Q. 15

[student]

The passage suggests that the main reason that TS type E. coli cannot grow above 42 oC, is that:

the answer is "the TS mutant gene causes the tertiary structure of Pol III to be lost at high temperatures." and the explanation is: "The Pol III enzyme is affected by heat in a way that causes it to stop functioning properly. The loss of an enzyme's tertiary structure will affect its function and can occur at elevated temperatures."

I'm not very satisfied with the explanation: wouldn't putting enough heat on ANY protein cause it to lose its tertiary structure?

Also please give me an ACTUAL explanation this time rather than saying "thats too bad, the MCATs are going to be like that".

[admin]

Yes, elevated temperatures can lead to any protein losing its tertiary structure. In so doing, the functionality of the protein can be lost. Thus answer choice D is a reasonable solution.

The other answer choices are not reasonable since any answer choice must link these facts as expressed by the passage:

Experiment 2: bacteria which already include a mutant gene -> the gene makes a temperature sensitive protein (a DNA polymerase) -> the protein is exposed to high temperature -> the protein does not work -> the cells will not grow.

Answer choice A: mutant gene -> dehydration

To begin with this is clearly an incomplete answer. Furthermore, there is nothing in the passage to relate dehydration with genes, DNA Polymerase nor cell growth.

Answer choice B: high temperatures -> mutant gene

The order is the reverse in Experiment 2.

Answer choice C: mutant gene -> no protein (DNA Pol)

This is incorrect since the passage states specifically: "Pol III enzyme that does not work at high temperatures"; in other words, some protein is indeed created but it does not work properly.

Answer choice D: mutant gene -> defective protein at high temperature which means -> at high temp -> protein to polymerize DNA does not work -> no cell growth.

[shaun.w.mc6504]

Any advice on a temperature range that we could expect a protein to denature at? I mean 42 degrees is not that hot considering body temperature is 37 degrees. I did not think that 42 degrees was hot enough to denature the protein?

[admin]

Though 36.5 is considered normal for humans, 39.5 is considered to be a very serious fever. Higher than that is life threatening if sustained (all the above, of course, is beyond the MCAT!). Not to mention, when discussing bacteria, the temperature range is quite extreme in terms of optimal function. There is no range that you should memorize for the MCAT but you must be comfortable, with time and practice, with your ability to deduce based on the information provided.

[quadalpha]

Perhaps the passage can be amended to remove this ambiguity: "contains a mutant gene (that codes for Pol III) that does not work at high temperatures" vs. "contains a mutant gene that codes for (Pol III that does not work at high temperatures)."

Something like this seems clearer: "has a mutation to the gene coding Pol III, so that the mutant Pol III does not work at high temperatures."

[mcat_premed3832]

How did you interpret the meaning of that part of the passage?

[quadalpha]