Theme A: A2 Cells - A2.3 Viruses Flashcards
Capsid
The protein shell that encloses the genetic material of a virus. It protects the viral genome and facilitates its delivery into host cells during infection.
Host cell
the cell used by a virus to carry out its metabolic and reproductive functions.
Obligatory intracellular parasite
(applies to all viruses) means they rely on living host cells to multiply and to carry out the metabolic functions required to exist
Virion
A virion is a fully-formed virus particle that is capable of infecting a host cell. The extracellular, infectious form of the virus.
Lysis
The breakdown or destruction of a cell, often due to the rupture of its membrane, releasing its contents. In this chapter, lysis is used in the lytic cycle which ruptures the host cell’s cell membrane by an enzyme called lysozyme.
Lysis only occurs after production of virions.
Convergent evolution
the process by which unrelated or distantly related species independently evolve similar structures and functions (traits and features) as a result of adapting to similar habitats and ecological roles. this term relates to the possible origin of viruses as they are all obligate parasites (they all depend on a host cell to survive, developing simialr traits by adapting to similar environements.)
Mutation
A change in the nucleotide sequence of an organism’s DNA that can result in altered proteins or functions. Mutations may occur spontaneously during DNA replication or be induced by mutagens (e.g., radiation, chemicals). this term relates to the rapid evolution of viruses
Antigenetic drift
Produces small incremental changes in viral genetic material over longer periods of time.
slowly produces variation in the virus’s surface proteins.
Result is accumulated changes that may eventually prevent immune system from recognising the virus.
influenza vaccine is adjusted every year due to antigenetic drift. successful approach as changes are small and relatively slow.
HIV undergoes extremely rapid antigenetic drift, where there is no successful vaccine for HIV yet
Antigenetic shift
2 or more different viral strain or viruses infect same cell and recombine genetic material. Major changes in relatively short amount of time.
Causes rapid + major changes in the virus’s surface proteins.
Result is new virus able to create pandemics as immune system does not recognise the new viral strain.
anti-genetic shift changes are not predictable enough for vaccine adjustments to be as effective
influenza can undergo antgenetic shift, creating seemingly new viruses that immune system doesn’t recognise.
characteristics shared by all viruses
1) small fixed size (40-100 nm around the size of a protein macromolecule but some are as large as bacteria)
size is a result of extreme SIMPLICITY: small number of organic molecules and the composition of these molecules prevents the formation of bonds to produce a large entity.
2) nucleic acid (DNA/RNA) as genetic material. can not replicate on their own. possess very few genes. the genetic material demonstrates variation by being linear or circle.
3) Genetic material enclosed by a capsid. each virus has a unique capsid based on presence of amino acids and the structures they produce when bonding. this unique composition and structure determines a viruses ability to infect a host cell. specialised sits on a capsid allow attachment to a specific host cell.
4) no cytoplasm inside the capsid. most space occupied by nucleic acid
5) few, if any, enzymes.
common virus structures
1) polyhedral (e.g. adenovirus)
2) spherical (e.g. influenza or coronavirus)
3) helical (tobacco mosaic virus)
4) complex (bacteriophage lambda)
to reproduce viruses must…?
APSAR:
1) attachment: attach to a site on specific host cell
2) penetrate: incorporate genetic material into host cell’s cytoplasm
3) synthesis: use host cell’s processes to produce components of themselves
4) assemble: assemble viral components into new functioning virus entities
5) release: release new virus entities into host cell’s environment (virion)
2 life cycles of a virus
1) lytic cycle
2) lysogenic cycle
recombination
Recombination is the process by which the viral genetic material integrates into the host cell’s genome during the lysogenic cycle.
prophage
A prophage is the viral DNA that has been integrated into the genome of a host cell during the lysogenic cycle of a bacteriophage.
The prophage remains dormant within the host’s DNA and is replicated alongside the host’s genome during cell division.
While in the prophage state, the virus does not cause harm to the host cell or produce new viral particles.
Under certain environmental triggers (e.g., stress, UV radiation), the prophage can excise itself from the host genome and enter the lytic cycle, leading to active virus replication and cell lysis.
evidence of convergent evolution in viruses
1) all obligate parasites
2) all have protein capsid with no cytoplasm
3) all of have genetic material in capsid and the code for this material is shared by all Earth’s organisms.
** hard to study their history as viruses don’t form fossils
key examples of rapidly evolving viruses + why
1) HIV 2) Influenza
they both have:
1) large population sizes
2) short generation times
3) high mutation rates: they both have RNA as their genetic material, therefore they produce DNA as part of their gene replication process. this added step increases chance of mutation. (viruses with DNA don’t undergo this step thus having lower mutation rates)
3 hypothesises on the origin of viruses
1) virus first hypothesis: viruses originated before cells. Viruses are more simple, where the course of evolution goes form simple –> complex. the ancestors of modern viruses could have provided material to develop first cell.
2) regressive hypothesis: viruses were once small cells and became parasites of larger cells. overtime, small cells shed their structures that they no longer need through gene reduction
3) escape hypothesis: portions of genetic material escaped from larger organisms like bacteria and became surrounded by an outer boundary
