CAS BI 114 Lecture Notes - Lecture 5: Protozoa, Phospholipid, Cell Nucleus
Viruses
What are Viruses?
•Have their own genetic information (DNA/RNA)
•No ribosomes, golgi, etc.
•Cannot make their own proteins and cannot reproduce themselves
•Obligate Parasites - parasites that cannot reproduce themselves
•They need a cell to replicate their genome/components for them
•Their only life cycle occurs inside a cell (cannot be grown in agar, air)
•They can infect bacteria, protozoa, fungi, algae, plants, animals
•All viruses are parasites
•Pathogen - a disease causing agent
•Parasite - something that requires a different organism in order to live
•The difference between parasites and pathogens are whether or not they elicit disease
Viral Structure
•3 central components
•Genome: DNA or RNA
•Capsid: protein
•Envelope: usually modifies host membrane
•Made of phospholipids (taken from the infected cell’s
membrane)
•Viruses can be separated into two groups: envelope or
non-envelope
•Enzymes: helps manipulate cells
Capsids and Envelopes
•Capsids - made up of individual proteins called cypsomers
•Has three shapes
•Envelopes - generally host cell membranes with a few viral proteins stuck in
•The proteins that sick out from the external capsid or envelope are called spikes
•Spikes - important for latching onto the surface of the cell
Genome
•Central Dogma: DNA —> RNA —> Protein
•All viruses have a nucleic acid genome
•Genomes can be kept in DNA/RNA single stranded form or double stranded form
•Some RNA viruses can go directly to translation while others are retroviruses
•Retroviruses - RNA — DNA —> RNA —> Protein
•Virus uses its own enzyme (embedded into it) in order to go from RNA to DNA
•RNA that can go directly into translation = positive sense
•RNA that needs to be transcribed first = negative sense
•Goal of viruses - to make many copies in order to infect as many cells as possible
•This means the genome must replicate into a form that can be replicated and attached to the cell
Viral Life
•When they are on their own they are inactive
•Have no environmental needs
•This enables viruses to be stationary for a long time without losing their ability to infect
•Carry few to no ingredients for replication
•They must infect a host cell in order to reproduce
•Viruses must
•1. Dock onto the surface of the cell
•2. Enter the cell
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•3. Replication within the cell
•4. Leave the cell
•How these events take place dictates everything about a virus (ex: how it is transmitted, its immune
response, its resulting symptoms)
Docking/Adsorption
•Involves a specific interaction between the spikes on the envelope/capsid of the virus with receptors
on the cell surface
•Essentially an interaction between spikes and cell receptors
•This interaction determines the host range and cell type specificity
•Host Range - what types of cells/species a virus can infect
Entry and Un-coating
•As the virus enters the cell it sheds its envelope
•This can take various forms
•One method is for the virus to be engulfed in a vesicle, where the envelops indigested away
•Another way is for the envelope to merge with the cell membrane
Synthesis: Replication and Protein Production
•Viral nucleic acids begin to synthesize the building blocks for new viruses
•Some viruses come equipped with necessary enzymes for synthesis of viral components while other
viruses utilize the host’s enzymes
•Proteins for the capsid, spikes and viral enzymes are synthesized on the host’s ribosomes using the
host’s amino acids
•The goals of synthesis and replication are to…
•Reproduce the viral genome and package it into new visions
•Make proteins for new capsids, spikes, etc.
Genome
•Viruses can have genomes composed of RNA or DNA
•They need more genomic copies for their offspring who have the same nucleic acid
•In order to integrate their genome into the host, viruses must have or create DNA
•In order to make proteins, viruses must use RNA
•***Main Point: viral genome comes in a variety of types
•At some point, we need to manufacture more copies of the gnome
•Positive sense RNA
•Ribosomes will ONLY read this kind
•Negative sense RNA
•Need to be converted to positive sense
•Double stranded RNA
•Need a positive sense to convert
•Can make another dsDNA as well as a mRNA strand
•Retroviruses - converting RNA —> DNA and making copies from there
•These have their own enzymes
•Most cells cannot do this
Assembly
•Mature virus particles are constructed from the growing pool of parts
•1. First the capsid is laid down as an empty shell and then filled with nucleic acid
•2. The viral spikes are inserted into the host’s cell membrane so they can be picked up as the virus
•3. Buds off with its envelope
Release
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Document Summary
Dock onto the surface of the cell: 2. Leave the cell: how these events take place dictates everything about a virus (ex: how it is transmitted, its immune response, its resulting symptoms) Assembly: mature virus particles are constructed from the growing pool of parts, 1. First the capsid is laid down as an empty shell and then filled with nucleic acid: 2. The viral spikes are inserted into the host"s cell membrane so they can be picked up as the virus: 3. Release: assembled viruses leave their host in two ways, enveloped viruses bud off, non-enveloped and complex enveloped viruses (viruses that reach maturation in the cell nucleus. Or cytoplasm) - attack the host cell to leave (ex: explode) The cell suffers and may die: cytopathic effects - virus-induced cellular damage, cells may join together to make large, non-functional, multi-nucleated cells (syncytia, it undergoes changes in cell size, function and reproduction, 2.