Zusammenfassung der Ressource
Viruses
- Structure
- Ways to classify
- Helical e.g
tobacco mosaic
- Icosahedral
e.g. adenovirus
- Enveloped
e.g. influenza
- Complex e.g.
T4
bacteriophage
- Genome - viral nucleic acid
- Types
- RNA
- DNA
- ss
- Linear
- Circular
- ds
- Single
- Multiple
- Function
- Synthesis of viral
components and
viral enzymes for
replication and
assembly of a virion.
- Capsid - Protein coat
Anmerkungen:
- Capsid + viral nucleic acid = nucleocapsid
- Protects, attaches
and introduces
viral genome into
host cells
- Protein Subunits -
capsomeres
- Nucleocapsid
Anmerkungen:
- capsid + nucleic acid (genome)
- Envelope
- Lipid bilayer
- Phospholipids
- Glycoproteins
- Derived from host
cell membranes
by budding
- Virus incorporates
its own proteins -
appearing as
glycoprotein spikes
- in most
animal
viruses
- Presence?
- Yes -
enveloped
virus
- No -
naked
virus
- Viral replication
- Bacteriophages -
viruses that only
infect bacteria
- Lytic
- Lyse the host
bacterium
- E.g. T4 Bacteriophage
(ds DNA)
- Lytic life cycle
- 1. Attachment
- attachment sites on tail
fibres recognise and adsorb
to complementary receptor
sites on bacterial surface
- Viral specificity:
specific strains of
bacteriophages can
only adsorb to specific
strains of host bacteria
- most adsorb to
cell wall (but
some attach to
flagella/pili)
- 2. Penetration
- Phage TAIL releases
lysozyme
- digests bacterial cell
wall, allowing
release of molecules
- molecules trigger a
change in shape of
phage base plate
- initiating
contraction of
tail sheath
- which drives
hollow core tube
through cell wall
- tip of core reaches
plasma membrane -->
phage DNA injected
into bacterial cell
- empty capsid
remains
outside cell
- 3. Replication
Anmerkungen:
- Early replication: Phage genes expressed
Enzymes produced to hydrolyse bacterium’s DNA to nucleotides.
Phage takes complete control of host cell macromolecular synthesising machinery to replicate its own genome. Early proteins digest host cell genome.
Late replication: Uses bacteria’s metabolic machinery + resources to express phage enzymes and phage components (late proteins)
- Eclipse period
- Period when complete,
infective virions are not
yet present in host cell
- only separate components
e.g. DNA, protein present
- Phage enzymes take over host cell's
macromolecular synthesising machinery
- Host cell DNA degraded
into nucleotides
- Nucleotides from host cell's
degraded DNA used to synthesise
many copies of phage DNA
- viral DNA escapes degradation
due to DNA methylation
- early proteins
- Phage DNA
immediately
transcribed
- synthesise mRNA using
host RNA polymerase
- Biosynthesis of viral proteins
- uses bacterium's
metabolic machinery
- synthesise phage enzymes and
phage structural components
- late proteins
- 4. Maturation
- Phage DNA + capsid
assemble into
DNA-filled head
- Head, tail, tail fibres
assembled independently,
joined in specific sequence
- tail fibres to tail,
then head to tail
- 5. Release
- phage lysozyme
breaks down
bacterial cell wall
- host cell plasma
membrane lyses
- newly formed mature
phage virions released,
ready to infect other
cells and repeat cycle
- Temperate
- Incorporates its DNA
into the host
bacterium's DNA and
becomes a prophage
- E.g. Lambda
bacteriophage (ds DNA)
- Lysogenic life cycle
- 3. Replication
- linear phage
DNA circularises
- inserted into host cell
genome by integrase
- integrase binds at
attachment site
sequences of viral
DNA and host cell
DNA, forming
lambda integrase
protein complex
- catalysis of
double strand
breakage and
rejoining
- integrase dissociates -->
phage DNA integrated into
bacterial chromosome
- integrated DNA = prophage
- prophage gene
expression repressed by
phage repressor proteins
- genes directing synthesis and
release of new virions are not
transcribed
- phage DNA not synthesised
- prophage DNA replicates along
with bacterial chromosome
- prophage remains latent
in all progeny cells
- 4. Spontaneous induction
- cellular proteases
activated
- Repressor proteins
destroyed
- prophage no
longer repressed
- prophage excised,
enters lytic cycle
- 5. Maturation -
same as lytic
- 1. Attachment -
same as Lytic
- 2. Penetration -
same as lytic
- 6. Release -
same as lytic
- Host cells immune to
re-infection by same phage
- capable of specialised
transduction
Anmerkungen:
- phage packages adjacent bacterial DNA together with its own viral DNA in same capsid, transferred to new bacterial cell along with prophage when virion infects a new cell.
