L5 Flashcards
What are viruses?
Biological entities unable to reproduce independently. They carry their nucleic acid within a protein shell. Not considered to be true living microbial organisms. Do not have 16S and 18S rRNA so lack ribosomes. No ability to make proteins so rely on hosts translation machinery to make viral proteins that are assembled in the viral progeny. Host dependent, infectious agents, vectors for treatment of disease, driver of evolution.
What is a virus particle (virion)?
Extracellular form of a virus. Exists outside host and facilitates transmission from one host cell to another. Contains nucleic acid genome surrounding by a protein coat and, in some cases, other layers of material.
How diverse are viruses?
Many diff shapes and sizes. smaller than prokaryotic cells and range from 20-300nm
What is a nucleic acid?
RNA or DNA, single stranded or double stranded.
What is a capsid?
A proteinaceous shell made from structural subunits known as capsomers. Some viruses have their capsid surrounded by an envelope of proteins, lipids and carbohydrates.
What is the structure of the virion?
Nucleocapsuds are constructed in highly symmetric ways. Helical symmetry : rod shaped viruses, length of virus determined by length of nucleic acid, width determined by size and packaging of protein subunits. Icosahedral symmetry: spherical viruses. Most efficient arrangement of subunits in a closed shell. May be combined as head and tail.
What is a nucleocapsid?
Complete complex of nucleic acid and protein packaged in the virion
What is an enveloped virus?
Virus that contains lipid bilayer with embedded proteins around the nucleocapsid
What is the viral envelope made of?
Proteins, lipids, and carbohydrates. The proteins are encoded by the viral genome. The lipids and carbohydrates derive from the host cellular membranes. These cellular components are picked up by the virus as it extrudes through the host memrbane.
What is the process of viral budding through the plasma membrane?
The host cell membrane before or early in the infection. Viral encoded DNA proteins associate with the plasma membrane. Viral glycoproteins spikes are incorporated into the membrane. Viral nucleic acids and proteins assemble near the membrane, budding begins. Budding continues with more viral spikes inserted into the membrane. Mature virion is released.
What enzymes does the virion contain?
Lysosomes - make hole in cell wall, lyses bacterial cell. Nucleic acid polymerases. Neuraminidases - enzymes that cleave glycosidic bonds, allows liberation of viruses from the cell.
How are viruses classified?
By nucleic acid, presence or absence of an envelope. What their host is : bacterial, archaeal, fungal, plant, animal. Their size and shape.
What are bacterial viruses?
Bacteriophages are very diverse, best studied bacteriophages infect enteric bacteria. Most phages contain dsDNA genomes. Most are naked, but some possess lipid envelopes. They are structurally complex, containing head, tails, and other components.
What are animal viruses?
Entire virion enters the animal cell. The eukaryotic nucleus is the site of replication for many animal viruses, there are many more kinds of enveloped animal viruses than enveloped bacterial viruses.
What do viruses consist of and vary in?
Consist of different strains. Vary in virulence and antigen properties.
What two pathways are used to infect their host?
Lytic pathway or lysogenic pathway
What is the Lytic pathway?
Circular dsDNA is replicated, transcribed and translated into proteins to make components of the viral progeny. Progeny is released.
What is the lysogenic pathway?
Circular dsDNA integrate in the chromosome. The prophage is replicated with the host chromosomes and passed to the daughter cells. Environmental stimuli induce phage excision. Lytic cycle begins
What is a virulent mode?
Viruses can lyse host cells after infection (lytic cycle)
What is a temperate mode?
Viruses can replicate their genomes in tandem with the host and without killing the host (lysogenic cycle)
What are temperate viruses?
Can undergo stable genetic relationship with the host. But can kill cells through lytic cycle.
What is lysogeny?
State where most virus genes are not expressed and virus genome is replicated in synchrony with host chromosome
What is the basic process of the lytic cycle?
Attachment of the virus to a susceptible host cell
Early penetration of the virion or its nucleic acid
Synthesis of virus nucleic acid and protein by cell metabolism as redirected by virus
Assembly of capsids and packaging of viral genomes into new virions
Release of mature virions from host cell
What happens in the lytic cell? - attachment and entry?
Attachment is highly specific, requires complementary receptors on the surface of a susceptible host. Receptor on host carry out normal functions for cell. Receptors include proteins, carbohydrates, glycoproteins, lipids, lipoproteins, or complexes.
What happens in the lytic cell? - attachment and entry of bacteriophage T4?
Bacteriophage T4 - virus of E. coli one of the most complex penetration mechanisms. Virions attach to cells via tail fibers that interact with polysaccharides on E. coli cell envelope. Tail fibers retract, and tail core makes contact with E. coli cell wall. Lysosomes-like enzyme forms small pore in peptidoglycan, tail sheath contracts, and viral DNA passes into cytoplasm, capsid is abandoned outside the cell.
