CAS BI 114 Lecture Notes - Lecture 5: Protozoa, Phospholipid, Cell Nucleus

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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.

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