- Animal viruses
- Influenza
(ss RNA)
- Structure
- Genome
- 8 different segments
of ss RNA associated
with nucleoproteins
- 3 RNA segments
code for 3 different
polymerases
- 3 polymerases form
RNA-dependent
RNA polymerase
enzyme complex
Anmerkungen:
- Function:
replication and
transcription of
viral genome
- Other 5
segments
- haemmagglutinin
- impt for
recognition and
attachment to
host cell
- neuraminidase
- impt for
leaving host
cell
- nucleoprotein,
matrix protein M1,
non-structural
proteins
- negative strand
genome
- sequence of viral
genome is
complementary
to sequence of
viral mRNA
- Capsid -
Present
- Envelope
- Embedded with
haemagglutinin (glycoprotein)
and neuraminidase (enzyme)
- diff types of h and n give rise
to diff strains of influenza
- Replication cycle
- 1. Attachment
- Protruding
glycoproteins bind
specific receptor
molecules on host cell
- Humans -
haemagglutinin
on influenza
binds sialic acid
receptor on host
cell membrane
- 2. Penetration
and uncoating
- Virus enters by
endocytosis
- Host plasma
membrane
invaginates and
pinches off
- Virus placed
in endocytic
vesicle/endosome
- Vesicle fuses with lysosome
--> pH of environment
lowered --> viral envelope
stimulated to fuse with lipid
bilayer of vesicle membrane
- Nucleocapsid
released into
cytoplasm
- Capsid degraded by
cellular enzymes
- Helical nucleoprotein
enters nucleus of cell
- 3. Replication
- Viral genome
used as template
to synthesise
viral mRNA (+
strand RNA)
- catalysed by viral
RNA-dependent
RNA polymerase
- mRNA acts as template
for synthesis of new
viral RNA genome
- mRNA strands
exit nucleus to
cytosol and rER
- translated into viral
structural components
e.g. glycoproteins and
capsid proteins
- 4. Maturation
- Viral glycoproteins
transported out of ER,
incorporated into
plasma membrane
- Associate
with capsid
proteins
- viral genome
associates with all
other proteins to
form helical
nucleoprotein
- interaction with
capsid proteins-->
budding initiated
- 5. Release by budding
- Evagination -
each new virus
buds from cell
- host cell
may/may
not lyse
- new viruses acquire
host membrane
with viral
glycoproteins
embedded
- neuraminidase
facilitates
release
- Cleaves sialic acid
and progeny virions
from cell surface
- HIV - Retrovirus
(ss RNA)
- Structure
- Genome
- 2 copies of ss RNA
- tightly bound to
nucleocapsid proteins
(nucleoprotein in HIV)
- both positive strands
- genome has
same sequence
as viral mRNA
- 3 major genes
5'-gag-pol-env-3'
- gag: structural proteins
- capsid
- matrix
- nucleocapsid
protein
- pol: viral enzymes
- integrase
- reverse
transcriptase
- HIV protease
- env:
glycoproteins
- gp120
- gp41
- Capsid
- conical-shaped
- contains reverse
transcriptase,
integrase and
protease
- Envelope
- embedded
with gp120
and gp41
- Have specific
conformation
allowing virus
to bind certain
receptors on T4
helper cells
- Replication cycle
- 2. Penetration
and uncoating
- gp41 helps fuse viral
envelope with host
cell membrane
- Capsid released
into cell, leaving
envelope behind
- Capsid +
nucleocapsid
protein degraded
- viral enzymes and
RNA released into
host cell cytoplasm
- 3. Replication
- Step 1
- Viral reverse
transcriptase
catalyses
conversion of viral
RNA into DNA
- First, DNA
strand
complementary
to viral RNA
strand is
synthesised
