A2.3 Viruses

What are viruses?

Viruses are non-cellular agents that infect cells and reproduce inside them.

Why are viruses thought to have multiple origins?

Viruses share relatively few features, and these shared features likely evolved through convergent evolution for functional reasons.

What is the typical size range of viruses?

Most viruses are between 20 and 300 nanometres in diameter.

Why must viruses be smaller than their host cells?

Viruses must be small to enter host cells and because they lack cytoplasm and other structural features.

Do viruses grow or increase in size?

No, viruses do not grow; they are assembled inside host cells to their full size.

What is the genetic material in viruses made of?

Viruses have genes made of DNA or RNA.

Why do viruses use the universal genetic code?

Viruses use the universal genetic code because they rely on the host cell's mechanisms for protein synthesis.

What is a capsid?

A capsid is a protein coat that encloses a virus's genetic material.

What is the purpose of the capsid in viruses?

The capsid protects the genetic material and gives the virus a symmetrical structure.

Do viruses have cytoplasm or enzymes?

Viruses released from host cells have no cytoplasm and contain few or no enzymes.

How do viruses produce enzymes during infection?

Viruses rely on the host cell's metabolism to produce enzymes needed for replication or cell lysis.

Why are viruses diverse in shape and structure?

Viruses evolved independently and have multiple evolutionary origins, leading to diversity.

What forms can viral genetic material take?

Viral genetic material can be single- or double-stranded DNA or RNA, and can be linear or circular.

What are positive-sense RNA viruses?

Positive-sense RNA viruses use their RNA directly as messenger RNA.

What are negative-sense RNA viruses?

Negative-sense RNA viruses transcribe their RNA to produce messenger RNA.

What are retroviruses?

Retroviruses make double-stranded DNA copies of their RNA and then transcribe it to produce mRNA.

What is the difference between enveloped and non-enveloped viruses?

Enveloped viruses have a membrane derived from the host cell, while non-enveloped viruses do not.

How does the membrane help enveloped viruses?

The membrane helps enveloped viruses contact and infect host cells.

What is a lytic cycle?

In the lytic cycle, viruses reproduce and burst the host cell, killing it.

What is a lysogenic cycle?

In the lysogenic cycle, viral DNA integrates into the host's DNA without killing the host.

What is a temperate virus?

A temperate virus does not harm its host and exists as a prophage in the lysogenic cycle.

How can lysogenic viruses benefit their host?

Lysogenic viruses can transfer genes from previous hosts, increasing the genetic diversity of the host.

Why is the genetic code in viruses and cells similar?

Viruses likely evolved from cells, inheriting the genetic code.

What is the progressive hypothesis of viral evolution?

Viruses evolved by taking and modifying cell components, such as retrotransposons.

What are retrotransposons?

Retrotransposons are nucleotide sequences in eukaryotic genomes that replicate using reverse transcription.

What is the regressive hypothesis of viral evolution?

Viruses evolved from cells through the loss of cell components.

What is an example of a virus-like bacterium?

Chlamydia, a parasitic bacterium with a small genome, was once thought to be a virus.

Why are viruses diverse according to the progressive and regressive hypotheses?

Viruses may have originated from different pathways, explaining their diversity.

Why can viruses evolve rapidly?

Viruses evolve rapidly due to short generation times, high mutation rates, and intense natural selection.

What contributes to high mutation rates in viruses?

Viruses, especially RNA viruses, lack proofreading mechanisms during replication.

How does natural selection act on viruses?

Natural selection favors viruses that evade the host immune system, promoting rapid evolution.

What is the genetic material of the influenza virus?

The influenza virus uses negative-sense single-stranded RNA as its genetic material.

Why does the influenza virus have a high mutation rate?

Its RNA replicase does not proofread or correct errors during replication.

What is unique about the influenza virus genome?

The genome consists of eight separate RNA molecules.

How do new strains of influenza virus appear?

New strains appear when RNA molecules from different strains combine in a host cell.

What are the antigens on the influenza virus?

Haemagglutinin and neuraminidase are antigens on the influenza virus.

What do haemagglutinin and neuraminidase do?

Haemagglutinin binds to host cells, and neuraminidase helps release viruses from host cells.

Why do influenza vaccines change every year?

Influenza evolves rapidly, producing new strains that require updated vaccines.

What type of virus is HIV?

HIV is a retrovirus that converts RNA to DNA using reverse transcriptase.

Why does HIV have a high mutation rate?

HIV's reverse transcriptase does not proofread errors, and cytidine deaminase introduces mutations.

How does HIV evade the immune system?

HIV evolves rapidly, producing new strains that avoid recognition by antibodies.

Why are HIV infections chronic?

HIV's rapid evolution and resistance to immune responses make infections chronic.

What is the role of the env gene in HIV?

The env gene codes for a protein that allows HIV to bind and enter host cells.

How does HIV develop drug resistance?

HIV mutations allow it to resist antiretroviral drugs, requiring combination therapy.

Why do viruses rely on host cells?

Viruses lack the machinery for replication and depend on host cells for metabolic functions.

What is convergent evolution in viruses?

Convergent evolution explains shared features of viruses that evolved independently for functional reasons.

What is the advantage of viral symmetry?

Symmetry in viruses allows efficient self-assembly of their capsids.

How are viral capsids formed?

Viral capsids form through the self-assembly of repeating protein subunits.

What happens during the lysis process?

Lysis bursts the host cell, releasing new viruses.

Why do lysogenic viruses remain undetectable?

Lysogenic viruses integrate into host DNA and do not produce new virus particles.

What triggers a lysogenic virus to enter the lytic cycle?

Stimuli from inside or outside the bacterial cell can activate prophage genes.

What is a prophage?

A prophage is viral DNA integrated into bacterial DNA during the lysogenic cycle.

