19 Viruses Flashcards
KEY CONCEPTS 19.1 A virus consists of a nucleic acid surrounded by a protein coat 19.2 Viruses replicate only in host cells 19.3 Viruses, viroids, and prions are formidable pathogens in animals and plants
Why are viruses not considered living?
They are unable to reproduce or undergo metabolic activities outside of their host.
What observation did Mayer make?
He could transfer disease from one plant to another by rubbing sap from an infected plant onto a health one.
What is ’TMV’?
The ’tobacco mosaic virus’ which causes the leaves to become mottled.
How is genetic information carried in a virus?
Their genomes consist of double-stranded DNA, single-stranded DNA or single stranded RNA depending on the type of virus.
The genome is typically a single linear or circular molecule of nucleic acid. Some viruses have multiple nucleic acids (like multiple chromosomes in eukaryotes)
How many genes does a virus have?
Anywhere from 4 to a thousand
How many genes does a bacterium have?
200 to a few thousand.
What are some examples of viruses with distinct structures?
’Tobacco mosaic viruses’, ‘adenoviruses’, ‘influenza viruses’ and ‘bacteriophage T4’
What are some basic structures that many viruses have?
‘Capsids’ and ‘viral envelopes’
What are capsids?
A protein shell which encloses the viral genome.
Capsids are made from a large number of protein subunits called ‘capsomeres’ (each capsid has very few unique types of protein)
What are the protein coats of viruses called?
Capsids
What are viral envelopes?
Envelopes derived from the membranes of the host cell which wrap around the virus.
They contain host cell phospholipids and membrane proteins.
They also contain proteins, glycoproteins and occasionally viral enzymes that are produced by the virus.
What are the basic shapes of viruses?
‘Helical viruses’, ‘icosahedral viruses’ and other more complex forms.
What is the structure of a ‘helical virus’?
It has a capsid made of a spiral of protein to form a tube. The viral genome is in the centre of this tube.
What is the structure of a ‘icosahedral virus’?
The protein capsid is arranged so that it forms a polyhedron with 20 triangular faces i.e. a icosahedron.
(Glycoproteins may protrude from the surface)
What is an example of a helical virus?
Tobacco mosaic virus
What is an an example of an icosahedral virus?
Adenovirus
What is the structure of ’tobacco mosaic virus’?
It is a ‘helical virus’
What is the structure of an adenovirus?
It is an icosahedral virus with exactly 252 protein molecules in its capsid.
At each vertex of the capsid glycoproteins protrude.
What are viruses that infect bacteria called?
Bacteriophages
What are ‘bacteriophages’?
Viruses that infect bacteria.
What is the structure of an influenza virus?
Influenza viruses have an outer ‘viral envelop’e studded with glycoprotein spikes.
The genome consists of eight different RNA molecules, each wrapped in a helical capsid.
What are some examples of bacteriophages?
The first 7 were named T1 (type 1), T2, T3, T4, T5, T6 and T7
What is a specific type of bacteriophage and why are they significant?
‘T-even’ phages (T2, T4 and T6) have a similar structure.
What does ’T-even’ refer to?
A T2, T4 or T6 bacteriophage (coincidentally they have similar structures)
What are bacteriophages also called?
Phages
What are ‘phages’?
Bacteriophages i.e. viruses which infect bacteria.
What is the structure of a T-even phage?
It has an elongated icosahedral head which contains the DNA.
Attached to the head is a protein tail piece with fibers by which the phages attach to a bacterium.
This tail consist of a ’Tail sheath’ which is kinda like a body. At the bottom ’tail fibres’ branch off like legs.
Generally speaking, how specific are viruses in what hosts they will infect?
Most are quite specific and will infect only a few,
What are the hosts that a particular virus infects called?
The ‘host range’
What does ’host range’ refer to?
The range of host species that a particular vital species is capable of infecting.
What is the basic vital replicative cycle?
Virus enters cell and is uncoated, releasing viral DNA and capsid proteins.
Host enzymes replicate the viral genome.
