viruses Flashcards
why may viruses be considered living
- contain genetic material
- their genetic material may undergo mutations
- able to evolve to adapt to new environments
why may viruses not be considered non living
- acellular and lack cellular organisms
- do not carry out metabolism
- unable to reproduce independently
- unable to respond to stimuli
why are viruses obligate parasites
depend on host cells to complete their reproductive cycle
negative vs positive sense RNA
negative: viral genome is complementary to viral mRNA
positive: viral genome has same sequence to viral mRNA
structure of capsid
protein coat that surrounds genome
composed of protei subunits called capsomeres
together with nucleic acid, forms nucleocapsid
function of capsid
surrounds nucleic acid and serves to protect, attach and introduce the genome into host cells
structure of envelope
a phospholipid bilayer, derived from host cell membrane, surrounding the nucleocapsid
embedded with viral glycoproteins
genome of bacteriophages
double stranded DNA
structure of genome of influenza
8 different segments of single stranded negative sense RNA, each associated with proteins
3 of the RNA segments are packed with 3 polymerase proteins which together form the enzyme RNA dependent RNA polymerase (RdRp)
what do 5/8 of the RNA strands in influenza’s genome code for
haemaglutinin
neuraminidase
nucleoprotein
matrix protein
non structural proteins
structure of genome of HIV
2 identical copies of single stranded positive sense RNA are bound to nucleocapsid proteins
what does the HIV genome code for
contains 3 major genes: 5’ gag-pol-env-3’
gag codes for structural proteins
pol codes for HIV’s viral enzymes
env codes for the glycoproreins gp 120 and gp41
structure of HIV’s capsid
conical shaped and contains reverse transcriptase, integrase and protease
structure of influenza’s envelope
has the glycoprotein haemagglutinin and the enzyme neuraminidase embedded
structure of HIV’s envelope
has the glycoproteins gp 120 and gp 41 embedded where gp 120 is attached to gp 41
describe attachment stage of lytic/lysogenic life cycle
attachment site on tail fibres adsorbs to complementary receptor sites on bacterial surface
describe penetration stage of the lytic/lysogenic life cycle
- bacteriophage releases lysozyme which digests bacterial cell wall
- molecules from bacterium are released, triggering a change in shape of proteins of base plate
- tail sheath contracts, driving the hollow core tube through the cell wall
- when the tip of the hollow core tube reaches the plasma membrane, phage DNA is injected into the bacterial cell
- empty capsid remains outside bacterial cell wall
describe replication of the lytic life/lysogenic life cycle after spontaneous induction
- Host cell macromolecular
synthesizing machinery is used
to synthesise phage proteins - phage proteins synthesied earlier will degrade host DNA
- Phage DNA is then synthesized using host cell nucleotides and phage proteins
- phage proteins synthesied later are phage enzymes and
structural components
describe replication of lysogenic life cycle
- Linear phage DNA circularizes
and inserted into host cell
genome by enzyme integrase - The integrated phage DNA is
known as a prophage - expression of phage genes is
repressed by phage repressor
proteins so new phages
are not synthesized - Prophage remains latent and replicates along with bacterial chromosome
- prophage will be found in all progeny cells, remaining latent or enters spontaneous induction
describe spontaneous induction (SI) of lysogenic life cycle
During spontaneous induction,
cellular proteases are activated, destroying the repressor
proteins
The prophage is then excised
from the bacterial genome
The replication phase of lytic
cycle then occurs
describe the maturation stage of the lytic/lysogenic after SI life cycle
Phage DNA and capsid assemble into a DNA-filled head
head, tail and tail fibers assemble independently & join in a
specific sequence.
