Genetics of Viruses Flashcards
1
Q
briefly describe the structural components of animal viruses
A
- contains only a single type of nucleic acid (DNA or RNA), and can be single-stranded or double-sranded
- enclosed in a capsid protein coat
- may be surrounded by a lipid envelope derived from the cell surface membrane of the host cell
2
Q
DNA genome of T4 and lambda phage
A
double-stranded and linear
3
Q
describe briefly the lytic cycle of the T4 bacteriophage
A
- T4 phage uses its tail fibres to attach to specific receptor sites on the cell surface of host bacterium
- phage-coded lysozyme hydrolyses a small region of the bacterial peptidoglycan cell wall
- sheath then contracts to drive a hollow tube into the bacterium, injecting the viral DNA genome into the bacterial cytoplasm
- host cell’ss DNA is degraded by phage-coded nucleases and the bacterium’s protein, RNA, DNA synthesis is shut down
- T4 phage uses the bacterium’s RNA polymerase to transcribe phage DNA into phage mRNA
- it uses the bacterium’s ribosomes, tRNA, amino acids, ATP, enzymes to translate the phage mRNA into phage proteins and enzymes
- T4 phage DNA is replicated using the bacterium’s DNA polymerase and packaged inside the capsid
- the proteins self-assemble to form phage heads, tails and tail fibres
- phage-coded lysozyme breaks down the bacterial peptidoglycan cell wall causing osmotic lysis
- intact T4 bacteriophages are released and proceed to infect other bacterial cells
4
Q
describe briefly the lysogenic cycle of the lambda phage
A
- lambda phage uses its tail tip to bind with specific receptor sites on the host bacterium
- tail tip interacts with host cell protein channels, injecting phage DNA into cell and leaving the empty capsid outside
- phage DNA circularises inside the bacterium
either undergoes lysogenic: - lambda phage does not shut down the host cell’s DNA, RNA, protein synthesis. instead, phage DNA integrates into host bacterium’s DNA using phage-encoded enzyme integrase as a prophage
- phage genes code for a repressor protein which blocks expression of other phage genes. no transcription of the phage DNA takes place
- phage DNA replicates along with the replication of bacterium DNA, enabling viruses to propagate without killing host cells
or undergoes lytic: - environmental signals (radiation) can cause damage to host DNA and stress to host cell, triggering breakdown of phage-coded repressor proteins
- without repressor proteins, phage genome is excised from bacterial genome (prophage induction), triggering the switchover from lysogenic to lytic cycle
- phage genome replicates and phage enzymes and proteins are synthesised using the host bacterium’s enzymes (DNA polymeraese, RNA polymerase), ribosomes, tRNA, amino acids, ATP
- lambda phage components self-assemble
- phage-coded lysozyme breaks down the peptidoglycan cell wall causing osmotic lysis and release of the intact lambda bacteriophages
5
Q
structure of influenza virus
A
- (-)-sense RNA genome is segmented into eight linear single-stranded molecules, each closely associated with nucleocapsid proteins. together, RNA segments and proteins are ribonucleoproteins (RNPs)
- proteins PB1, PB2 and PA form the viral RNA-dependent RNA polymerae is associated with each RNP
- nucleocapsid is of helical symmetry and is enclosed in an envelope (derived from host cell and has a number of viral-specific glycoproteins)
- beneath the viral envelope is a layer of matrix protein, M1, which holds the RNPs on one side and to the glycoproteins on the other
- two types of short glycoproteins spides projecting from outer surface of envelope: haemagglutinin (HA) and neuraminidase (NA)
- third integral membrane protein, M2, serves as an ion channel and is important in release of RNPs into the host cell cytoplasm after virus has entered host cell
6
Q
describe the major differences between the lysogenic and lytic cycles of viruses (9 pts)
A
- in lytic cycle, phage DNA does not integrate into the bacterium chromosome while in lysogenic cycle, phage DNA integrates into the bacterium chromosome, forming a prophage
- in lytic cycle, phage DNA is replicated separately from bacterial genome to form new phage DNA while in lysogenic cycle, phage DNA is replicated as part of bacterial genome (as a prophage) during binary fission
