viruses Flashcards
how do viruses challenge the cell theory?
1 - cells are the smallest unit of life
- viruses lack the necessary molecular machinery to conduct many of the biochemical reactions a normal cell would need
- but viruses contain the genetic material necessary to form the next generation and can evolve in response to the environment
2 - all cells come from pre-existing cells
- viruses can replicate but rely on host cells to provide the energy and materials needed for replicating their genomes and synthesising their proteins
- viruses cannot replicate without entering a suitable host cell
3 - all living organisms are composed of cells
- viruses are acellular and do not have protoplasm or organelles so they are not considered cells
- when in the extracellular virion state, viruses are metabolically inert and don’t carry out respiration or biosynthesis
what structures are present in all viruses?
- genome (comprising DNA/RNA)
- capsid (protein coat) comprising protein subunits, capsomeres
the following structures are NOT present in all viruses
- envelope comprising phospholipids from host cell
- enzymes
describe the viral genome
- viral genomes contain either DNA or RNA, but not both
- viral genomes may be single-stranded, double-stranded, linear or circular
- viral genomes are small, typically encoding functions that the virus cannot adapt from the host
- all viral genomes contain genes coding for essential proteins like respiratory proteins (regulate the action of host genes) and structural proteins (like viral capsid proteins)
describe the capsid (protein coat)
- the capsid encloses the viral genome
- each capsid is constructed from identical protein subunit called capsomeres
- capsomeres are made up of capsid proteins
describe the viral envelope (present in enveloped viruses only)
- the envelope is derived from host cells: when viruses are released from the host cell by budding, they take with them the host’s cell surface membrane (phospholipid bilayer) and insert proteins of viral origins into the membrane
- the proteins include viral glycoproteins that are essential for the attachment of viruses to the next host cell
- the envelope protects the virion’s nucleic acid from the effects of various enzymes and chemicals
viruses that are not surrounded by the lipid membrane envelope are referred to as naked or non-enveloped viruses
what are bacteriophages?
bacteriophages are DNA viruses that infect bacteria
what are 2 examples of bacteriophages?
- T4 phage
- lambda phage
what are structural features of the T4 phage?
genome - linear double-stranded DNA
capsid - capsomeres surrounds the nucleic acid, contained in the phage’s head
tail - consisting of tail sheath, tail fibres and base plate.
tail fibres - allows phage to adsorb onto surface of bacterial cell
tail sheath - surrounds a central tube and enables central tube to pass through host cell wall and membrane
base plate - comes into contact with host cell surface, allowing DNA to enter host cell
what are structural features of the lambda phage?
genome - linear double-stranded DNA
capsid - capsomeres surrounds the nucleic acid, contained in the head of the phage
head - the 5’ terminus of each DNA strand is a single-stranded tail that is 12 nucleotides long, which is important in prophage formation
single tail fibre - allows phage to adsorb onto the surface of the bacterial cell by binding to the specific receptor found on the cell surface
lambda phages’ tails are NOT CONTRACTILE and serves to deliver the viral DNA to the cell membrane
define lytic cycle and lysogenic cycle
lytic cycle: a phage reproductive cycle that finally results in the death of the host cell
lysogenic cycle: involves replication of the phage genome without destroying the host in the initial steps.
VIRULENT phages (eg T4 phage) reproduce through the lytic cycle
TEMPERATE phages (eg lambda phage) reproduce through the lysogenic cycle)
true or false: the T4 phage reproduces through the lytic cycle
TRUE
T4 phage, a virulent phage, reproduces using the LYTIC CYCLE
lytic or lysogenic?
which reproductive cycle does the lambda phage use to reproduce?
lysogenic.
lambda phage, a temperate phage, reproduces through the lysogenic cycle.
APSAR
what are the 5 step of the lytic cycle?
