Viruses and Their Reproduction Cycles

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Questions and Answers

What are the two main components of a virus?

DNA and RNA only

Protein subunits and lipid layers

Genomic sequence and metabolic machinery

Outer capsid and inner core (correct)

Which cycle results in the death of the host cell immediately after virus release?

Enzootic cycle

Lysogenic cycle

Epizootic cycle

Lytic cycle (correct)

What is a reservoir species in the context of viruses?

A living host of a virus without causing disease (correct)

Any organism that carries a virus

A host that dies from the infection

An organism that spreads the virus between hosts

What is the purpose of spikes on a virus?

To attach to the host cell Signup and view all the answers

Which characteristic makes viruses different from living organisms?

Viruses cannot reproduce without a host. Signup and view all the answers

What is the primary difference between the lytic and lysogenic cycles?

The lytic cycle is faster than the lysogenic cycle. Signup and view all the answers

What triggers a virus in the lysogenic cycle to enter the lytic cycle?

Changes in environmental conditions Signup and view all the answers

In viral classification, what complicates the process compared to living organisms?

There is no common genomic sequence shared by all viruses. Signup and view all the answers

Which type of genome is found in a virus classified as type II in the Baltimore classification system?

Single-stranded DNA Signup and view all the answers

What characterizes the capsid structure of a naked icosahedral virus?

It is made up of protein subunits Signup and view all the answers

Which of the following examples is classified as a type IV virus?

Common cold Signup and view all the answers

In the Baltimore classification, how is mRNA produced for a type V virus?

Transcribed from the RNA genome Signup and view all the answers

What is a distinguishing feature of segmented genomes in virus classification?

They consist of multiple RNA molecules Signup and view all the answers

Which virus is an example of a type III virus according to the Baltimore classification?

Rotavirus Signup and view all the answers

Which classification system was first developed by David Baltimore in the early 1970s?

Baltimore viral classification Signup and view all the answers

Non-segmented genomes in viruses typically indicate what kind of replication process?

The genome can be transcribed directly into mRNA Signup and view all the answers

What is the main reason anti-viral medications are difficult to develop?

They depend on the host's cellular machinery. Signup and view all the answers

Which of the following is true regarding viroids?

They consist only of naked strands of RNA. Signup and view all the answers

What is a common characteristic of prions?

They induce misfolding of normal proteins. Signup and view all the answers

Kuru is mainly associated with which cultural practice?

Cooked and consumed dead family members. Signup and view all the answers

Which of the following diseases is NOT caused by prions?

HIV. Signup and view all the answers

What is the role of reverse transcriptase in ssRNA viruses like HIV?

Synthesizes DNA from the RNA genome. Signup and view all the answers

Which of the following statements about dsDNA viruses is correct?

Their viral DNA is replicated through an RNA intermediate. Signup and view all the answers

Animal viruses can be classified primarily based on which aspect?

The type of genetic material they contain. Signup and view all the answers

What distinguishes a bacteriophage from other types of viruses?

They specifically attack bacterial cells. Signup and view all the answers

Which of the following is an example of an emerging virus?

COVID-19. Signup and view all the answers

How can emerging viruses spread to new locations?

Through air travel. Signup and view all the answers

Which of the following is NOT a characteristic of plant viruses?

They can infect human cells. Signup and view all the answers

What is a common feature of emerging diseases?

They can infect new species. Signup and view all the answers

Study Notes

Viruses

Viruses are not considered to be living organisms, and they lack a cellular structure, which means they can't perform metabolic functions on their own
They're obligate intracellular parasites, meaning they require a host cell to replicate
The main components of a virus are a protein coat called the capsid and a core containing either DNA or RNA as its genetic material
Some viruses also possess spikes for attaching to host cells and an outer envelope
Viruses have a specific host cell that they infect
Once inside the host cell, the viral genome exploits the host's cellular machinery to produce more viruses
Viruses are unable to replicate independently and rely on a host cell for their survival

