NextStep FL 4 C/P #19

[jeep1010]

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The light blue appearance of S157T is most directly attributable to what other phenotypic change versus the wild type?


A.

Increased λ max

B.

Decreased λmax

C.

Decreased ɛ

D.

Increased ɛ


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I chose C because the table shows the wild type has a wavelength of 608, whereas the S157T substitution has a wavelength of 611. The molar absorptivity values are 122, 923 for the wild type and 85, 654 for the S157T substitution. The wild type color was Blue and the S157T subsitution color was light Blue. The question asks what change caused the light blue appearance. I figured that because the wavelength only changed by 3 units of 608 --> 611, that the molar absorptivity would be the bigger factor in the color change because it went from 122,923 --> 85, 654 which is a much bigger decrease and change than 3 units of the wavelength.

If someone could possibly clarify this that would help a lot. I remember a similar question on the section bank where the greater change caused the effect observed in the experiment

@NextStepTutor_2 @NextStepTutor_3

 

[aldol16]

One thing you should know is that the extinction coefficient does not give you the color. It only tells you how strongly something absorbs. Where it absorbs tells you the color and that where is always given by a lambda max.

 

[jeep1010]

@aldol16 Oh, interesting, I did not know that. Do you think that this concept is something that should be covered in content review? I figured that concept sounds very small and I figured critical reasoning would be the way to go about this problem?

 

[aldol16]

@aldol16 Oh, interesting, I did not know that. Do you think that this concept is something that should be covered in content review? I figured that concept sounds very small and I figured critical reasoning would be the way to go about this problem?

Well, for this particular problem, critical reasoning doesn't work because you actually need to know the difference between lambda max and the extinction coefficient. The critical reasoning part comes in when you're trying to figure out increasing or decreasing. You're trying to apply critical reasoning too broadly.

Knowing this is low-yield, but for this question it's relevant.

 

[NextStepTutor_3]

Hi @jeep1010 ! As usual, @aldol16 is absolutely right about the science content, but this question actually can be answered using reasoning alone. Here's how I would do it, even assuming no knowledge about extinction coefficient vs lambda max:

The question asks us to explain the light blue color of S157T. The wild-type was blue, as you said, so we're trying to explain why S157T has a lighter blue color than the wild-type does. We're not given information about statistical significance, so it's generally not a good idea to simply choose the larger change - sure, a 3-nm change in wavelength may seem like a minor change, but it could very well be significant. So let's actually look at both A and C in the context of the entire table instead.

Let's first look at the answer you chose, choice C. This choice is saying that a decrease in extinction coefficient caused the color change the question stem is asking about. However, look one line further down in the table, at the S157C mutant. This mutant had an even LOWER extinction coefficient, but it appears DARK blue, not light blue. Thus, the light blue phenotype of the S157T mutant can't be explained - at least solely - by the decrease in extinction coefficient. If it could, we'd expect the S157 mutant to be even LIGHTER blue, since it has a lower coefficient. Choice C must be incorrect, and we're left with choice A.

 

[NextStepTutor_3]

Choice A makes sense too, by the way, when we look at the table as a whole. Compared to the wild type, the S157C mutant has a higher lambda max value, and it has a lighter color. The S157T mutant has a LOWER lambda max value, and it has a DARKER color - exactly what we'd expect.

The main takeaway from this is not to simply rely on numbers when we don't know anything about them. Here, the changes in lambda max look small, but for all we know, a 3-nm or 7-nm change could produce an enormous difference in phenotype. Similarly, the differences between the extinction coefficient values look large, but we don't know if they're large enough to produce any difference at all. Instead, we need to look at the table as a whole and choose the answer that goes along with all the information we're given.

 

[jeep1010]

That was a really great response, I really should have looked at the next mutation down to see if the pattern continued. Thank you very much for this, and all the other, thorough responses.