Why is the influenza virus prone to pandemics?

New strains from antigenic changes can spread rapidly across populations.

What is antigenic shift in influenza?

Antigenic shift occurs when influenza viruses exchange genetic material, creating novel strains.

What are the two main hypotheses for the origin of viruses?

The progressive and regressive hypotheses explain viral origins through evolution from or loss of cellular components.

What is the role of antibodies in viral infections?

Antibodies target viral antigens, aiding in the destruction of viruses.

Why do RNA viruses have higher mutation rates than DNA viruses?

RNA viruses lack error-checking mechanisms during replication.

What is the significance of retroviruses in evolution?

Retroviruses may have evolved from retrotransposons, sharing similar mechanisms of integration.

How do new variants of HIV evolve in a single host?

HIV mutations and recombination between strains produce new variants within an infected individual.

What is a disadvantage of viral virulence?

Virulent viruses may kill their host or be detected and destroyed by the immune system.

Why do viruses with high virulence risk extinction?

Highly virulent viruses may destroy all available hosts, leading to their own extinction.

What are the functional reasons for the small size of viruses?

Small size allows viruses to enter host cells and reduces structural complexity.

What is the significance of fixed size in viruses?

Viruses are assembled to a specific size using a fixed number of components.

What is the function of glycoproteins in enveloped viruses?

Glycoproteins aid in binding and infecting host cells.

How do bacteriophages infect host cells?

Bacteriophages bind to the host surface and inject their DNA through a tubular tail.

What is the role of RNA replicase in the influenza virus?

RNA replicase replicates the viral genome but introduces mutations due to lack of proofreading.

Why do retroviruses require reverse transcriptase?

Reverse transcriptase converts RNA to DNA, allowing integration into the host genome.

How do cytidine deaminase mutations affect HIV?

Cytidine deaminase introduces mutations in HIV, some of which may protect the host.

Why is the universal genetic code important for viruses?

The universal genetic code allows viruses to hijack host cellular machinery for protein synthesis.

What is the structure of a non-enveloped virus?

Non-enveloped viruses lack a membrane and consist of a capsid enclosing genetic material.

Why do viruses evolve through natural selection?

Viruses that evade immune responses or adapt to hosts survive and reproduce.

How do viruses spread within multicellular hosts?

Viruses following the lytic cycle spread from cell to cell, increasing infection.

What is the difference between virulent and temperate viruses?

Virulent viruses kill hosts, while temperate viruses integrate into host DNA without immediate harm.

Why is HIV difficult to treat?

HIV's rapid mutation and drug resistance make treatment challenging.

What is the role of phospholipids in enveloped viruses?

Phospholipids form the membrane around enveloped viruses, derived from the host cell.

How do viral mutations lead to immune evasion?

Mutations alter viral antigens, preventing recognition by host antibodies.

Why is vaccination less effective against rapidly evolving viruses?

Rapid evolution creates new viral strains not covered by existing vaccines.

What are retrotransposons?

Retrotransposons are sequences of nucleotides that replicate within genomes by reverse transcription of RNA into DNA and inserting copies into chromosomes.

How are retrotransposons similar to retroviruses?

Both use reverse transcription of RNA into DNA and integration into host chromosomes.

What would retroviruses need to evolve from retrotransposons?

Capsid proteins would need to evolve from host cell proteins.

What is the regressive hypothesis of virus evolution?

Viruses evolved by losing cell components in a series of steps from parasitic cells.

What evidence supports the regressive hypothesis of virus evolution?

Examples of small parasitic bacteria like Chlamydia, which have lost some self-reliant metabolic functions.

What are mimiviruses?

Mimiviruses are large viruses with a diameter of 0.75 micrometers and a genome of 1.2 million base pairs.

How do mimiviruses differ from smaller viruses?

Mimiviruses have some of their own enzymes and perform functions usually left to host cells.

How do viruses show diversity in complexity?

Some viruses are very small with simple structures, while others are larger and have more complex functions.

What is the connection between parasitic bacteria and viruses?

Some parasitic bacteria, like Chlamydia, might represent evolutionary intermediates toward viruses.

What is convergent evolution in viruses?

Convergent evolution occurs when viruses share features for functional reasons, not due to common ancestry.

Why can viruses evolve rapidly?

Viruses have short generation times, high mutation rates, and intense natural selection.

What is the relationship between mutation rates and virus evolution?

High mutation rates increase genetic variation, which accelerates virus evolution.

Why do RNA viruses evolve faster than DNA viruses?

RNA viruses lack proofreading during replication, resulting in higher mutation rates.

What is the role of natural selection in viral evolution?

Natural selection favors virus variants that evade host immune systems and survive.

What are the three reasons for the rapid evolution of viruses?

Short generation times, high mutation rates, and strong natural selection pressures.

How does influenza virus replicate its genome?

It uses RNA replicase, which lacks proofreading, causing high mutation rates.

Why does the influenza virus have a segmented genome?

The genome has eight separate RNA molecules, enabling recombination between strains.

How do new influenza strains appear?

New strains appear when two different strains infect a host and recombine their RNA.

What role do haemagglutinin and neuraminidase play in influenza infection?

Haemagglutinin helps bind to host cells, and neuraminidase helps release viruses from host cells.

What is the significance of haemagglutinin and neuraminidase in influenza strains?

Different combinations of these proteins define strains, such as H1N1 or H3N2.

Why do influenza vaccines need to be updated yearly?

New influenza strains emerge frequently due to mutation and recombination.

What causes the high mutation rate in HIV?

HIV uses reverse transcriptase, which does not proofread, and is also affected by the host enzyme cytidine deaminase.

How does cytidine deaminase affect HIV?

It converts cytosine to uracil, introducing mutations in the viral genome.