- adsorption
- penetration
- synthesis (of phage proteins) & replication (of phage nucleic acid)
- assembly
- release
using the T4 phage as an example, describe, in detail, the lytic cycle.
step 1: adsorption
- T4 phage’s multiple tail fibres attach to specific receptor sites on the surface of a bacterial host cell
- the base plate settles down on the host cell surface
step 2: penetration
- conformational changes occur in the tail sheath, causing it to contract and its tube pierces through the bacterial cell wall and cell membrane
- T4 uses lysozyme to hydrolyse peptidoglycan, degrading a portion of the bacterial cell wall for insertion of the tail core
- DNA is extruded from the head, through the tail tube, into the host cell
- the capsid is left on the outside of the bacterial cell wall
step 3: synthesis & replication
- after phage DNA is injected into the host cell, synthesis of host DNA, RNA and proteins is halted. the host cell machinery is taken over by the virus for synthesis of viral nucleic acids and viral proteins
- host DNA is degraded into nucleotides, providing raw materials for T4 phage DNA replication by host DNA polymerase
- T4 phage mRNAs are synthesised by host RNA polymerases through transcription
- T4 phage mRNAs are translated by host cell ribosomes, tRNAs and translation factors into viral proteins and enzymes required to take over the host cell and replicate phage nucleic acids.
step 4: assembly
- viral proteins are assembled to form phage heads, tails and tail fibres
- these components are assembled into the complete bacteriophage
step 5: release
- the T4 phages LYSE the host cell using the lysozyme, which digests the bacterial cell wall
- water enters the cell by osmosis, causing it to swell and burst
- lysozyme (an enzyme) makes a small hole in the bacterial cell wall, allowing the viral nucleic acid to enter
- proteins translated from T4 phage mRNAs include enzymes for viral replication and inhibitory factors that stop host cell RNA and protein synthesis
APPS
what are the 4 steps of the lysogenic cycle?
- adsorption
- penetration
- prophage formation
- switch to lytic cycle
using the lambda phage as an example, describe, in detail, the lysogenic cycle.
step 1: adsorption
- the single tail fibre of the lambda phage attaches to a specific receptor site on the surface of a bacterial host cell
- the base plate settles down on the host’s cell surface
step 2: penetration
- DNA is extruded from the head, through the tail tube and injected into the host cell, passing through the bacterial cell wall and cell membrane
- the capsid is left on the outside of the bacterial cell wall
step 3: prophage formation
- the lambda phage genome circularises & inserts itself into a specific site on the bacterial chromosome (prophage insertion site) by genetic recombination. there is NO LOSS of host DNA.
- the integrated lambda phage is known as a prophage
- in the integrated state, the viral DNA is replicated along with the chromosome each time the host cell divides, and is passed on to generations of the host daughter cells
- a single infected cell can soon give rise to a large population of bacteria carrying the viral DNA in prophage form
step 4: switch to lytic cycle
- when there is an environmental trigger (eg UV radiation) the virus switches from the lysogenic cycle to the lytic cycle
- lysis genes which were repressed during lysogeny are activated, allowing the lambda phage genome to be excised from the bacterial chromosome, giving rise to new active phages
- steps 3-5 (synthesis and replication, assembly, release) of the lytic cycle resumes
lambda phages’ tails are NOT CONTRACTILE and serves todeliver the viral DNA to the cell membrane
what are enveloped animal viruses and state two examples of it.
enveloped animal viruses are viruses with a membranous envelope surrounding their nucleocapsids.
examples of enveloped animal viruses include the influenza virus and human immunodeficiency virus (HIV)
what is the influenza virus?
- the influenza virus is an enveloped virus in which the viral negative (-) sense single-stranded RNA is present in the virion in 8 separate pieces.
- the negative (-) sense viral RNAs must first be transcribed to positive (+) sense single-stranded RNA which serves as mRNA for translation
- (-) sense single-stranded RNA is subsequently synthesised from (+) sense single-stranded RNA for use as genomic material and packaged with the viral proteins to form new virions
what are structural features of the influenza virus?
genome - 8 different segments of negative (-) sense single-strand RNA. (-) sense RNA must be transcribed to complementary (+) sense RNA before it can be used for translation of viral proteins
capsid - nucleoprotein (NP) associate with the viral nucleic acid to form nucleocapsid
membrane/viral envelope - phospholipid bilayer obtained from host cell surface membrane upon budding
surface glycoproteins - haemagglutinin (HA) and neuraminidase (NA)
protein envelope - matrix protein (M1 & M2) forms second layer of envelope, enclosing the nucleocapsid
enzymes - PB1, PB2 & PA forms RNA-dependent RNA polymerase (replicase), NS1 regulates viral replication mechanisms
details of protein envelope & enzymes dn memo
haemagglutinin - binds to sialic acid containing receptors, attaching virus to receptor on host cell membrane
neuraminidase - hydrolyses mucus, allowing virus to entercells of the respiratory tract & facilitates budding by cleaving sialic acid containing receptors
APSAR
what are the 5 steps of the reproductive cycle of influenza virus?
