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Summary Viruses and Vaccines COMPLETE NOTES
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Complete summary of the viruses and vaccines module for MCB3024S
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Viruses and Vaccines (Module 4)Lecture 1: Viruses
History of Virology
- 1892: Dmitri Iwanowski tried to filter sap of infiltrated tobacco plants
o Source of TMV (small bacteria or spore)
- 1899: Dutch botanist Martin Beijerik rules out filterable toxin conclusion because
filtered sap is capable of causing undiluted infection
o Not bacteria but a “poison”
- 1935: Wendell Meredith Stanley discovered this after crystallization
- First human virus = yellow fever
- DiTicult to identify initially because of tiny size of viruses
Tobacco Mosaic Virus
- Rod shaped virus with RNA genome that infects tobacco mosaic plants
- First virus viewed with transmission electron microscope
Discovery and Detection
- Next = Bacteriophages
o Virus that infects and replicates in bacteria
§ Viruses are much smaller than bacteria that they infect
- Twort and d’Herelle in early 1900s
Bacteriophage
- Capsid surrounding viral genome
o Genome is RNA or DNA
§ Single or double stranded
- STRUCTURE =
o Head + protein structure that helps bind the virus to the host cells surface
is the tail.
o (1) Capsid head - Genetic material is packaged into the capsid head
which is made up from protein
o (2) Collar - The sheath is connected to a thin disc or collar at the head end
and to a base plate at the tip end
o (3) Sheath - It is an external contractile-envelope wrapping the tail tube in
some viruses
o (4) Baseplate - The baseplate of bacteriophage is a multiprotein
molecular machine that controls host cell recognition
Characteristics of Viruses
- Small infectious agents of 20-300nn in diameter
- Acellular pathogens
- Obligate intracellular parasites with host and cell type specificity
o Require a host to survive and replicate
- DNA or RNA genome (never both)
, - Lack the gene products needed for replication/reproduction à co-opt host
machinery
- Infect all types of cells
o Humans, plants, animals, archaea, protozoa, bacteria
- Virology is a branch of science that deals with viruses and viral diseases.
o Viruses require the presence of a host cell to multiply.
- Some scientists have claimed they have found viruses that infect other viruses
- The genome is surrounded by a protein capsid and, in some cases, a
phospholipid membrane studded with viral glycoproteins
- Lack genes for many products needed for successful reproduction, requiring
exploitation of host-cell genomes to reproduce new virus particles (virions)
Structure of a Virus
- Viruses can be extremely simple in design, consisting of nucleic acid surrounded
by a protein coat (capsid)
o The capsid = composed of smaller protein components referred to
as capsomers.
o The capsid + genome combination is called a nucleocapsid.
- Viruses can also possess additional components, with the most common being
an additional membranous layer that surrounds the nucleocapsid, called
an envelope.
o The envelope = acquired from the nuclear or plasma membrane of the
infected host cell, and then modified with viral proteins
called peplomers.
o Some viruses contain viral enzymes that are necessary for infection of a
host cell and coded for within the viral genome.
- A complete virus, with all the components needed for host cell infection = virion.
- Envelope protein = spike protein
- Envelope = made of lipidic membrane
- Nucleocapsid = many capsomers coming together
- Can have a matrix protein
, Non-living Living
- Acellular with no cell nucleus, - Can only reproduce in living cells
organelles or cytoplasm - Obligate intracellular parasite
- Cannot move or reproduce - Can evolve
(without a host cell) - Genetic material
According to sandiswa
Viruses Bacteria
- Obligate intracellular pathogens - Usually free living but can be
- No ribosomes – need host parasite
machinery for transcription and - Ribosomes
translation - DNA and RNA
- DNA or RNA - Light microscope to view
- Electron microscope to view - 100-1000s of genes
- 10-100 of genes - Natural phylogeny
- Tangled phylogeny
DiTerent Shapes
- Helical
o TMV with RNA genome
o Elongated tube like structure
o Nucleic acid is spiral
o Capsomers arranged helically around coil
- Polyhedral
o Adenovirus
o Icosahedral à spherical shape with nucleic acid inside capsid
o Composed of 20 triangular faces
- Spherical
o Influenza, SARS Co-V2, HIV
o Round and some have spikes
- Complex
o Bacteriophage + poxvirus
o Do not conform to any category
, Enveloped or Naked Viruses
Baltimore Scheme
All viruses have to make mRNA to organize all the known viral genomes.
• Group I viruses = dsDNA genome
• mRNA produced by transcription in same way as with cellular DNA
• Group II viruses = ssDNA genome
• Convert their single-stranded genomes into a dsDNA intermediate before
transcription to mRNA can occur.
• Group III viruses = dsRNA genome.
• The strands separate, and one of them is used as a template for the
generation of mRNA using the RNA-dependent RNA polymerase encoded
by the virus.
• Group IV viruses = ssRNA with a positive polarity.
• Positive polarity means that the genomic RNA can serve directly as
mRNA.
• Intermediates of dsRNA (replicative intermediates) made in the process
of copying genomic RNA
• Multiple, full-length RNA strands of negative polarity (complementary to
the positive-stranded genomic RNA) are formed from these
intermediates, which may then serve as templates for the production of
RNA with positive polarity, including both full-length genomic RNA and
shorter viral mRNAs.
• Group V viruses = contain ssRNA genomes with a negative polarity,
• Sequence is complementary to the mRNA
• dsRNA intermediates are used to make copies of the genome and
produce mRNA. In this case, the negative-stranded genome can be
converted directly to mRNA.
• Full-length positive RNA strands are made to serve as templates for the
production of the negative-stranded genome.
• Group VI viruses have diploid (two copies) ssRNA genomes that must be
converted, using the enzyme reverse transcriptase, to dsDNA; the dsDNA is then
transported to the nucleus of the host cell and inserted into the host genome.
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