Genetics of Viruses 2 (MJC)
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State the name and chemical composition of structure X and Y.
X is the capsid which is made up of identical repeating protein subunits called capsomeres.
Y is the nucleic acid which can be single or double-stranded DNA or RNA which is made up of nucleosides.
State two reason why all viruses rely on host cells for their reproduction.
1)Viruses lack organelles such as ribosomes so as to synthesize it's own proteins.
2) Virsuses also lack enzymes such as DNA polymerase III neccessary for DNA replication, thus it relies on host cells for its reproduction.
State two similarities between the lytic and lysogenic cycles.
- 1) Both requires recognition and attachment of phages to specific receptor sites on host's cell surface.
- 2) Both requires penetration that injects the phage DNA into the host.
Explain the significance of bacteriophage co-existing with their bacterial host cells during the lytic cycle.
- They do not kill or destroy the bacterial host cells.
- The host cells can replicate the viral DNA along with its own genome during binary fission such that the viral DNA can be found in each of its progeny.
Explain why the host range of swine influenza virus is restricted only to a fixed species.
The hemagglutinin glycoproteins found on the envelope of avian influenza virus are specific or complementary only to the protein receptors found on the surface of swine respiratory epithelial cells.
Outline the role of the viral genome of HIV virus upon its entry into a host cell in forming new viral particles.
- -Upon entry into the host cell, the viral RNA is first reverse-transcribed to give a complementay DNA which is then used as a template for replication to give a double-stranded viral DNA strand which is then integrated into the host cell's genome.
- - The viral double-stranded DNA functions as a template for making the complementary mRNA strands.
- -Some of the mRNA strands are used as templates for the replication of the viral RNA genome of new viruses
- - The rest of the mRNA strands are used in the translation of new viral proteins such as gp41.
State the differences between the exit of a bacteriophage and a retrovirus from the host cell.
- 1) The release of a bacteriophage results in the lysis of the cell while a retrovirus buds off the host cell, leaving it intact.
- 2) The exit of a retrovirus results in the aquirement of the host cell membrane to form an envelope while bacteriophage do not acquire the host cell's membrane.
- 3) A retrovirus incoporates its viral glycoproteins into the host cell membrane while a bacteriophage does not.
- 4) The progeny of a retrovirus is exits out of the cell one at a time while all the progeny of a bacteriophage is released due to the lysis of the cell.
How does a viral infection results in the systoms of a disease or illness to occur?
- -Viruses cases the release of hydrolytic enzymes from the cell's lysosomes causing the lysis of the host cell.
- -Viruses produce toxins in which will result in an immune response to eradicate the poison.
- -Hence temporary symptoms usually arise due to the immune defence against viral infections.
Explain how viruses such as the human papillomavirus results in the formation of cancer.
- -The viruses may carry oncogenes that trigger continuous uncontrolled cellular division
- -Hence, the integration of the viral genome into the host genome will result in the expression of the oncogens and may eventually result in the formation of cancer.
- - The intergration of viral genome into the host cell's genome may cause disrupt tumor supressor genes and hence the tumor supressor proteins expressed are non-functional and hence is unable to regulate the cell cycle.
What are the differences between the lytic and lysogenic cycles?
- 1)Lytic cycle results in the death of the host cell while in lysogenic cycle, the phage genome replicates without destroying the host cell.
- 2) Lytic cycle results in the hydrolysis of the host cell's DNA while the viral DNA is incoporated into the host cell's genome in the lysogenic cycle.
- 3) In the lytic cycle, the phage DNA exploits the host machinery for protein synthesis and DNA replication while in the lysogenic cycle, the viral DNA only replicates when the host cell divides
Certain viruses such as the influenza viruses contains negative sense strand RNA.
Explain the term negative sense strand RNA.
- -Negative sense strand RNA cannot be directly translated into proteins.
- -It is complementary to the positive sense strand RNA which can be used for translation for synthesis of the viral proteins.
- -Thus, negative sense strand RNA must be converted to a positive sense strand RNA via RNA polymerase prior to translation.
State 2 differences in the adsorption and penetration of the influenza viruses and the HIV viruses into the target cells.
- 1) The hemagglutinin found on the envelope of the influenza virus binds to the complimentary shaped receptor sites found on the cell surface membrane of its target host cell which is the lung epithelium cells while in HIV, the gp120 glycoproteins on its envelope binds to its specific receptors on the cell surface membrane of the white blood cells(target cells).
- 2) The influenza virus enters the cell via receptor-mediated endocytosis while for HIV, its envelope fuses with the host cell's surface membrane, hence releasing the capsids and the viral RNA(nucleocapsid) into the host cell.
Explain the function of the following proteins in viral replication.
ii) Reverse Transcriptase
i) Hemagglutinin is a glycoprotein found on the envelope of influenza viruses and its function is to recognise the specific receptor binding sites found on the cell surface membrane of the lung epithelial cells which is complementary in shape so as to initiate receptor-mediated endocytosis.
ii) Reverse transcriptase is an enzyme that is found in HIV viruses which functions to synthesize complementary DNA using a positive sense strand of RNA as a template.
Explain why the HIV virus is stated to have a more rapid evolution as compared to the human genome.
- -This is due to the high mutation rate of the HIV virus
- -The high mutation rate is due to the fact that the reverse transcription process is error-prone as it lacks proofreading in which there are no mechanisms to correct errors such as the placing of wrong non-complementary nucleosides during this process.
Explain how HIV protease inhibitors which are complementary in shape to the binding site of the protease prevents new viruses from forming.
- -These protease inhibitors are structurally similar to the substrate (viral polyprotein) and hence competes to bind to the complementary-shaped active site of the protease to form the enzyme-inhibitor complex.
- -The formation of the complex prevents the substrate from entering the active site of the protease,thus inhibiting the forming of the enzyme-substrate complex
- -Thus the polyprotein cannot be cleaved and the new virus cannot be formed.
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