Viral Reproduction Cycles

Lytic Cycle: The virus commandeers the host cell immediately, leading to the destruction of the infected cell and its membrane.
Lysogenic Cycle: The virus remains dormant until specific conditions trigger its entry into the lytic cycle.
Lytic Cycle Stages:
- Absorption: Attachment of the virus to the host cell
- Entry: Penetration of the virus into the host cell
- Replication: Replication of the viral genome using host cell resources
- Assembly: Formation of new viral particles
- Release: Release of newly formed viral particles from the host cell, often leading to host cell death
Lysogenic Cycle:
- The virus integrates its genetic material into the host cell's chromosome as a prophage
- The phage remains dormant until a trigger activates its transition to the lytic cycle, resulting in replication, assembly, and release, ultimately killing the host cell

Viral Classification

Classifying viruses is more intricate than classifying living organisms due to their diverse evolutionary origins.
Viruses are often described as "remnants" of other organisms, making traditional genomic or protein analysis less effective.
Viruses lack a common universally shared genomic sequence.
Previous classification methods included:
- Genome structure: DNA vs RNA, single-stranded vs double-stranded, linear vs circular, non-segmented vs segmented genomes
- Capsid structure: Naked icosahedral, enveloped icosahedral, enveloped helical, naked helical, complex with multiple proteins
Baltimore Classification: This method, named after Nobel laureate David Baltimore, classifies viruses based on their mRNA production methods during their replicative cycle.
- Group I: dsDNA - mRNA is transcribed directly from DNA
- Group II: ssDNA - DNA is converted to dsDNA before mRNA transcription
- Group III: dsRNA - mRNA is transcribed from the RNA genome
- Group IV: ssRNA (+) - Genome functions as mRNA
- Group V: ssRNA (-) - mRNA is transcribed from the RNA genome
- Group VI: ssRNA (reverse transcriptase) - Reverse transcriptase produces DNA from RNA, which is then integrated into the host genome; mRNA is transcribed from the incorporated DNA
- Group VII: dsDNA (reverse transcriptase) - Viral DNA replicates through an RNA intermediate; RNA can serve as direct mRNA or a template for mRNA production

Host Domain Viral Classification

Viruses can be categorized based on the types of hosts they infect:
- Animal viruses: Viruses that infect animal cells, including humans (e.g., influenza virus, rabies virus, mumps virus, poliovirus). They typically have RNA or DNA genomes.
- Plant viruses: Viruses that infect plants. Their genetic material is usually RNA, enclosed within a protein coat. (e.g., tobacco mosaic virus, potato virus, beet yellow virus, turnip yellow virus).
- Bacteriophages: Viruses that infect bacterial cells. They typically contain DNA as their genetic material. Each bacteriophage usually attacks only one specific species or strain of bacteria.

Emerging Viruses

Emerging viruses cause diseases that have recently infected large populations.
Examples include: COVID-19, HIV, West Nile virus, SARS virus, Hantavirus, Ebola virus, Avian influenza (H5N1) virus, Zika Virus.
They emerge through various mechanisms:
- Transportation to new locations: Often facilitated by air travel
- Infection of new species: Ability to infect a different host species
- New modes of transmission: Development of novel transmission pathways

Anti-Viral Medication

Developing anti-viral medication is challenging because viruses use their host's cellular machinery to replicate.
Some drugs are structurally similar to nucleotides, interfering with viral genome synthesis.
Other drugs target viral enzymes like reverse transcriptase in HIV.
Prevention is the most effective defense:
- Vaccinations
- Social distancing
- Mask wearing

Viroids

Exclusively infect plants
Consist of naked strands of RNA, not enclosed within a capsid
They take over the host cell to produce more viroids
Responsible for about a dozen crop diseases

Prions

Proteinaceous infectious particles
Malformed proteins that convert normal proteins into more misshaped proteins
Discovered as the cause of kuru
Also responsible for mad cow disease (bovine spongiform encephalopathy) and Creutzfeldt-Jakob disease

Kuru

A deadly prion disease that affected many people in Papua New Guinea
The disease was spread through the traditional practice of consuming the brains of deceased family members.
The brain was most likely to transmit kuru.

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Description

This quiz covers the nature of viruses, highlighting their non-living status and essential components such as the capsid and genetic material. It also explores the lytic and lysogenic reproduction cycles, providing insights into how viruses replicate inside host cells.