Question

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Question 13 Answer saved This phylogeny from Tang et al. 2020 shows the relationship between the human coronavirus SARS-CoV-2 (top right) and other related coronaviruses with non-human hosts, including bats and pangolins. Points out of 1.00 P Flag question 0.083(0.031) 0.158(0.034) SARS-CoV-2 0.091(0.041) 0.094(0.036) Bat RaTG13 GD Pangolin-CoV 0.196(0.053) 0.313(0.034) GX Pangolin-CoV_P2V GX Pangolin-CoV_P5E 0.644(0.045) GX Pangolin-CoV P1E 0.428(0.098) GX Pangolin-CoV P5L GX Pangolin-CoV P4L 0.051(0.090) Bat SARS-CoV ZXC21 GX Pangolin-CoV P3B 0.183(0.061 0.058(0.049) _ 0.412(0.076) Bat SARSr-CoV ZC45 SARS-CoV Bat SARS-CoV BM48-31 From this relationship, we can conclude that: Select one: a. SARS-CoV-2 is more closely related to the other human virus, SARS-CoV • b. The human virus is more closely related to the bat virus RaTG13 C. SARS-CoV-2 evolved from a human virus d. There is no evidence for transmission between non-human hosts e. GD Pangolin-CoV is the ancestor to SARS-CoV-2

And the two questions that use the photos

Answer 1

Q.14.

Answer - Human SARS- CoV-2 is experiencing more purifying selection than the GX Pangolin-CoV.

Explanation :

Since the ratio of dN/dS of Human SARS -CoV-2 is lower than that of GX Pangolin-CoV, it implies that number of synonymous substitutions are higher than non-synonymous substitutions. Thus there is a selection against the non-synonymous substitutions. This type of selection is called purifying selection.

Q.15.

Answer - Selection removes deleterious mutations at non-synonymous sites.

Explanation :

The total divergence of the sequence was smaller than the divergence observed only at synonymous sites due to the selection against deleterious mutations at non-synonymous sites. The selection of synonymous mutations is in general more than the non-synonymous ones and thus the evolution is faster in these sites. As a result, the mutations are fixed faster.

The answer could also be that genetic drift has occurred at synoymous sites. The reason that it is not is because genetic drift acts on both synonymous and non-synonymous substitutions and generally leads to their fixation. In this case, genetic drift has lead to the fixation of both synonymous and non-synonymous substituions. However natural selection is the reason behind decreased proportion of fixed non-synonymous substitution in comparison with the favoured synonymous substitution.