TBR Biology #98, §10: Cancer/Mutated Genes

[Gauss44]

98. Mutated gene A has been shown to induce uncontrolled growth. In vivo studies of the gene have demonstrated normal levels of mRNA and protein product from gene A. From this information, it can be concluded that:

A. the mutation in gene A has resulted in a depressed RNA polymerase binding frequency.
B. protein product A is necessary for entering mitosis.
C. the mutation in gene A has resulted in a significant structural change in protein A.
D. the cell line used possesses a ******ed cellular ribosome level.

Correct Answer is C. Can anyone explain?

(Answer key doesn't help. All the answer key does is repeat the question stem, labels the question stem as "straight forward," mentions that non-functioning proteins can result from mutations (true but irrelevant to question as far as I can tell), and gives reasons why other answer choices are wrong.)

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[sillyjoe]

98. Mutated gene A has been shown to induce uncontrolled growth. In vivo studies of the gene have demonstrated normal levels of mRNA and protein product from gene A. From this information, it can be concluded that:

A. the mutation in gene A has resulted in a depressed RNA polymerase binding frequency.
B. protein product A is necessary for entering mitosis.
C. the mutation in gene A has resulted in a significant structural change in protein A.
D. the cell line used possesses a ******ed cellular ribosome level.

Correct Answer is C. Can anyone explain?

(Answer key doesn't help. All the answer key does is repeat the question stem, labels the question stem as "straight forward," mentions that non-functioning proteins can result from mutations (true but irrelevant to question as far as I can tell), and gives reasons why other answer choices are wrong.)

Think of it this way: If the concentration of the protein and mRNA for gene A are "normal" then out of the given answer choices what would be the most likely answer? Let's look at the answers in plain english:

A. The mutation effects mRNA levels specifically by decreasing it's levels This is in direct conflict with the question stem.
B. Protein A is a cell-cycle protein This is irrelevant information and does not explain why a protein with normal levels in the cell would cause uncontrolled cell growth. Who cares if it necessary for the cell-cycle in this context? It might be in an inactive state until the cell is ready to go into the cell cycle so this definitely does not answer the question.
C. The mutation in protein A caused the protein to change structure This is a plausible explanation for why the cell now has entered uncontrolled cell growth if the protein and mRNA level are still normal. Let's just say you had a protein that is normally inactive that at certain points in a cell's life it can get activated and cause the cell to grow and replicate. Now if you were to mutate that protein so it is always in the active state you will get uncontrolled cell growth. Remember, structure dictates function. This answer certainly seems like the best choice for the scenario given.
D. Protein synthesis in the cell is lower than normal This answer contradicts the question stem because it specifically says that the protein concentration is normal.

While this may not be the best question, it is certainly obvious that only one answer choice can be correct. Unfortunately, a lot of time you need to give the "best" answer to a crappy question. This is when you must think the most critically.

 

[Gauss44]

Think of it this way: If the concentration of the protein and mRNA for gene A are "normal" then out of the given answer choices what would be the most likely answer? Let's look at the answers in plain english:

A. The mutation effects mRNA levels specifically by decreasing it's levels This is in direct conflict with the question stem.
B. Protein A is a cell-cycle protein This is irrelevant information and does not explain why a protein with normal levels in the cell would cause uncontrolled cell growth. Who cares if it necessary for the cell-cycle in this context? It might be in an inactive state until the cell is ready to go into the cell cycle so this definitely does not answer the question.
C. The mutation in protein A caused the protein to change structure This is a plausible explanation for why the cell now has entered uncontrolled cell growth if the protein and mRNA level are still normal. Let's just say you had a protein that is normally inactive that at certain points in a cell's life it can get activated and cause the cell to grow and replicate. Now if you were to mutate that protein so it is always in the active state you will get uncontrolled cell growth. Remember, structure dictates function. This answer certainly seems like the best choice for the scenario given.
D. Protein synthesis in the cell is lower than normal This answer contradicts the question stem because it specifically says that the protein concentration is normal.

While this may not be the best question, it is certainly obvious that only one answer choice can be correct. Unfortunately, a lot of time you need to give the "best" answer to a crappy question. This is when you must think the most critically.

Thanks for your reply, but I'm still confused about C: How can mutating a protein lead to uncontrolled cell growth without changing the levels of mRNA or protein product from gene A?

Was it just POE/process of elimination? I agree about A and D. B and C were more confusing for me.

 

[sillyjoe]

Thanks for your reply, but I'm still confused about C: How can mutating a protein lead to uncontrolled cell growth without changing the levels of mRNA or protein product from gene A?

Was it just POE/process of elimination? I agree about A and D. B and C were more confusing for me.

Let me give you an example to better elucidate this concept: Let's say there is a specific protein that is always expressed in the cell at a constant level. Let's call this protein gauss44. Now lets say gauss44 is normally responsible for phosphorylating the nuclear lamins (intermediate filaments that are structural proteins in the nucleus) of the nucleus that will then cause the dissolution of the nuclear membrane that occurs during prophase of mitosis. Therefore, we can say that gauss44 is an essential protein that regulates cell division. Now gauss44 is normally at a constant level in the cell at 5nM, but INACTIVE because a certain regulatory portion of gauss44 covers the active portion of the enzyme. When the cell is ready to go into the cell cycle a specific enzyme/protease will come and cleave the regulatory portion of gauss44 and allow for it to become active and continue in prophase.

Now let's say there was a genetic mutation of gauss44 that caused the regulatory subunit on gauss44 to not get translated or is translated in a truncated form. You now have protein that is ALWAYS active and causing the cell to enter the cell cycle and have uncontrolled growth. In this scenario gauss44 mRNA levels and protein levels are NORMAL, but the actual protein gauss44 is mutated and always in it's active form causing the cell to have unregulated cell growth.

Again, this is just a made up example I came up with based off similar mechanisms that do occur in the cell. Hopefully this gives you a taste of how choice C could come about.

 

[Evisju7]

genes eventually make proteins: gene codes for mRNA which may translate into a a protein. IF that gene is mutated, then the resulting protein will be messed-up too.