridae (130–375 kbp).
- Structures and expression processes vary across families.

RNA Virus Classification and Replication

+ Strand RNA Viruses:
- Families include Picornaviridae and Togaviridae.
- Preferentially translatable into proteins by host ribosomes.
- Replication occurs in two steps: copying the + strand into a full-length - strand, then back to + strand genomes.
Retroviruses:
- Defining trait includes converting the + strand RNA genome into a dsDNA intermediate via reverse transcriptase, a hallmark of Retroviridae.

Genome Size and Diversity

The largest known viral genomes belong to Pandoravirus salinus and Pandoravirus dulcis, with 2.4 million and 1.9 million bases of dsDNA, respectively.
Viroids represent the smallest known viral genomes, with lengths of 246 to 401 nucleotides and no coding capacity.
Viiral genomes exhibit a wide range in sizes, impacting their evolutionary trajectories and adaptations.

Importance of Viral Genome Sequences

Viral genome sequences are crucial for cataloging and classifying viruses.
Partial genome sequences can assist in identifying responsible viruses during outbreaks and tracking their spread.
Some viroid and anellovirus genomes indicate pollution in agricultural sectors, emphasizing their economic impact.

Origin of Viral Genomes

No solid evidence of viral fossils complicates the understanding of the origins of viral genomes.
Evidence suggests that viral replication enzymes may have originated from precellular self-replicating elements in ancient cells.
Capsid proteins have been found to derive from unrelated genes, highlighting the complex evolutionary pathways of viruses.

Terminology and Concepts in Virology

The term "wild type" refers to the natural strain from which mutants are derived, differentiated from laboratory adaptations.
Viral strains are categorized based on genetic heritability and pathogenesis through terms such as "strains," "variants," and "mutants."
Clinical isolates are distinct from laboratory wild types, as they originate from natural infections.

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Description

This quiz explores the processes involved in viral genome replication and mRNA synthesis, highlighting the importance of double-stranded DNA and the role of host RNA polymerase. Gain insights into various strategies for viral gene expression as discussed in Figures 3.2 to 3.8.