The human angiotensin converting enzyme B (ACE2) gene encodes a cell surface receptor that regulates the constriction of blood vessels. The ACE2 receptor is the target of the SARS-CoV-2 (the virus responsible for COVID-19) spike protein during infection. Binding of the SARS-CoV-2 spike protein to amino acids #19-83 of the ACE2 receptor is required for viral entry via endocytosis into the human host cell [Renzi and Ghersi, (2020) bioRxiv]. The human ACE2 gene is expressed in several cell types including alveolar, heart, brain, kidney, pancreas, intestine, stomach and testis, thereby allowing for potential SARS-CoV-2 infection in multiple tissues of an individual and producing the complex symptoms and illness of COVID-19. There are two alleles of the human ACE2 gene. One allele (referred as "+" or wild type) produces normal levels of ACE2 mRNA or protein in the cell. In contrast, the other allele (referred as "OE") produces two-fold more ACE2 mRNA and ACE2 protein than the wild-type allele.
Quiz - BIOL 243 - Winter 2022 - DNA, Inheritance and Evolution - University of Calgary Question 1 (6 points) The figure below shows the phenotypes (COVID-19 Fold Infection) of the three ACE2 genotypes (+/+, OE/+, OE/OE). Given the previous information describing the two types of alleles of the ACE2 gene, explain how these three genotypes produce the distinct phenotypes. Your answer should indicate the relative levels (fold change or % difference compared to normal) of ACE2 protein produced and the phenotype of each genotype. Question 2 (3 points) Explain why individuals possessing the ACE2OE allele show a higher frequency of SARS-CoV-2 infections compared to those possessing the ACE2+ allele. ACE2 Gene structure (Q3-Q14) Paragraph Lato (Recomm… 19px (… Paragraph Lato (Recomm… 19px (… Refer to the transcript map below for the human ACE2 pre-mRNA for your answers to Questions 3-14. It is recommended that you download the figure or open it in a new window to view a larger version (right-click to open in a new window or download image). Question 3 (2 points) Question 4 (2 points) Question 5 (1 point) How many introns are there in the ACE2 pre-mRNA? How many exons are there in the ACE2 pre-mRNA? Which letter on the transcript map corresponds to the transcriptional start site? Which letter on the transcript map corresponds to the transcriptional stop site? Question 6 (1 point) Question 7 (2 points) Question 8 (1 point) Which of the following exons (numbered from the 5' end) do not contain any amino acid coding sequence? Select all that apply. What do the hollow (unfilled) exonic regions at either end of the transcript map represent? Choose the best answer. 1 2 18 19 All exons contain coding sequences Start and stop codons Poly-A tail 5' m7G cap Untranslated regions (UTRs) Based on this map, in which direction does transcription occur? (L to R or R to L) Based on this map, in which direction does translation occur? (L to R or R to L) Question 9 (1 point) Question 10 (1 point) Question 11 (2 points) Which of the following choices best accounts for the size difference in nucleotides between the ACE2 pre-mRNA and mature mRNA? Choose the best answer. If an amino acid is encoded by a 3-nucleotide codon, why is the nucleotide length of the ACE2 mature mRNA not three times the amino acid length of the ACE2 protein? Choose the best answer. A base substitution mutation in region "i" results in a stop codon. What is the effect of this mutation? Choose the best answer. Addition of the poly-A tail to the mature mRNA The mature mRNA contains introns The exons have been removed in the mature mRNA The pre-mRNA contains introns The poly-A tail is not translated The stop codon isn't factored into the calculation The final protein product is cleaved The mature mRNA contains large UTRs How many nucleotides in length is the ACE2 open reading frame (start codon to stop codon)? Enter a number only. Question 12 (2 points) Question 13 (1 point) A base substitution mutation in the intron-exon junction "g" prevents splicing of the intron between "g" to "h". Which of the following are possible effects of this mutation? Select all that apply. A synonymous mutation occurs in region "e". What is the effect of this mutation on the ACE2 protein? Choose the best answer. Lower mRNA expression, so less ACE2 protein is produced Normal mRNA expression, but the ACE2 protein is truncated The mRNA is truncated, so the ACE2 protein is truncated Normal mRNA expression, but no ACE2 protein is produced The resulting protein might have a longer amino acid sequence The amino acid sequence after the mutation will be altered because the intron sequence will be translated The pre-mRNA nucleotide sequence length will be unaffected The mature mRNA might have a shorter nucleotide sequence length The resulting protein might have a truncated amino acid sequence The protein will be unaffected because intron sequences are non-coding The mature mRNA nucleotide sequence length will be unaffected The mature mRNA might have a longer nucleotide sequence length Question 14 (4 points) Question 15 (2 points) Alternative splicing of the ACE2 pre-mRNA results in the removal of a large intron encompassing all nucleotides between "g" and "l" on the transcript map. If this ACE2 protein is expressed on the surface of the cell, what effect would there be on the SARS-CoV-2 infection rate and spike protein binding? Briefly explain why. ACE2 Allele Variants The higher protein abundance observed in the ACE2OE allele compared to the wild- type (ACE2+) allele is caused by altered regulation of gene expression at the post- transcriptional level. Which of the following mechanisms are a likely explanation for the abnormally high protein expression from the ACE2OE allele? Select all that apply. An amino acid change that may affect function No effect A truncated protein resulting from an early stop codon Paragraph Lato (Recomm… 19px (… Question 16 (5 points) You have identified a new COVID-19-resistant ACE2 allele, which you designated ACE2CR. To determine the nature of COVID-19 resistance from the ACE2CR allele, you compare the nucleotide sequence with the gene sequence of the ACE2+ allele. Remarkably, you discover that there is only a single nucleotide difference between the alleles, but this difference is not located within the transcriptional unit of both ACE2 alleles. Given this information, where is the nucleotide difference in the two alleles of the ACE2 gene likely to be? Propose an explanation for how this difference produces the COVID-19 resistance phenotype of the ACE2CR allele. Your answer should not exceed 8 sentences. Increased length of the poly-A tail increases mRNA stability and translational efficiency Alternative splicing makes a shorter protein that is translated faster Faster 5' capping increases mRNA stability and translational efficiency Decreased microRNA expression reduces translational inhibition and ACE2 mRNA degradation All of the above mechanisms are likely explanations Paragraph Lato (Recomm… 19px (… Submit Quiz 0 of 16 questions saved