- adsorption
- penetration
- synthesis (of viral components) and replication (of viral genome)
- assembly of new virion
- release
describe, in detail, the reproductive cycle of influenza virus
step 1: adsorption
- haemagglutinin (HA) molecules on the viral membrane binds to sialic acid containing receptors on the membrane of the host cell (epithelial cells of the respiratory tract)
- (HA molecules have a complementary 3D shape with the receptor)
step 2: penetration
- the virus is taken in by receptor-mediated endocytosis, forming an endocytic vesicle within the host cell, aka endosome, with the influenza virus attached to its inner surface
- fusion of the endosome with an acidic lysosome lowers the pH of the vesicle, triggering conformational changes in the HA protein, causing the viral envelope and endosome membranes to fuse, releasing the 8 viral segments of the influenza genome directly into the host cell cytoplasm.
- viral RNAs are transported into the nucleus
step 3: synthesis & replication
- viral replicase (incuded as part of the virion) copies the (-) sense RNA template into complementary (+) sense RNAs for synthesis of viral nucleic acids and viral proteins.
- (+) sense RNAs are used as templates for synthesis of full length (-) sense strand viral RNAs by viral replicase. the (-) sense viral RNAs can be packaged into new viral particles as their nucleic acid
- (+) sense RNAs are used as mRNA which are translated in the cytoplasms by host ribosomes, synthesising enzymes, matrix proteins capsomere proteins and glycoproteins.
- free ribosomes synthesise enyzmes, matrix and capsomeres as they are ultimately folded into final conformation in the cytoplasm and packaged into the new virion. rER-bound ribosomes synthesise viral transmembrane surface glycoproteins, which are transported to the golgi apparatus for glycosylation before being incorporated into the host cell membrane via a vesicle which fuses with the host cell membrane
step 4: assembly
- 8 (-) sense viral RNAs associate with nucleoprotein (NP), and enzymes (eg viral replicase) are packaged, completely assembling viral particle.
- the glycoprotein studded membran envelope is acquired during the release of the virus
step 5: release
- the virus is released from the host cell by budding, acquiring the host cell’s lipid bilayer as the virus’ envelope
- the host membrane containing HA, NA and M2 (matrix protein) buds off from the host cell with the virion components
- because HA is present on the viral envelope and sialic-containing cellular receptors are on the host cell’s membrane, budding brings the virus and host cell together, resulting in the new viral particle remaining attached to the host cell
- neuraminidase (NA), helps release the virus by cleaving sialic acid residues on the cellular receptor that binded the newly formed virions to the cell
- this releases the virions, allowing infection to continue
virion: new virus that is synthesised
what is human immunodeficiency virus (HIV)?
HIV (human immunodeficiency virus) is a retrovirus that causes AIDS (acquired immunodeficiency syndrome)
a retrovirus is an enveloped RNA virus which replicates by means of a DNA intermediate synthesised by the enzyme reverse transcriptase
what are structural features of HIV?
genome - 2 identical single-stranded RNA. the single-stranded RNA is reverse transcribed to produce DNA for integration into the host genome. the DNA is then used for the transcription of viral mRNA which is translated into viral proteins and for use as the viral genome in the progeny virus
capsid - surrounds the nucleic acid
viral envelope - is the phospholipid bilayer obtained from the host cell upon budding
surface glycoproteins - gp120 (binds to CD4 receptors on WBCs like macrophages & T helper cells) and gp41 (helps HIV envelope & host cell membrane fuse)
protein coat - made of matrix protein, forming the 2nd layer of the protein envelope and enclosing the capsid
enzymes - reverse transcriptase (reverse transcribes RNA into DNA), integrase (facilitates incorporation of double-stranded DNA into host cell’s genome) and protease (cleaves viral polypeptide into functional proteins)
APSAR
what are the 5 steps of the reproductive cycle of HIV?
- adsorption/attachment
- penetration
- synthesis (of viral components) & replication (of viral genome)
- assembly of new virions
- release