- in lytic cycle, phage shuts down macromolecular synthesis (RNA, DNA, protein) in host cell while in lysogenic cycle, phage does not shut down macromolecular synthesis (RNA, DNA, protein) in host cell
- in lytic cycle, phage uses host’s RNA polymerase and ribosomes to transcribe and translate a large number of phage genes (to express phage structural proteins e.g. capsid, tail, sheath, and enzymes) while in lysogenic cycle, phage uses host’s RNA polymerase and ribosomes to transcribe and translate a small number of pahge genes (to express enzyme integrase and repressor proteins)
- in lytic cycle, structural proteins are assembled with viral genome to form new phages while in lysogenic cycle, there is no assembly of phage components
- in lytic cycle, phage-coded lysozyme hydrolyses the bacterial peptidoglycan cell wall to release new phages through osmotic lysis while in lysogenic cycle, bacterium does not lyse/remains intact and alive
- in lytic cycle, phage DNA does not circularise and remains linear inside the bacterium while in lysogenic cycle, phage DNA circularises inside the bacterium
- in lytic cycle, no repressor protein is involved in blocking the expression of phage genes while in lysogenic cycle, repressor proteins are involved in blocking the expression of phage genes
- in lytic cycle, no prophage induction occurs while in lysogenic cycle, prophage induction occurs
7
Q
outline the role of proteins in the influenza virus reproductive cycle
A
- HA binds to sialic acid residues of specific glycoprotein receptors on the CSM of epithelial cells lining the respiratory tract, allowing attachment of virus to host cell’s surface during infection
- NA helps virus penetrate host cell by breaking down mucoproteins in mucus layer lining respiratory tract and aids in release of virus at the end of reproductive cycle by cleaving sialic acid residues on host CSM
- M2 ion channel allows acidification of virion interior and release of M1 proteins from RNPs so that RNPs can be released into host cell cytoplasm to enter the nucleus of the host cell
- M1 proteins provide structural support for the virus by holding the RNPs on one side and to glycoproteins in the viral envelope on the other
- viral RNA-dependent RNA polymerase transcribes single-stranded (-)-sense RNA molecules into (+)-sense mRNA so that viral structural proteins and enzymes can be translated by host ribosomes
- viral RNA-dependent RNA polymerase also catalyses the replication of new viral (-)-sense RNA genome using the mRNA transcribed
- hence, viral RNPs and proteins can be released into cell cytoplasm for assembly of the components
8
Q
describe the reproductive cycle of influenza virus
A
- NA breaks down mucoproteins allowing the virus to penetrate the mucus layer and HA binds to specific receptors containing sialic acid on the host CSM
- receptor-bound viruses are taken into the cell by receptor-mediated endocytosis
- viral envelope fuses with endocytic vesicle membrane, M2 ion channel allows virion interior to be acidified, releasing M1 proteins from the RNPs, releasing the the capsid and the 8 RNPs into the cytoplasm
- released RNPs then enter the nucleus of the epithelial cell, releasing viral (-)-sense RNA genome
- within the nucleus, single-stranded (-)-sense viral RNA genome is used as a template and transcribed to (+)-sense mRNA by viral RNA-dependent RNA polymerase
- mRNA transported to cytoplasm and translated to viral structural proteins (capsid, matrix proteins, envelope glycoproteins) and viral enzymes by host cell ribosomes
- newly synthesised viral proteins (NA and HA) are transported through the GA and incorporated into the CSM
- nucleocapsid proteins are transported back to the nucleus to bind with viral RNA to form RNPs
- (+)-sense mRNA in nucleus also used as templates to replicate (-)-sense RNA genomes for new influenza viruses
- RNPs transported to sites at CSM where HA and NA components have been incorporated into host CSM
- mature viruses formed and released by budding: host CSM with incorporated HA and NA evaginates and pinches off to form the viral envelope
