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.

[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.

[mcat_premed3832]

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

[meweber6502]

The passage says that a mutant gene is being produced at high temperatures. This does not mean that the protein is NOT PRODUCED by the cell. What it means is that at the temperature the protein has been genetically altered to be mutant at that temperature. Remember basic genetic engineering which is required for the MCAT. This has nothing to do with memorizing temperature which enzymes denature at. This is about reading the information contained in the passage and connecting it to your basic biology knowledge.