Synthesis of viral
components and
viral enzymes for
replication and
assembly of a virion.
Capsid - Protein coat
Nota:
Capsid + viral nucleic acid = nucleocapsid
Protects, attaches
and introduces
viral genome into
host cells
Protein Subunits -
capsomeres
Nucleocapsid
Nota:
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
Nota:
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
Nota:
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
Nota:
or RNA replicase
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
Nota:
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
Nota:
the segments of genes that code for haemagglutinin and neuraminidase,
sudden and major
change in the surface
antigens of a virus