Viruses chapter

• Viruses were first discovered in the late 1800s by Russian scientist Dmitry Ivanovsky, who demonstrated the existence of viral particles by filtering sap from a diseased plant and using that filtrate to infect healthy plants
• Viruses are obligate intracellular parasites, meaning they must replicate within a host cell
• Viruses are composed of a nucleic acid genome (DNA or RNA) protected by a protein coat called a capsid
• Some viruses are enclosed by an envelope derived from the host cell's membrane, and some have protein spikes that allow them to attach to specific host cells
• Viruses infect specific types of cells, known as their host range, based on specific molecules on the virus surface that interact with molecules on the host cell surface
• Viruses can have diverse morphologies, ranging from spherical to helical to icosahedral structures
• Virophages are viruses that infect other viruses, using the larger virus as a vehicle to move between cells
• Viral classification is based on genetic makeup using the Baltimore classification system, which categorizes viruses by the type of genome they possess (DNA or RNA)
• Viruses can be grown in culture using living hosts like plants or animal model systems, or in cell culture for studying their biology and effects on cells
• Antibodies can be used to detect viruses in serological tests, as they specifically attach to certain chemical groups on the surface of a virus
• Viruses replicate based on the nature of their genome, with different types of viruses having distinct replication cycles
• Electron microscopy is used to understand the infectious cycles of viruses, providing dramatic images of different stages in the replication cycle
• All viruses follow a general replication cycle: attachment, entry, biosynthesis, maturation, and release
• Attachment is when the virus first attaches to the cell membrane of the host and enters the host cell
• During entry, the capsid is degraded, and the genomic instructions are released
• Biosynthesis occurs next, where the viral instructions are interpreted by the host cell to produce more virus particles
• Maturation follows biosynthesis, where the new viral particles assemble and mature
• Release is the final stage, where viruses exit either by budding (enveloped viruses) or by rupturing the cell (non-enveloped viruses)
• Enveloped viruses exit by budding, taking a piece of the host cell membrane as an envelope
• Non-enveloped viruses exit by rupturing the cell, destroying it
• Pinocytosis is a process where the virus is internalized by the cell through infolding of the membrane
• Enveloped viruses use fusion to exit, carrying a bilayer as they leave the infected host cell
• DNA viruses replicate by targeting the nucleus to use the host cell's DNA polymerase to make copies of their DNA genome
• RNA viruses, with plus strand genomes, are immediately infectious upon entry as they can be translated by the host ribosomes
• RNA viruses with negative strand genomes need to carry their own RNA-dependent RNA polymerase to activate replication inside the host cell
• The flu virus is an example of a negative strand, segmented genome virus that uses antigenic shifting and antigenic drift for increased diversity
• Fragmented genome segments of the flu virus are covalently attached to an RNA polymerase, allowing for the production of the coding strand for protein synthesis
• Antigenic shifting involves shuffling genome fragments to create hybrid viruses, while antigenic drift refers to the high mutation rate of RNA viruses
• RNA viruses mutate at a high rate due to less proofreading capacity of RNA polymerases compared to DNA polymerases
• RNA replication is an imperfect process with mutations accumulating each time an RNA molecule is copied
• Spike proteins on viruses change with each mutation, leading to different strains each flu season
• Flu viruses infect various animals, with each virus having a preference for its specific host based on interactions with sugary compounds on the host's cell membrane
• Avian flu viruses prefer the avian glycosidic bond configuration (alpha 2,3), while pigs have both alpha 2,3 and alpha 2,6 configurations in their cyalic acids, and humans predominantly have alpha 2,6
• Pigs can serve as incubators for both avian and mammalian viruses due to having both glycosidic bond configurations, leading to potential recombination and transmission to humans
• Retroviruses like HIV carry their own enzymes for replication inside the host, using reverse transcriptase to make a DNA copy of the RNA genome, which is then integrated into the host chromosome as a provirus
• HIV recognizes the CD4 receptor on white blood cells, with a co-receptor called CCR5 facilitating viral docking and internalization
• Antivirals target specific stages of viral replication, such as attachment, uncoating, reverse transcriptase, integrase, transcription, and protease, but can lead to rapid development of resistant viral populations
• Antiviral cocktails are often used for HIV treatment to target multiple stages simultaneously and reduce the likelihood of resistance development
• Bacteriophages can switch between lytic and lysogenic cycles, with the lytic cycle involving viral attachment, injection of DNA, replication, and cell lysis, while the lysogenic cycle integrates the viral DNA into the host chromosome as a prophage
• Virus replication


  The nature of the virus genome dictates the replication cycle, which varies depending on the type of virus