Meanwhile, host enzymes transcribe the viral genome into viral mRNA, which host ribosomes use to make more capsid proteins.
Viral genomes and capsid proteins self-assemble into new virus particles, which exit the cell.
What species can the measles virus infect?
Only humans
What is an example of a virus that infects only one species?
Measles (only humans)
How does viral DNA enter a cell?
T-even phages use their tails to inject DNA.
Other viruses enter by endocytosis and release the DNA held in their capsid
What is the purpose of the tail of a T-even phage?
It injects viral DNA into a cell.
How is the viral genome replicated?
Many DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA. In contrast, to replicate their genomes, RNA viruses use virally encoded RNA polymerases that can use RNA as a template. (Uninfected cells generally make no enzymes for carrying out this process.)
Why is the host needed for viral replication?
It provides nucleotides for making viral nucleic acids, as well as enzymes, ribosomes, tRNAs, amino acids, ATP, and other components needed for making the viral proteins.
These components are not made by the virus as it can not undergo metabolic processes.
What allows the specificity of viruses and thus their limited ‘host range’?
Viruses usually identify host cells by a “lock- and-key” fit between viral surface proteins and specific receptor molecules on the outside of cells.
What can the reproductive cycles of phages be divided into?
The ‘lytic cycle’ and the ‘lysogenic cycle
What is the fundamental difference between the lytic cycle and the lysogenic cycle?
The lytic cycle eventually leads to the death of the cell whereas in the lysogenic cycle the cell continues to live as an infected cell.
What are the stages of the lytic cycle? (in the context of a T4 phage)
Attachment, Entry of Phage DNA & Degradation of Host DNA, Synthesis of Viral genome/proteins, Assembly and Release.
Attachment: The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell.
Entry of phage DNA and degradation of host DNA: The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolysed.
Synthesis of viral genomes and proteins: The phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using components within the cell.
Assembly: Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms.
Release: The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and bursts.
What is a phage that can only use the lytic cycle?
A virulent phage
What is a ‘virulent phage’?
A phage that reproduces exclusively by the lytic cycle
How does a bacteria protect itself from phages?
It release ‘restriction enzymes’ which breakdown the viral DNA.
What are ‘restriction enzymes’?
Enzymes that are realised by bacteria to destroy the DNA of phages that are infecting them.
(also used in biotechnology)
How does a bacteria protect its own DNA from destruction by the restriction enzymes its uses to defend against phages?
Its own DNA is methylated in a way which prevents attack
What are phages which can use both the lytic and lysogenic cycles called?
Temperate phages.
What are ’temperate phages’
Phages which can use both the lytic and lysogenic cycles
What is an example of a temperate phage?
Lambda (phage λ)
What happens during the ‘lysogenic cycle’?
Phage DNA integrates into the bacterial chromosome, becoming a ‘prophage’.
The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells.
Many cell divisions produce a large population of bacteria infected with the prophage.
Occasionally, a prophage
exits the bacterial chromosome, initiating a lytic cycle.
What does ’prophage’ refer to?
Viral DNA that has integrated into the bacterial chromosome during the lysogenic cycle
What is viral DNA that has integrated into the bacterial chromosome during the lysogenic cycle called?
‘Prophage’
Why is the lysogenic cycle important?
Each time the infected bacterium replicates exponentially more viruses are created.
What triggers the change form lysogenic to lytic cycle?
Environmental factors.
What are animal viruses grouped in?
Classes (I, II, III, IV, V and VI)
What characterises Class I viruses?
They have ‘Double-Stranded DNA (dsDNA)
What characterises Class II viruses?
They have ’Single-Stranded DNA (ssDNA)’
What characterises Class III viruses?
They have ‘Double-Stranded RNA (dsRNA)’
What characterises Class IV viruses?
They have ’Single-Stranded RNA (ssRNA)’ which serves as mRNA
What characterises Class V viruses?
They have ’ssRNA’ which acts as a template for mRNA synthesis.
What characterises Class VI viruses?
They have ‘ssRNA’ which serves as a template for DNA synthesis