- viruses remain bound to the sialic acid receptors on host cell through HA, they detach once their NA has cleaved sialic acid residues from host cell
9
Q
RNA segments containing genes PB2, PB1 and PA code for RNA polymerase. explain why such RNA segments are needed by the virus
A
- RNA-dependent RNA polymerase required by the virus are not present in host cell
- these polymerases are requird to transcribe (-)-sense viral RNA genome into (+)-sense mRNA, which is then translated to viral structural proteins and this mRNA is also used as a template for the replication of (-)-sense viral RNA genome
10
Q
state how, despite having no ribosomes, the influenza virus is able to trigger the synthesis of viral structural proteins
A
virus uses host cell’s ribosomes to translate viral genes (in RNA segments) into viral proteins
11
Q
explain how inhibiting NA can prevent influenza
A
- cannot cleave sialic acid residues of the receptors on the host CSM that still bind the mature viruses
- new viruses cannot bud off successfully from the infected cells to infect other cells
- viruses cannot break down the mucoprotein and penetrate mucus layer of the host cell
12
Q
suggest why it may be advantageous for a bacteriophage to have a lysogenic cycle
A
- a phage-infected bacteria culture that is subjected to conditions unfavourable for rapid replication of phage components will enter dormancy
- a phage that is able to switch to lysogenic cycle and become dormant simultaneously with the host cell can wait for the opportunity to replicate when conditions are favourable. lytic phage will be unable to continue to reproduce since it can only replicate in actively metabolising bacteria
- viral genome/prophage can also be replicated each time the host cell divides hence allowing a large number of viral particles to be assembled upon prophage induction
13
Q
explain why the reproductive cycle of the influenza virus is not a life cycle
A
- do not grow
- do not feed
- no respiration
- no protein synthesis on their own
- not able to replicate/reproduce independently
- acellular
- do not move/rely on passive modes of transport
- not affected by freezing
14
Q
name the structural components of HIV
A
- two identical single-stranded (+)-sense RNA containing gag, pol, env genes that are enclosed by a conical capsid
- reverse transcriptase, protease and integrase enzymes
- capsid and envelope with glycoproteins gp41 and gp120
15
Q
describe reproductive cycle of HIV
A
- binding of gp120 to specific CD4 receptors and co-receptors on macrophages and T helper cells, triggering allosteric change (change in 3D conformation) in gp41 that penetrates host CSM
- HIV envelope fuses with host CSM, releasing the nucleocapsid (capsid and RNPs) into the cytoplasm
- capsid and nucleoproteins degraded and HIV genome (two single-stranded (+)-sense RNA molecules) and enzymes (reverse transcriptase) released
- reverse transcriptase synthesises double-stranded DNA using viral RNA as the template
- viral double-stranded DNA then enters the. nucleus, where integrase catalyses its integration into the host chromosome, forming a provirus and the provirus remains in a dormant stage
- upon activation of viral genes by production of specific host cell proteins, HIV double-stranded DNA will be transcribed into HIV (+)-stranded RNA
- HIV (+)-sense RNA serves as mRNA for translation in the cytoplasm by host ribosomes to form HIV proteins and enzymes, and also serves as a template for HIV RNA genome replication
- gag and pol mRNA are translated to give polyprotiens – long polypeptide chains that are then cleaved by viral proteases into individual functional proteins
- gag mRNA is translated into protein that forms the nucleocapsid
- pol mRNA is translated into reverse transcriptase, integrase and viral protease
- env mRNA is translated into molecules of envelope protein gp160, which is then cleaved by host cell proteases into gp120 and gp41
- envelope proteins are formed at the RER and then transported to the GA to be processed into gp120 and gp41 proteins, which are then transported to the CSM via vesicles
- capsid proteins assemble around viral enzymes, viral RNA genome and nucleoproteins
- new viruses bud off from the CSM, with gp120 and gp41 proteins on the surface
