Question

1.In early March, it was reported that two main strains of SARS-CoV-2 had been identified, termed S and L, with the S type being the less virulent ancestral form representing about 30% of prevalence and the more virulent L type having descended from the S type and representing about 70% of prevalence. However, the prevalence of the more virulent L type seems to have decreased after early January, and it is thought that this resulted from human intervention against this more virulent strain; meanwhile, the milder S form seems to have increased in relative frequency. Explain these findings in terms of evolution by natural selection and the relative fitness of these two strains.

2.The SARS-CoV-2 is an enveloped virus, meaning it is surrounded by a bilayer of membrane that it acquires as it exits the host cell.

1. What is the host membrane bilayer made of?

2. What is the other main macromolecule that comprises the “fluid mosaic” model of a cell membrane?

3.To gain entry, spikes of the SARS-CoV-2 bind to angiotensin-converting enzyme 2 (ACE2) found in epithelial cells especially of the respiratory and oral passageways. As such, these cells are susceptible to infection by SARS-CoV-2. ACE2 is a protein consisting of multiple alpha-helices that is embedded in, and spans, the membrane. What is this type of protein called, and what structure do the alpha helices represent?

4.Following binding of spike proteins to ACE2, SARS-CoV-2 likely gains entry into the cell either via fusion of its envelope with the cell membrane, or via endocytosis. If the latter, the virus must escape the endosome before fusion with the cellular lysosome. Once inside the cytoplasm, the virus unpackages and its positive stranded RNA genome is released.

1. What is endocytosis?

2. What is a lysosome, and what is its importance in the cell?

3. What type of macromolecule is RNA?

5.The virus hijacks the cell, and RNA polymerases produce the complement to the positive stranded RNA genome. We can call these strands negative strands, and they then serve as templates for RNA polymerases to produce their complement. How does the sequence of these strands, the complement to the negative strands, compare with the original viral genome?

6.RNA polymerases lack proofreading ability. Define proofreading ability and describe its importance in replication of DNA genomes.

1. Why is this a critical function for human DNA polymerases?

2. What advantage might its absence give RNA viruses like SARS-CoV-2?

7.The positive stranded RNA serves as the template for protein synthesis.

1. What is the process called that involves converting the RNA sequence into a sequence of amino acids?

2. What is the cellular “workbench” of this process, and what is this “workbench” composed of (i.e., what structure accomplishes protein synthesis)?

3. How is the sequence of RNA converted into a sequence of amino acids?

4. Where does this process take place in the human cell?

8.Following protein synthesis, RNA genome and proteins assemble to form virus particles that then bud into the endoplasmic reticulum (ER) lumen and then on to the Golgi apparatus. Next, they are transported via Golgi vesicles to the cell membrane, where they are released via exocytosis. Define ER, Golgi apparatus, and exocytosis, and explain how this process undertaken by these viruses is comparable to processes regularly used by the cell.

Answer 1

1. The evolution is an ongoing process powered by random mutations and natural selection. Virus has evolved into two different strains in short span leading to pandemic outbreak. But since human intervention including personal hygiene and symptomatic medications lead to decline in L strain spread. But this process lead to selection advantage of S strain, which slowly increased in frequency. The human intervention acted like selection pressure that worked against L form and favoured S strains. The rapid spread of L form shows it has higher fitness, but due to changes selection pressure it's relative fitness reduced and that of S strain increased. Now due to increased relative fitness S strain will regain its dominance and would reduce once selection pressure is directed against it.

2. A. The lips bilayer consists of phospholipids arranged in two layers. Hence it is called bilayer. The lips are arranged with thsir polar hydrophilic head group facing outside and hydrophobic tail facing inside.

B. Other than phospholipids, bilayer also contains several glycoprotiens, proteins and carbohydrate units. It consists of enzymes, protein receptors to recieve ligands and hormones. It consists of protein channels that allow selective passage to various ions and macromolecules. It also consists of glycoprotiens that face outside to work as antigens and markers outside a cell.

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