- RNA-DNA
hybrid formed
- Step 2
- Viral DNA
enters host
cell nucleus -->
integrated
into host's
genetic
material
- becomes
provirus
- may remain
latent for years
afterwards
- catalysed
by
integrase
- Activation of
host cell --> viral
DNA transcribed
into viral RNA
- serves as mRNA
- Step 3
- mRNA exits
nucleus into
cytoplasm
- translated
into viral
polyproteins
- gp120 &
gp41 made
in ER
- transported
by vesicles to
plasma
membrane
- 4. Maturation
- Polyproteins and
HIV RNA genome
assemble at inner
surface of host cell's
plasma membrane
- 1. Attachment
- virus must
contact cell with
CD4 protein on
surface
- T lymphocytes/T-
helper cells
- macrophages
- gp120 binds
complementary CD4
receptors on cells, with
the help of co-receptor
- 5. Release
- virus buds
off/evaginates
- Viral
envelope
derived from
host cell
membrane
- HIV protease
cleaves
polyproteins
into functional
proteins
- structural
proteins
- viral
enzymes
- mature
virions
- Difference between
prophage and provirus:
Prophage is DNA that has
been integrated into
prokaryote's genetic
material. Provirus is DNA
that has entered a
eukaryotic cell's nucleus
and is integrated into the
host cell's genome.
- Variation
- Antigenic drift
- accumulation of
mutations in genes
encoding surface
glycoproteins
- Influenza:
Haemagglutinin
and
neuraminidase
- surface
antigens/glycoproteins
have
different
conformations
- Results from
- RNA-dependent
RNA polymerase:
lack of
proof-reading
ability
- Virus:
Fast/high rate
of replication
- Virus is ss RNA,
no backup for
replication
- Antigenic shift
Anmerkungen:
- When a bird strain and a human strain of the same virus infect a single cell of an intermediate host e.g. a pig, genetic reassortment can occur.
When new viruses are assembled in the host cell, they can have new combinations of RNA segments. Sometimes genetic reassortment produces viruses with new antigens. Host becomes susceptible to the virus if the host does not have antibodies that recognises these modified antigens once infected.
- Results from
- 2 ot more different strains of a
virus/different viruses combine
to form a new subtype
- having a mixture
of surface
antigens of the
original types.
- segmented RNA
genome allows for
reassortment
Anmerkungen:
- the segments of genes that code for haemagglutinin and neuraminidase,
- sudden and major
change in the surface
antigens of a virus
- genetic
reassortment,
conferring
phenotypic change
Anmerkungen:
- http://www.virology.ws/2009/06/29/reassortment-of-the-influenza-virus-genome/
- host cell forms new
viruses combining
surface antigens of
the two different
strains that infect
the same cell
- may further
evolve to
spread from
person to
person
- enables strains to jump
from species to species
- mostly
influenza
- How immune
system recognises
viruses
- antigens are on
virus particles
- antibodies and other
receptors are in our
immune systems
- immune receptors are
virus-specific -->
recognise and bind to
antigens on viruses
- body produces
more after an
infection -->
re-infection
prevented -->
acquired immunity
- antigens constantly
mutate to produce new
forms --> no longer
specific to receptors -->
no longer bind -->
evade immunity
- Note COMPARISON TABLE
- Types
- Naked viruses -
only
nucleocapsid
- Enveloped
viruses
- Characteristics