What happens in the lytic cell? - attachment and entry In human diseases?
Herpesvirus uses proteins in its envelope to bind the host receptors. Binding of the hosts receptors triggers fusion of the viral envelope with the hosts cell membrane. Other viruses fuse their envelopes with the cell membrane or enter the cell by endocytosis.
What happens in the lytic cell? - replication of viral nucleic acid?
DNA virus can be - double stranded DNA (dsDNA), single stranded DNA (+sense) ((-) ssDNA), single stranded DNA (-sense) ((-) ssDNA)
RNA virus can be - double stranded RNA (dsRNA), single stranded RNA (+sense) ((-) ssRNA), single stranded DNA (-sense) ((-) ssRNA)
What is the central dogma of molecular biology?
Fundamental framework that describes the flow of genetic information within a biological system. Replication -> transcription -> translation.
RNA viruses - what is a retro virus?
Virus that uses RNA as its genetic material. When a retrovirus infects a cell, it makes a DNA copy of its genome that is inserted into the DNA of the host cell. Retroviral dsDNA is integrated in the host genome by viral intergrase. Viral mRNAs and proteins are made by the host.
Lytic cycle - how does virion assembly and release from the host occur in nonenveloped viruses?
Viral capsomers self-assemble forming the capsid or the head. Viral DNA is packed in the capsid/head. In phages, the tail and tail fibers are assembled. Viral proteins degrade the cell membrane and virions are released.
Lytic cycle - how does virion assembly and release from the host occur in phages?
Holing make pores in the cell membrane, enlysins (with lysosome activity) degrade the peptidoglycan layer. Outer membrane bursts because of the internal osmotic pressure of the cell. The cell membrane and the cell wall are debilitated.
Lytic cycle - how does virion assembly and release from the host occur in enveloped viruses?
Viral capsomers self-assemble forming the capsid. Viral DNA is packed in the capsid. Virions are released by budding. The release of virions does not kill the cell.
What are host defences against viruses in eukaryotes and prokaryotes?
Eukaryotes - possess mechanisms to diminish viral infections. E.g. immune defence mechanisms, RNA interference.
Prokaryotes - CRISPR, restriction enzymes cleave double-stranded DNA at specific sequences. Modifications of hosts own DNA at restriction enzyme recognition sites prevents cleavage of own DNA.
What viral counterdefence mechanisms are there?
Viral mechanisms to evade bacterial restriction systems. Chemical modification of viral DNA. DNA contains the modified base 5-hydroxymethylcytosine. Production of proteins that inhibit host cell restriction system.
How do antiviral therapies work? E.g. acyclovir
A synthetic analogue of guanosine, blocks elongation of viral DNA by the virus-encoded DNA polymerase. Effective against herpesviruses
Why do viruses have mutations?
Viruses do not always replicated precisely so mutations arise with high frequency. Cumulative small mutations or one major can give rise to a different strain. Mutations can make virus resistant to drugs.
How do viruses adapt to by mutation?
RNA viruses adapt faster, there are no RNA repair mechanisms. RNA replicate does not have proof-reading activity.
Why do viruses matter?
Only a small proportion of viruses cause diseases in humans, animals and plants. Most of the viruses infect bacteria and archaea only. Viruses are the most abundant biological entities in the ocean. There are 10 times more viruses than cells in seawater. In soil and marine sediments, viruses are more abundant than in seawater
What key role do viruses play in ecology?
Virus predation keeps the population of microbes at bay, preventing blooms that would disrupt the balance of the ecosystem. Prophages alter the host metabolism. Phages are major vehicles for horizontal gene transfer between prokaryotes.
How does Virus predation keeps the population of microbes at bay, preventing blooms that would disrupt the balance of the ecosystem?
Like protists, viruses are important bacterial and phytoplankton predators in the ocean.
How is virus predation a cure?
Phage therapy or biocontrol. Higher specificity than an antibiotic. Typically harmless to human/animal and beneficial to microbiota. Currently not authorised for humans.
How do prophase alter the host metbolism?
Phage influences on bacterial behaviours including motility, biofilm formation, defence, toxicity, replication, metabolism, sporulation, stress response, and quorum sensing.
How are phages major vehicles for horizontal gene transfer between prokaryotes?
HGT is the movement of genetic material between organisms. The process in which bacterial DNA is moved from one bacterium to another by a virus is transduction. During synthesis of new phages the phage incorporates some host DNA. Infecting a new host incorporates the phage and the previous host DNA into the host.