describe the release stage of lytic/lysogenic after SI life cycle
Phage lysozyme synthesised within the cell breaks down
the bacterial cell wall
Bacterial cell membrane lyses and release the newly formed
virions
how can bacteria defend themselves against phages
- develop lysogenic relationship with phage
- develop receptor sites that are no longer complementary to phage attachment sites
- develop restriction enzymes which recognise foreign phage DNA and cleave the DNA off
describe attachment stage of influenza’s life cycle
the glycoprotein haemagglutinin binds to complementary sialic acid receptor on host cell
membrane
describe the penetration stage og influenza’s life cycle
- host plasma membrane invaginates and pinches off, placing the virus in a endocytic vesicle, so the virus enters host cell by endocytosis
- Endocytic vesicle fuses with lysosome, which lowers the pH
- the low pH environment stimulates viral envelope
to fuse with lipid bilayer of vesicle - nucleocapsid is released into cytosol
describe the uncoating stage of influenza’s life cycle
Capsid degraded by cellular enzymes, releasing the
8 viral RNA segments into the
cytosol
the RNA segments will then enter the nucleus
describe what happens in the replication stage of influenza’s life cycle
Viral RNA-dependent RNA polymerase uses
viral genome as a template to synthesise
mRNA
fate of syntheised mRNA
1. enters cytosol where it will be translated into viral
structural components
Capsid proteins are
made in the cytosol and Envelope glycoproteins are made in the RER & eventually are embedded in host cell membrane
- acts as template for synthesis of new viral RNA genome in the nucleus
after synthesis, it will exit nucleus
describe what happens in the maturation stage of influenza’s life cycle
- viral glycoproteins are transported by vesicles from the ER and get incorporated into the plasma membrane
- capsid proteins associate with these glycoproteins at the plasma membrane
- Nucleoproteins associate with the RNA genome to form the helical nucleoprotein and then interact with capsid proteins
that have associated with the glycoproteins
embedded on the plasma membrane. - the interaction of the capsid with the nucleoprotein initiates the budding process.
describe what happens in the release stage of influenza’s life cycle
the new virions are released by budding
Newly formed viruses bud off by evagination, acquiring host cell membrane with embedded
viral glycoproteins
Neuraminidase facilitates the release of the
new virions from the host cell membrane by cleaving sialic acid from the host cell receptor.
describe what happens in the attachment stage of HIV’s life cycle
gp120 binds to complementary CD4 receptors on T
helper cells or (macrophages) with the help of a co-
receptor.
describe what happens in the penetration stage of HIV’s life cycle
With the help of gp41, the viral envelope fuses with
host cell membrane, so nucleocapsid is released into
cytosol, leaving envelope behind
describe what happens in the uncoating stage of HIV’s life cycle
Capsid gets degraded by cellular enzymes and the 2 viral RNA
strands and enzymes are released into the cytosol
describe what happens in the replication stage of HIV’s life cycle (before activation version)
- Reverse transcriptase makes DNA strand using viral RNA as template to form a DNA-RNA hybrid.
- the RNA is
then degraded and the remaining DNA strand is used as a template to synthesise a 2nd DNA strand, producing a double stranded DNA molecule
3.Viral DNA enters nucleus inserted into host cell
genome by integrase
4. the integrated Viral DNA is known as provirus
5. the provirus can remain latent for a long time
describe what happens in the replication stage of HIV’s life cycle (after activation version)
- Upon provirus being activated, viral DNA transcribed to viral RNA
which enters cytosol - Viral RNA can either act as mRNA and be translated into
proteins or become part of the genome of the new virions
fate of mRNA - is translated in the ribosome in the cytosol to form viral polyproteins
- is translated in the RER. the proteins are glycosylated in the RER into envelope glycoproteins gp120 and gp 41 and eventually are embedded in the host
cell surface membrane.
describe what happens in the maturation stage of HIV’s life cycle
For HIV, maturation is completed only after release of virus.
The viral RNA genome and polyprotein assembles at
the cell surface membrane where viral glycoproteins gp41 and gp120 have been inserted.
describe what happens during the release stage of HIV’s life cycle
Newly formed viruses bud off by evagination, acquiring
host cell membrane with embedded viral glycoproteins
Viral protease cleaves polyproteins, forming viral
enzymes and proteins.
The viral RNA genome and enzymes are then encapsulated by a protein coat to form a capsid
The HIV virus (virion) is now mature and able to infect
neighbouring cells.
describe antigenic drift
When the influenza virus replicates in its host cell, mutations frequently occur due to the poor proofreading mechanism of the viral RNA-dependent RNA polymerase and the fast replication rate of the virus. Over time, there is an accumutation of mutations in the viral genome.
Sometimes, these mutations produce viruses with MODIFIED surface antigens with different conformation.
If these viruses infect a host that does not have the antibodies that recognise these modified surface antigens, thehost becomes susceptible to the virus.
describe antigenic shift
When a bird strain of influenza A and human strain of influenza A infect a single cell of an intermediate host (e.g.a pig), genetic reassortment can occur. Genetic reassortment is when new viruses are being assembled in the host cell, new combinations of RNA segments come together.
Sometimes, genetic reassortment produces viruses with NEW surface antigens such as new glycoproteins
If these viruses infect a human host the host becomes susceptiblet to the virus, as the host will not have the antibodies that recognise these new surface antigens,
condition for antigenic shift to occur
as long as species barrier is crossed
genetic assortment may or may not be involved
