a2.3 viruses Flashcards
what are viruses?
viruses are non-cellular agents that infect cells and reproduce inside them. they are made up of nucleic acids and proteins only.
viruses share relatively few features, so they probably have multiple origins.
what are the structural features common to all viruses?
- small size: most are 20 - 300 nm in diametre, so smaller than almost all bacteria and eukaryotes. they must be smaller than host cells so that they can enter them.
- fixed size: full size as soon as assembled in host cells
- nucleic acid genetic code: viruses must be compatible with the host cells protein synthesis mechanism
- protein capsid: before release from host cell, genetic material is enclosed in a protein coat
- no cytoplasm or enzymes: they rely on the hosts metabolism
what is the diversity of the genetic material in viruses?
viruses are classified based on:
1. the type of genetic material they contain (DNA or RNA)
2. whether the genetic material is double stranded or single stranded
3. whether the genetic material is linear or circular
how do viruses use RNA?
- positive sense RNA viruses have viral mRNA which can be directly translated into virus proteins
- negative sense RNA viruses have RNA which must first be transcribed before being translated into virus proteins
- retroviruses have RNA that is first ‘reverse transcribed’ to make DNA. the DNA is then transcribed and translated to make virus proteins
what’s the difference between enveloped & non-enveloped viruses?
viruses burst most cells (lysis) to be released. some viruses, especially animal viruses become membrane covered. the membrane (phospholipids come from the host, proteins from the virus) helps the virus make contact with and infect the host. most bacteria or plant viruses are non-enveloped.
what are the ways viruses can infect other cells?
- receptor mediated - injecting DNA
- endocytosis - engulfing the virus
once the virus or its genome has entered the host cell, the viral genome will undergo replication. the two main methods of viral genome reproduction are the lytic cycle and the lysogenic cycle. some viruses only use one method, but some can switch between dependent on stressors such as chemicals or UV.
what’s the structure of a bacteriophage?
host: e. coli bacteria
genetic material: double stranded, linear DNA
structure: complex capsid structure with an icosahedral head bound to a helical tail. non-enveloped
lifecycle: alternates between lytic and lysogenic
what’s the structure of coronavirus?
host: human and other mammal cells
genetic material: single stranded, linear, positive sense RNA
structure: complex capsid composed of a helical nucleocapsid surrounded by an icosahedral structure that has distinctive club-shaped spikes. enveloped
lifecyle: lytic
what’s the structure of HIV?
host: t-helper cells in the immune system
genetic material: two copies of single stranded DNA
structure: icosahedral capsid with an envelope. retrovirus
lifecycle: lytic
what is the lytic cycle?
- bacteriophage attaches to the host cell and inserts its genetic material into the host cell
- the bacteriophage DNA circularises and enters the lytic cycle
- bacteriophages are assembled inside the host cell
- bacteriophages cause the host cell to lyse, causing the new viral particles to be released from the cell
plant and animal viruses often follow the lytic cycle, spreading from cell to cell within host organisation. infection becomes increasingly widespread and effects more severe. usually an animal host will be able to fight off these viruses so virulence has disadvantages. it can only persist if it finds a new host
what is the lysogenic cycle?
- the viral DNA is inside the host cell
- the bacteriophage DNA integrates with the host cell genome
- the infected host cell divides producing daughter cells which also contain the bacteriophage DNA integrated into their genome
- occasionally the viral genome is excised from the cell genome
lysogenic viruses are temperate and causes minimal harm. it remains undetectable as a prophage in host DNA. it’s inherited by daughter cells but cannot spread.
called lysogenic as it could change to the lytic state at any moment. for this to happen, prophage genes need to be activated by internal or external stimuli.
temperature viruses can benefit their hosts as their DNA may have genes from a previous host - gene become integrated into host DNA, increasing genetic diversity and facilitating evolution
how did viruses orginate?
- viruses may have originated from ancient RNA or DNA molecules that became encapsulated in a protective protein coat. the encapsulated genetic material or ‘proto-virus’ may have been able to replicate and evolve, eventually forming modern viruses
- viruses may have evolved from viroids, which are small infectious agents that consist of only a short strand of RNA and infect angiosperms (flowering plants)
- viruses may have originated from transposons, which are genetic elements that can move around in an organisms genome
- viruses may have evolved from ancient cells that lost their ability to live independently and became dependent on cells for reproduction
what is the progressive hypothesis?
for viruses
-> viruses are built up in a series of steps by taking and modifying cell components
-> fits with observation that there are virus-like components in some cells, eg retrotransposons
-> there are nucleotide sequences that occur widely in eukaryotic genomes
-> when transcribed to produce RNA, which is then translated, several enzymes are produced
what is the regressive hypothesis?
for viruses
-> viruses develop from cells in a series of steps by loss of cell components. this fits with observation that both viruses and bacteria show variation in complexity and self-reliance
-> some viruses are small and simple, others are much larger and more complex
-> bacteria are expected to be self-reliant but there are parasitic ones which replicate inside a host cell
-> some have lost the ability to perform certain metabolic functions
-> it is likely to have evolved from an independent organism that became parasitic
-> these observations suggest that viruses may have originated from intraparasitic bacteria by loss of more life functions, including respiration and photosynthesis
-> could be a result of convergent evolution - shared for functional reasons rather than due to ancestry
