Studied by 1 person
Are viruses alive?
No
Why are viruses not alive?
Because they are non-cellular, don’t metabolise and cant grow
What do viruses do to cells?
They
Do viruses share many features?
No
Why do viruses not share many features?
Because they probably have multiple origins (dont share one common ancestor/ LUCA)
List the 5 few shared features
Small size
Fixed size
Nucleic acid as genetic material
Capsids made of protein
No cytoplasm & few/no enzymes
Between what sizes are viruses?
20 to 300 nanometres in diameter
Why must viruses be small in size?
So they can enter their host cell
What makes viruses so small?
Lack cytoplasm and other structural features
Why do viruses have a fixed shape?
They dont grow but assemble inside the host cell
When do viruses reach their full size?
As soon as they assemble
What are viruses often composed of?
A fixed number of components
What is the genetic material of viruses /what are their genes made of?
Nucleic acids (RNA & DNA)
What do all viruses genetic material use?
The universal genetic code
Why is it essential that viruses use the universal genetic code? (hint- synthesis)
Because their proteins are synthesized by the nucleic acid-to-polypeptide translation mechanism of their host cell
What is the viruses capsid made of?
Proteins
Where is the genetic material of a virus stored?
It is packed into a protein coat called a capsid
What is the virus capsid made of?
Repeating protein subunits
What creates the symmetrical structure of viruses?
Self-assembly of the repeating subunits of the capsid
What functions of host cells do viruses rely on?
Metabolism
What are the few enzymes present in viruses used for? (3 things)
Replication of the genetic material
Infecting host cells
Lysis (bursting host cells)
What does the diversity of viruses suggest?
They have multiple evolutionary origins
Can some genes occur in all viruses?
No
What is the genetic diversity of viruses?
Their genetic material can be RNA or DNA - single stranded or double stranded for either
What shape can be molecule be if the viruses genetic material is DNA?
Linear or circular
If the viruses genetic material is RNA, what can the genes be?
Positive-sense or negative-sense
What is positive sense of RNA in viruses?
The RNA can be used directly as mRNA and translated by the host cell
What is negative sense of RNA in viruses?
The RNA needs to be transcribed before translation
When do viruses become enveloped in a membrane?
During lysis
What makes the membrane of enveloped viruses?
Phospholipids from the plasma membrane of the host cells and proteins
What does the membrane of viruses do?
Helps the virus to make contact with host cells and infect them
Name the 3 examples of viruses
Bacteriophage Lambda
COVID - 19
HIV
Draw a bacteriophage lambda
What type of virus and host does bacteriophage use?
DNA virus in prokaryote hosts (E-Coli)
Describe bacteriophage’s DNA strand
1 double stranded DNA with 32 genes
What cycle does bacteriophage use?
The lytic or lysogenic cycle
Draw covid-19
What type if virus is covid-19?
RNA virus with a crown shape and animal hosts
Is covid-19 enveloped?
Yes
Describe covid-19’s RNA strand
1 single stranded positive sense RNA with 16 genes
What type of disease is covid-19?
Zootonic
Define zootonic?
The virus comes from animals
What is the host cell of covid-19?
Epithelium cells in the airway and lungs of humans
Draw HIV
What type of virus is HIV?
A retrovirus
HIV converts _______ to _______ after entering the host cell
DNA to RNA
What is HIV’s host cell?
T-helper cells in the human immune system
Is HIV virus enveloped?
Yes
Describe the RNA molecule in HIV
2 copies of a single-stranded positive sense with 9 genes
What does HIV virus contain?
Reverse transcriptase
What does reverse transcriptase in HIV make?
A double stranded copy of the viral RNA genome, which is integrated into the host cells chromosome
Draw and label the lytic cycle
Describe the lytic cycle of bacteriophage lambda in detail
Bacteriophage lambda binds to its host cells plasma membrane (E-coli) using proteins at the tip of its tail
It injects its DNA into the host cell through the tubular tail
The viral DNA has single-stranded ends, which link by base pairing which convert the molecule from a linear to circular form
DNA replicates
The viral proteins synthesise using mRNA transcribed from viral DNA
The new viruses with DNA assemble inside the protein coat
Lysis (bursting) occurs to release the new virus
Draw the lysogenic cycle
Describe the lysogenic cycle in detail using bacteriophage lambda
The virus attaches to the host cell and injects its DNA
INSTEAD of replication, the viruses DNA becomes integrated into the host cells DNA molecule
Its stays undetected and inactive
Each time the host cell replicated its DNA before cell division, the viral DNA is also replicated
This means all the daughter cells will inherit the viral DNA but dont produce viral proteins
At this stage the viral DNA is known as prophage and is temperate (inactive) but it can become lytic and cause lysis
The stimulus causing this can be internal or external
What does viruses being obligate parasites mean?
They need a host cell to replicate in, suggesting that cells evolved before viruses
What do viruses use that is the same as all living organisms
The universal genetic code, suggesting that they evolved from cells
What is extremely diverse in viruses?
Their structure and genetic constitution, suggesting multiple origins from living cell rather than just once common ancestor.
What are the similarities between viruses from?
Convergent evolution - when organisms that aren’t closely related evolve similar features or behaviours
How could viruses have evolved (a series of…)?
In a series of steps by taking and modifying cell components, fitting with the hypothesis of the occurrence of virus-like components in some cells
How could viruses have evolved from cells?
By loosing cell components and life functions
State the 3 main reasons why viruses have rapid rates of evolution
Very short generation times of <1 hour in the lytic cycle
High mutation rates, especially in RNA viruses
Intense natural selection due to host organisms evolving defences (antibodies for destroying viruses)
What is influenza caused by?
Rapidly evolving RNA virus
What does influenza use RNA replication for?
To replicate the hosts genetic material
What does influenza not do and what does it lead to?
Proofread or correct errors leading to high mutation rates
How many separate RNA molecules does the flu virus have
8
What happens if a host cell is invaded by 2 different strains of the virus (genetic variants)?
A new strain can be formed with a combination of RNA from the 2 strains
What does the transmission of the flue between humans and animals teigger?
Evolution
Because of the rapid evolution of the flu virus, what is required?
Annual vaccinations to give immunity to new strains
What does HIV have?
The highest known mutation rate of any virus
What does HIV not do?
Proofread or correct errors, leading to mutations
What causes mutations in HIV?
Cytidine deaminase
What happens to a person infected by a HIV strain?
Mutations will produce many genetically different strains, helping the virus to evade the immune system and become resistant to antiretroviral drugs, making HIV infections almost always chronic
