Viruses Flashcards
What is a virus?
- infectious particle
- genetic material packaged in a protein case
Is a virus alive?
- viruses are somewhere between being alive and being chemicals
- they cannot reproduce outside of a host cell
- outside of a host cell, they exist in an inert / non-living state
Viral Genome Diversity
- viruses can have DNA or RNA genomes
- they can be single stranded or double stranded
- a single linear or circular molecule of nucleic acid
Pox Virus
- up to 450nm
- double stranded
- circular
Parvovirus
- less than 20nm
- single stranded
- linear
Capsid
Definition
Protein case that surrounds the viral genome
Capsomeres
Defintion
protein subunits that make up the capsid
Host Range
Definition
the range of species that a virus is able to infect
Bacteriophages / Phages
Definition
viruses that infect bacteria
The Lytic Cycle
Definition
the replicative cycle of some double stranded DNA phages, the cycle results in the lysis of the host cell i.e. host cell death
The Lytic Cycle
Steps
1) the phage attatches to the host cell surface
2) the phage enters the host cell and hydrolyses the host DNA
3) more viral DNA and proteins are synthesised using host cell organelles
4) the viral proteins and DNA spontaneously form new phage particles
5) The viral DNA also results in the production of an enzyme which allows excess fluid to enter the cell
6) the cell swells and busts releasing the phage particles
Virulent Phage
Definition
a phage that reproduces only by the lytic cycle
Lysogenic Cycle
Definition
Replicative cycle of a phage during which viral DNA is integrated into the host cell genome, this replication does not result in host cell death
Lysogenic Cycle
Steps
1) the phage enters the host cell
2) the phage DNA integrates into the host cell genome
3) when the bacterium reproduces, the prophage is passed to the daughter cells
4) the prophage contains a gene which supresses transcription of other genes in the prophage so that the prophage can spread through the population without killing the host
5) it is possible for the prophage to leave the bacterial chromosome and initiate a lytic cycle
Animal Viruses
-one key distinguishing feature between the different animal viruses is the genome: DNA double stranded DNA single stranded RNA double stranded RNA single stranded
Envelope
Some viruses have a lipid bilayer surrounding their capsid, these viruses are said to be enveloped
Glycoproteins
some viruses have glycoproteins on their outer surface which allows them to interact with target cells
Animal Viruses - DNA
Characteristics
- 5kb - 1.2Mb
- mainly double stranded, 1 family of single stranded
- liner and circular
- most use host polymerase to replicate in host nucleus
- enveloped and non-enveloped
- often splice viral mRNA into host
- genomes often contain overlapping genes
Animal Viruses - RNA
Characteristics
- small genomes ~ 3.3kb
- have to use their own polymerase as eukaryotes cannot replicate RNA
- replicate quickly
- error prone polymerase
- no proof reading
- mostly single stranded, 1 family of double stranded
- are either positive or negative sense
What is the name of the polymerase enzyme in RNA animal viruses?
RNA dependent RNA polymerase
Double Stranded DNA Replication Cycle
1) virus enters host cell
2) capsid breaks down
3) viral DNA is replicated
4) transcription of viral DNA -> RNA
5) translation and synthesis of capsid proteins
6) self assembly of new virus particles from DNA and capsomeres
7) new viral particles exit cell
Positive Single Stranded RNA Viruses - Replication Cycle
1) viral genome is already mRNA, it is replicated
2) Some of this mRNA is transcribed in to capsomeres and glycoproteins
3) some negative (template) RNA strands are also synthesised and used as templates to make more positive strands
4) the capsomeres self assemble to form capsids around the copied RNA
5) the glycoproteins are synthesised in the ER
6) sections of the RER membrane fuse with the plasma membrane so that the glycoproteins are on the surface of the cell
7) when the capsid buds from the cell, part of the plasma membrane forms an envelope around it, the glycoproteins are on the outside of the viral particle
Retrovirus
Definition
Single stranded positive sense RNA virus with a DNA intermediate
Negative Single Stranded RNA Virus - Replication Cycle
1) Virus enter host cell
2) it uses its own polymerase to transcribe the negative RNA into positive RNA that can be translated
3) some of the positive strands are used as templates for the synthesis of more negative strands
4) the other positive strands are translated and capsomeres and glycoproteins are synthesised
5) the capsomeres and RNA self assemble to from new virus particles which are released from the cell
Lock and Key
There is a lock and key model for the interaction between viral glycoproteins and host cell surface receptors
How does a virus insert its genome into a host chromosome?
using an integrase enzyme
Viral Envelopes
Charactersitics
- associated with glycoproteins
- derived from host plasma membrane
Purpose of Glycoproteins on the Surface of a Virus
- used to enter the host cell
- protect the capsid by hiding it from immunofactors
Host Tropism
the range of cell types within a host that a virus can infect e.g. come viruses are tissue tropic, they only infect certain tissues
What affects host range?
- cell surface recptors
- viral protein receptors
- (host) intracellular conditions effecting availability of enzymes
What effects the efficiency of viral attachment?
- density of receptors on host cell surface
- density of ligands on viral surface
- concentration of viral particles and host cells
- temperature
- pH
- presence/absence of specific ions
Emerging Pathogen
Definition
completely new/unseen before pathogen that rapidly increases in frequency in a susceptible population
Causes of Pathogen Emergence
- mutation (especially RNA viruses)
- dissemination (transfer from a small isolated population to a larger population)
- jump to a new species
What are the candidates for the source of viral genomes?
plasmids and transposons
What are the two methods for change in a viral genome?
- Recombination
- Re-assortment
Re-assortment
- two different viruses infect the same host cell
- when the new viral particles spontaneously self assemble inside the host, a new capsid forms around a combination of genetic material from each of the two viral genomes
- this creates a new viral particle with a new genome
Recombination
- two viruses infect the same host cell
- parts of their genetic material are combined to form a single strand
- the capsid forms around the recombined genome and a new virus is created
Viral Replication and Quasispecies
- viral replication, especiallly RNA viruses, are highly error prone
- this means that there are many slight differences between viruses classified as the same speices, these are called quasispecies
Lysis
- mature viruses released by destruction of the infected host cell and its membrane
- this results in acute infection of the host
Budding
- virions released by budding leave the host cell membrane intact
- -can still damage the host by the cytopathic effect
- may result in an acute or chronic infection
