Reasons why all viruses rely on host cells for their reproduction
1. Absence of translation machinery such as ribosomes and the building blocks amino acids, have to rely on host ribosomes and the building blocks amino acids for production of viral proteins
2. Absence of transcription machinery such as RNA polymerase and free ribonucleotides, have to rely on host RNA polymerase and free ribonucleotides for formation of mRNA for the purpose of translation
3. Absence of DNA replication machinery such as DNA polymerase and free deoxyribonucleotides, have to rely on host DNA polymerase and free deoxyribonucleotides to replicate genome
Role of viral DNA in the infection process of the T4 bacteriophage
1. Viral DNA codes for/is transcribed and translated to produce enzymes and structural components of phage which are important for infection of bacterial cell
2. Enzymes hydrolyse the DNA of host cell to nucleotides which can be used by host DNA polymerase to replicate viral DNA
3. Enzyme lysozyme which is released by phage to digest bacterial cell wall to trigger lysis of host cell for release of newly formed virions/to trigger injection of viral DNA into host cell
4. Phage proteins which form tail fibres which can recognise and bind to receptors on surface of bacterial cell
5. Phage proteins which form base plate which can change shape in response to molecules released from bacterial cell to cause contraction of contractile sheath for injection of viral DNA
New viral strains are formed when 2 or more strains of influenza viruses infect a common/same host cell<|>Reassortment of the different RNA segments occur resulting in a new combination of RNA segments in a virion
Viral glycoproteins (gp120 and gp41) are translated at rough endoplasmic reticulum from the mRNA of the provirus transcribed in the host
These glycoproteins are then transported and inserted into the host cell membrane via vesicles
The nucleocapsid will then be assembled near the cell surface, and it will bud off from host cell membrane that is studded with viral glycoproteins to form a new virus
Explain how the 2reproductive cycles of bacteriophages – lytic and lysogenic cycle, differ in bacterial cells (Control of host cell's protein synthesising machinery)
Lytic Cycle
Host cell DNA is degraded immediately upon virus entry allowing
Enzymes encoded by the virus take complete control of the bacterium’s macromolecular (DNA, RNA, protein) synthesis.
2. Lysogenic Cycle
Host cell DNA is not degraded upon virus entry.
Phage partially control the bacterium’s macromolecular synthesis to synthesize repressor proteins.
Explain how the 2 reproductive cycles of bacteriophages – lytic and lysogenic cycle, differ in bacterial cells (Integration into host cell genome)
Lytic Cycle
Phage does not incorporate genome into host cell DNA
2. Lysogenic Cycle
Phage incorporates its genome into the host cell DNA. Virus is now called a prophage
Explain how the 2 reproductive cycles of bacteriophages – lytic and lysogenic cycle, differ in bacterial cells (Repressor protein)
Lytic cycle
No repressor protein is encoded or synthesised
2. Lysogenic cycle
Prophage gene expresses two repressor proteins which block expression of other phage genes involved in phage replication.
Explain how the 2 reproductive cycles of bacteriophages – lytic and lysogenic cycle, differ in bacterial cells (Latent stage/ induction into lytic cycle)
Lytic Cycle
Phage synthesizes enzymesandphage components immediately after infection. /There is no latent stage as the virus replicates once inside the host cell.
2. Lysogenic Cycle
Phage synthesizes enzymesandphage components after the induction event in which prophage is excised from host genome and phagegenes are activated /Phage undergoes a latent stage where it doesnot undergo replication
Explain how the 2 reproductive cycles of bacteriophages – lytic and lysogenic cycle, differ in bacterial cells (Propagation of virus)
Lytic Cycle
The viruses are released by lysing the host cell and infect new bacteria cell
2. Lysgenic cycle
Virus replicate as the host cell genome is replicated and will be found in the bacteria progenies during latent stage
Lamba phage replication process (1. Attachment)
Tail fiber adsorb to complementary receptor site on host bacterial surface (same as T4 phages)
Lamba phage replication process (2. Entry - penetration)
Unlike T4 phage, lambda phage has a non-contractiletail without a contractilesheath
Phage genome enters the bacterium via hollowtube through the bacterial cell wall
Lamba phage replication process
1. Penetration into bacteria cell
2. Linear phage DNA forms a circle
3. Circular DNA can be replicated and transcribed leading to production of newphage and celllysis (lytic cycle)
4. Circular DNA can integrate into and become part of circular bacterial DNA (lysogenic cycle)
Occurs in one of every million to every billion bacteria containing a prophage
Induction occursspontaneously but its frequency is enhanced by irradiation with ultravioletlight or exposure to agents that damageDNA
Activates the cellular proteases (cleave repressor proteins)
Under these conditions, repressor protein is degraded by increasedprotein activity
Prophage is no longer repressed but is instead excised and enters the lytic cycle
Lambda phage replication process (5. Assembly)
Since phagegenome is nolongerrepressed, phage components are produced using the hostbacterium’smetabolicmachinery
Morecopies of viral genome are produced by DNAreplication using hostcellmachinery
Bacteriophage components then assemble into completevirions (same as T4 phages)
Lambda phage replication process (6. release)
