Viruses (HL) Flashcards
Why are viruses not living things?
No cells, dependent on host cells, no growth, can’t replicate independently, no independent metabolism
Define obligate intracellular parasites
Viruses are struct molecular parasites — they must be infect cells in order to reproduce
Define virion
Outside the host
Differentiate virions, viruses and viroids
Virions:
- outside the host; infectious; nucleic acid and protein coat
Viroid:
- smaller virus; only RNA; plant cells only
Virus:
- inside the host; nucleic acid and protein coat
(general: viruses)
How do viruses vary
- size
- shape
- genetic material type
- genes present
- enveloping
What are the characteristics of those descended from luca
proteins, nucleic acids
Give the multiple possible evolutionary origins of viruses
regressive hypo, progressive, and virus-first
What are the common structures ampong all viruses
- nucleic acid
- capsid: protein coat
What are the features common to all viruses? [7]
- intracellular parasites
- small
- fixed in size
- nucleic acid
- capsid
- no cytoplasm
- no or few enzymes
Viruses vary in…
- size
- shape
- genetic material
- genes present
- enveloping
How are viruses’ genetic material (?) classified?
1.) RNA or DNA
2.) single or dble stranded
3.) linear or circular
Define zoonosis
Disease of animals to humans
Define convergent evolution
Evolving from different ancestors acquire similarities between one another
Why are viruses said to go through convergent evolution?
Because they are highly diverse
Why do we say that viruses may not be part of LUCA?
Convergent evolution
Outline the missions of viral genomes
1.) Make more virus nucleic acid
2.) More virus proteins
3.) Assemble new virus molecules
Define viral genome
composed of DNA or RNA in a virus
Outline the three types of viral nucleic acids
1.) Positive sense
2.) Negative sense
3.) Retrovirus
Describe positive sense RNA
Viral RNA -> protein
Describe negative sense DNA
DNA -> RNA -> protein
(transcription first then translation)
Describe retroviruses RNA
- “retro”: going back
- RNA -> DNA [reverse transcriptase]
- Viral DNA injected into cell’s DNA
- Carries out the functions of the viral genome
- WBCs cannot detect because of the cell DNA
- can attack WBC (HIV)
Outline the three possible structures of capsids
1.) Helical
2.) Polyhedral/Icosahedral
3.) Complex
Describe virus envelopes or “enveloping”
- Lipid bilayer
- from the host cell due to “budding” (exocytosis)
- provides additional protection
- round things: spikes;
- spikes used to know which receptor proteins are compatible.
Do all viruses have enveloping?
No
- some say it might be hereditary if the envelope is in the genetic material (??)
How do viruses without an envelope exit the host cell?
- BY lysis
- they make the cell burst sp they could exit
- therefore, more resistant to extreme pH, dryness and disinfectants
Describe the example virus of bacteriophage lambda
- due to feces consumption
How do viruses infect host cells?
- They attach to compatible recepor proteins of the host cell
- inject their nucleic acids
Describe SARS-CoV 2
Describe HIV
Due to excessive amt of sexual partners (?)
Briefly define the process of lysogenic cycle
- Virus assimilates (‘fits in’) with the host cell genome [retroviruses]
- Doesn’t kill the host
Define the lytic cycle
Virus reproduces and does lysis out of the cell
- kills the cell
Outline the lytic acid cycle
- also known as “spread”
Describe the phage attachment to the host cell (Lytic cycle)
- virus attached to receptor protein;
- occurs via random collisions
Describe the phage nucleic acid entry into host cell (Lytic cycle)
- phage’s genetic material injected into cell
- phage tails wildly vary
- but usually a tube
- tail contracts like a coiled spring that drives the genetic material in the cell
Describe phage DNA replication (Lytic cycle)
- cloning of phage DNA via rolling circle replication
- one strand is nicked
- enzymes extend the free 3’ end
- complimentary strand is made around the circular DNA: the 5’ end is peeled off
Describe protein synthesis in viruses
- transcription of phage DNA into phage RNA via cell’s polymerase (HC’s “machinery”)
- translates phage RNA -> phage proteins
Describe the assembly of new phage viruses (lytic cycle)
- phage proteins -> phage viruses (assembling)
- empty heads into which condensed phage is packed
- tails made independently
- last step: heads + tails
Describe how lysis happens in bacteria due to viruses
- enzymes produced by the phage gradually weaken the bacterial cell wall
- bacteria cells lyse
- releases 100-200 phage progeny (“children”)
