Genomas Virales

Viral Genomes

• Viruses can have either DNA or RNA genomes.
• Different types of viruses have different genomic structures (single-stranded or double-stranded).
• RNA viruses have faster evolution capacity, leading to rapid adaptation.
• RNA viruses have plastic genomes that can increase coding capacity through segmentation and polyproteins.
• dsDNA viruses have greater storage capacity which results in a broader protein arsenal.
• dsDNA viruses are more stable and harder to detect in the nucleus, leading to persistent infections through latency.

Viral Origins

• Early virology studies were linked to observations of plant diseases.
• The infectious agent of tobacco mosaic disease was characterized by Martinus Beijerinck in 1899.
• The pathogen was recognized as a virus, distinct from bacteria.
• Viruses were also initially studied through observations of animal disorders.

The Puzzle of Virus Structure

• Early research struggled with understanding the structural nature of viruses.
• Stanley (1935) concluded that a virus was essentially protein in nature.
• Bawden (1936) proposed the concept that viruses are ribonucleoprotein.

Peter Medawar on Viruses

• Medawar described a simplistic but insightful quote expressing that a virus is malicious news wrapped up in a protein.

Viral Lifecycle

• The fundamental aim of a virus is reproduction.
• A general virus lifecycle includes attachment, entry, replication and expression of copies of the genome, assembly, and release.

Genomic Flavors of Viruses

• Viruses exhibit diversity in their genomic configurations, categorized as single-stranded RNA, double-stranded RNA, single-stranded DNA, and double-stranded DNA.
• This includes examples of various known viruses such as SARS-CoV-2, HIV, influenza, rabies, reovirus, rotavirus, parvovirus, hepatitis C, measles, Zika virus, poliovirus and the rift valley fever virus.

Viral Molecular Differences

• RNA viruses typically use uracil.
• RNA viruses typically consist of single-stranded genetic material.
• RNA usually resides in the nucleus and/or cytoplasm.
• RNA has an OH at the 2' ribose position.
• DNA viruses typically use thymine
• DNA viruses typically consist of double-stranded genetic material
• DNA mainly resides in the nucleus.
• DNA has an H at the 2' ribose position.

Biological Implications of Viruses

• DNA is considered more stable than RNA.
• RNA viruses have a higher mutation rate and are more susceptible to mutations, while DNA viruses have a lower mutation rate
• RNA viruses have a much smaller genome size compared to DNA viruses
• Different genome sizes confer varying mutation rates across numerous virus families.

Viral Heterogeneity

• Viruses can exhibit genetic variability termed quasispecies.
• Viral heterogenicity depends on mutation rate.
• Viral fitness drives selection, with only beneficial mutations being retained.

Mutation Incidence and Antigenic Drift

• Mutations may cause significant changes or antigenic drift in virus structure.
• Antigenic drift is a stochastic process of accumulated small mutations, where significant advantages arise through selective pressure.
• Prominent changes can occur in various parts of the virus such as the antibody scape and MHC scape.

Evolution of SARS-CoV-2 Spike

• The N439K mutation enhances binding affinity for the ACE2 receptor.
• The Y453F mutation increases ACE2 binding affinity.
• Mutations can contribute to increased viral infectivity.
• Key amino acid mutations can impact the receptor-binding domain (RBD), leading to decreased antibody effectiveness.

Segmented Viruses

• Segmented viruses have genomes with genes distributed across multiple RNA or DNA molecules .
• All segments must be incorporated into the viral particle for infectivity.
• Examples include influenza virus and Rift Valley fever virus.

Genome Reassortment and Antigenic Shift

• Genome reassortment is the mixing of genomic segments across different viral strains.
• Antigenic shift is a significant change in viral antigen due to reassortment or inter-strain recombination.

Recombination

• Recombination is the genetic exchange of material between viruses or with the host.
• This process contributes to significant alterations in viral genes.
• Recombination can cause significant changes, especially in RNA viruses where replication is discontinuous.

Genetic Storage Capacity

• Genetic storage capacity is correlated with mutation rates in viruses.
• Viruses that store genetic information in RNA have a higher mutation rate, while viruses with DNA have a lower mutation rate relative to genome size.

Viral Mimicry

• Viruses employ strategies to mimic host cell receptors or inducers, manipulating the immune response by mimicking host factors.
• Viruses may mimic or employ immuno-modulators to interfere with the host immune response

LIR1 Inhibitors

• CMV UL18 interacts with natural killer receptors, blocking their activation, resulting in the loss of MHCs in infected cells preventing immune recognition.

Disabling the Immune System

• DNA viruses frequently feature large genomes, enabling them to deploy various mechanisms designed to disable the host’s immune response. These may include decoys and disabling parts of the immune cell signaling pathways.

Absence of Antigens/Latency

• DNA viruses can employ a latency mechanism, where the virus remains dormant and quiescent within the host cell.
• Lack of viral antigens for a prolonged period can evade the host’s immune response.

Este quiz explora la struktur e origines de virus. Discurre sobre genomas viral e lor capacitate evolutive, como tamque lor storages de information. Preparea te pro un examen de virologia e apprende sobre virus RNA e DNA.