The completevirions are then released from the hostcell in the samemanner as the lytic cycle (same as T4 phages)
Retrovirus- HIV structure
2 copies of positive-sense single-stranded RNA
RNA tightly bound to nucleocapsid proteins
contains 3 major genes, 5’ gag-pol-env 3’ which encode major structural proteins and enzymes (Gag: structural proteins, Pol: viral enzymes, Env: glycoproteins)
concial-shaped capsid containing 2 molecules of enzyme reverse transcriptase + integrase and protease
envelope surrounds the capsid, which contains gp120 and gp41 with specific conformation that allows the virus to bind to certain receptors on T-helper cells
HIV replication process (1. Attachment)
Process begins when a viral particles comes into contact with a cell that carries on its surface a special protein called CD4
gp120 interacts with the receptor on the target cell (usually T-lymphocytes) with the help of a co-receptor
a conformational change in shape is triggered which involves a receptor
HIV replication process (2. Entry - penetration and uncoating)
With the help of gp41, the viral envelope will fuse with the host cell membrane and the capsid is then released into the cell, leaving the envelope behind (alternatively can also enter via endocytosis)
Capsid and nucleocapsid protein are then degraded, releasing viral enzymes and the RNA into the cytoplasm
HIV replication process
1. Reverse transcriptase catalyses conversion of viral RNA into DNA
2. Reverse transcriptase catalyses synthesis of DNA strand complementary to viral RNA, forming RNA-DNA hybrid
3. RNA strand is degraded and second DNA strand complementary to first is synthesised to form double-stranded DNA molecule
4. Viral DNA enters host cell nucleus and is integrated into host genetic material
5. Enzyme integrase catalyses integration process
6. Integrated viral DNA may persist in latent state for years
7. Activation of host cell results in transcription of viral DNA into viral RNA which serves as mRNA
8. mRNA exits nucleus into cytoplasm where it is translated into viral polyproteins
9. Envelope glycoproteins gp120 and gp41 are made in ER and transported to cell membrane
10. Env polyprotein is cleaved by host cell protease in ER
11. Viral RNA forms genetic material for next generation of viruses
Polyproteins and viral genome assemble at the inner surface of the host cell surface membrane
HIV replication process (5. Release)
After assembly at the host cell surface membrane, the virus buds off from the cell
Host cell surface membrane evaginates and pinches off, forming the viral envelope with viral glycoproteins (gp120 & gp41) embedded
Polyproteins will be cleaved into the functional proteins by HIV protease
The functional proteins include structural proteins (matrix, capsid, nucleocapsid proteins) and viral enzymes (reverse transcriptase, integrase, HIV protease)
The virion is now considered mature and ready to infect another cell
Envelope viruses - influenza structure
spherical/ ovoid in shape
organised into 8 segments of single-stranded negative sense RNA strands
RNA is packaged with protein into a helical nucleoprotein form, with 8 RNA molecules
3 polymerases form an enzyme complex, RNA-dependent RNA polymerase/ RNA replicate which functions in both replication and transcription of the viral genome
other 5 RNA segments code for other viral proteins such as glycoprotein (haemagglutinin) and enzymes (neuraminidase)and non-structural proteins
derived lipid bilayer from cell surface membrane
Influenza replication process (1. attachment)
Protruding glycoproteins bind to specific receptor molecules on surface of host cell
In humans, haemagglutinin on the influenza virus binds to sialic acid receptor on the host cell surface membrane
Influenza replication process (2. Penetration and unloading — entry)
The virus usually enters by endocytosis
Host cell surface membrane invaginates and pinches off, placing the virus in an endocytic vesicle/ endosome
Within the vesicle, the low pH environment will stimulate the viral envelope to fuse with lipid bilayer of the vesicle membrane and nucleocapsid is released into the cytoplasm
The capsid is degraded by cellular enzymes, leaving behind the helical nucleoprotein
The helical nucleoprotein then enters the nucleus of the cell
Influenza replication process (3. Replication)
The viral genome is used as a template to synthesise the viral mRNA/ (+) strand RNA catalysed by the viral RNA-dependentRNA polymerase
mRNA produced in turn acts as a template for the synthesis of new viral RNA genome
mRNA strand then exit the nucleus to the cytosol and RER where they are translated into viral structural components, such as the glycoproteins to be incorporated into the viral envelope (at the ER) and capsid proteins (in the cytosol)
Influenza replication process (4. Maturation - assembly)
Viral glycoproteins are transported by the vesicles from the ER
Incorporated into the host cell surface membrane
Capsid proteins then associate with these glycoproteins at the host cell surface membrane
Viral genome associates with proteins to form the helical nucleoprotein which then interacts with the capsid proteins at the host cell surface membrane
Interaction of the capsid with the nucleoprotein will initiate the budding process
Influenza replication process (5. Release by budding)
Each new virus buds off from the cell
Host cell surface membrane evaginates and pinches off, forming the viral envelope with viral glycoproteins (haemagglutinin & neuraminidase) embedded
With enveloped viruses, host cells may or may not be lysed
The release is facilitated by neuraminidase
Neuraminidase cleaves sialic acid from cell surface and progeny virions, facilitating virus release from infected cells