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Viruses Module Complete Summary
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Complete notes covering the viruses and virus reproduction mechanism module of MCB2020f
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Viruses• Nb!! QUESTION 1 FOR TESTS!!!
• Virus = acellular organisms whose genomes consist of nucleic acid (DNA or RNA), which
obligately replicate inside host cells using host metabolic machinery and ribosomes to form a
pool of components which assemble into particles called virions which serve to protect the
genome and to transfer it to other cells
• = acellular organisms with nucleic acid genomes which make particles in infected cells to
protect the virus genome and transfer it between host cells
• = infectious cellular entity composed of compatible genomic components derived from a pool
of genetic elements which infect cells and make particles to transfer its genome between cells
• Distinct from other virus like agents such as viroids, plasmids + prions
• Prion = infectious protein (molecular mimicry disrupts cellular functioning)
• Plasmids = encode machinery for their own transmission
• Viroids = infectious circular RNA
• Do not encode for anything except their own structure which they can force hosts
to replicate (no capsids, no genes)
• Genome = entire genetic constitution of an organism
• Eg. Human genome = mitochondrial + nucleic DNA
• Viruses = obligate intracellular parasites
• Virion function = get nucleic acid inside other cells
• Alive by virtue of being inside another cell
• Allows for replication/reproduction
• Take over a host cell’s metabolic machinery
• Utilise host ribosomes to create mRNA for export
• Adaptation, selection and mutation by selection of viruses allows for retention of traits that are
favourable to increase reproduction
• R0 = number of people that can be infected from one infected person
• Virocell —> cell infected with a virus where there is no delineation between the host cell and
virion
• Molecular motors —> can pull DNA into and out of cells
VIROMICS
• Total number of phage particles in biosphere = 10^31
• Suggests there are more phage particles than all other biological forms added together
• Viruses exist wherever life is found
• Major cause of mortality
• Driver of global geochemical cycles
• Keeping O2 levels up
• Reservoir of diversity
• A ect available form available nutrients
DISCOVERY OF VIRUSES
• Discovered as an excluded entity rather than by culture
• Cell free extracts from diseased plants + animals could still cause disease —> idea of a
“ lterable virus”
• Tobacco mosaic virus!!
VIRUSES AS ORGANISMS
• Acellular organisms w/ nucleic acid genomes which make particles to protect the genome and
transfer it between cells
• Do not exhibit all attributes of cellular organism but are independent entities that are not limited
to one host
• Virus like agents = plasmids, transposable elements, satellite viruses, satellite nucleic acids,
viroids, retroelements
fiff
, • Organism = unit element of a continuous lineage with an individual evolutionary history
• Virus IS the cell it infects —> why?
• Takes over the cell and uses it to make portable versions of the genome that can infect other
cells
• Viral infection transforms the infected cells into a virocell whose aim is to produce virions
• Size ranges from 1800 nucleotides up to 2.5 million nucleotides
• ssDNA circoviruses to pandoraviruses
• Only organisms on the planet to have RNA as sole genetic material
• Only autonomously replicating organisms to have ssDNA
VIRAL GENOMES
DNA RNA
dsDNA ssDNA dsRNA ssRNA
Linear Circular Linear Circular Linear Linear
Single Single or Single Single or Single or multiple + sense - sense
component multiple multiple (Equivalent (Antisense
to sense equivalent
strand) and cannot
be
recognised
by a
ribosome)
Single or Single or
multiple multiple
• Viruses have the largest variety of genome types of all organisms
• Multiple components mean that viruses can have chromosomes
REPLICATION OF VIRUSES
• Pirates various machineries of the host cell (result of acellular nature)
• All viruses use host ribosomes to make their proteins
• Many use intracellular transport machinery + derive envelopes + components from cell
membrane complexes
• All genomes replicate via semi-conservatie model
• Every double stranded helix = parent and daughter strand
• All cells replicate dsDNA genomes using self encoded DNA pol.
• Transcribe mana and other RNA using their own RNA pol
• Translation of mRNA occurs using self encoded tRNAs and ribosomes
—> CENTRAL DOGMA FOR CELLS!!
*reverse transcription —> making DNA from RNA
• Cellular RNA dependent DNA pol
• Occurs in all eukaryotes but more rare in prokaryotes
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