Describe the step of spread (lytic cycle)
- new virus particles infect new cells
- actively infecting viruses: “virulent” viruses
Outline the steps in the lysogenic cycle
1.) phage attachment to host cell
2.) phage DNA entry
3.) Phage DNA integrated into hist genome
5.) cell dovision
Describe Phage DNA integration (lysogenic chcle)
- circularization: linear phage DNA -> circular after infection
- has an attachment site
- prophage: when the virus exists as part of HC genome
Describe DNA replication (lysogenic cycle)
- prophage genome DNA replication
- passively replicates: so small, bacterial hosts unharmed
- WBCs can’t detect
describe cell division (lysogenic cycle)
- cell divides
- 2 daughter cells with the prophage
- “temperate”: prophage virus that doesn’t kill or infect bac cells, “dormant”
describe return to lytic cycle (lysogenic cycle)
- Under exterme conditions:UV light, low nutrient conditions, chemicals
- induction: entry into lytic cycle
Describe the virus-first hypothesis
- Viruses existed before the LUCA (main claim)
- HOWEVER, viruses are obligate parasites
Describe the progressive hypothesis
- Viruses came from modified cell parts
- Viruses escaped from LUCA and evolved
- HOWEVER, virus structures not aligned with cell structures (?)
- HOWEVER, viruses are very diverse
Describe regressive hypothesis
- Similar to endosymbiosis
- Viruses were other cells that entered the host cell
- became highly dependent
- lost all cell components
- (now just the capsid and the nucleic acid)
- HOWEVER, no cellular counterparts
- HOWEVER, no resemblance to ___
- HOWEVER, unique genetic make up
How fast do viruses evolve?
- very fast
- This means we are “delayed” with our vaccines
Define evolution
a cumulative (slow and gradual) change in inheritable traits in a population
Define generation time
Average time between consecutive generations
Define natural seleciton
When nature selects the traits that allow survival via environmental conditions
- e.g. predators
What is the generation time for humans
20-30 years (average age for reproduction)
What is the relationship between generation time and rate of natural selection
Inverse relationship
- less time to pass on traits
Outline the characteristics of viruses (regarding reproduction and natural selection)
- small generation time
- each virion; many offspring
- high rates of mutations
Define eclipse period
the time of maturation of a virus
- inside the host
What is the generation time of SARS-COV
10 hours
What is needed in order for natural selection to happen [3]
1.) Variation in a population
- to get the best traits
2.) Production of viruses (VIRUS SPECIFIC)
- competition in host cell
- determines the ones who can survive
3.) Passing of trait to the next generation
- the “best” virion reproduce
What is the phrase “survival of the fittest” really about
The traits that could allow populations to survive in an env. condition
Define viral burst size
- how many virion can go out of a cell
- e.g. HIV = 6,000 virions
- SARS-Cov = 103 virions
Define mutations
- change of intergenetic sequence
- the difference between original and new sequence
Define DNA polymerase III
- adds and pairs nucleotides during replication
Why are the VIRUSES with RNA more susceptible to mutations?
RNA polymerase has no proofreading abillity
- ^ chance for “incorrect” pairings -> diff types of amino acids
Why do viruses with DNa less suceptible to mutattions?
- DNA polymerase has a proff-reading ability
Describe the D6-14G mutation of Sars-COV
- GAT -> CGT
- different amino acid -> changed the shape of the spike protein
- more infectious bc of ^
- made very dangerous alongside high reproduction rates
How would natural selection affect the amount of beneficial mutations in a population?
Increases it because the ones with more beneficial traits survive
Why does HIV (>) have the highest mutation rate?
- because they attack the white blood cells
- some of them are more resistant to drugs
- they reproduce over time
Is the independent variable x-axis or y-axis?
x-axis
Outline the two different strands of viruses:
1.) anti-genic drift
2.) anti-genic shift
Define antigens
The surface protein of the viruses
Describe anti-genic drift
- cumulative small changes of antigens
- over time
- (influenza): can make people have the flu again due to changes
- changes a lot -> need for annual vaccines (?)
Describe antigenic shifts
- abrupt and major changes to the antigen
- antigens combined
- e.g. the swines (to infect humans)
Define temperate
Does not cause cell death
Define passive
Does not assemble viruses
Define induction
lysogenic cycle -> lytic cycle
