SA Biology MCAT AAMC #25 Q25

[tshank]

I didn't quite understand this one. I understand that in theory II the rat lives longer, but, it doesn't say that the rat will live longer because of more life cycles, can someone explain? I've got an inkling, but I'd like some walk through.

See attachments for passage, problem, answer, and explanation.

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[techfan]

Well Theory I says that aging is predetermined and can't be changed. Theory II says that free radicals speed up aging and if you lower free radicals you slow down aging.

In Theory I it says that tissue divides 50 times before death, period.

Theory II says that by taking Vitamin E (or C) you can slow down the process so you'd have more than 50 divisions before you die if you take the proper amount.

This question is designed for you to read into the passage, then eliminate answer choices rather than try to reason through the process (hence the hypothesis question type).

As for your question, Theory I is merely saying that tissue divides 50 times before death. Since cells are continuously dividing (that's kind of a necessary for life, with the exception of cells stuck in G_0) there would be more divisions if life is longer, and Theory II says that taking Vitamin E makes life longer (or aging slower).

 

[tshank]

So, two assumptions/deductions you @techfan made from the passage are that:
A) the cells will divide more if they live longer
B) a normal cell divides 50 times before death

I was figuring that since there are two separate theories, they are mutually exclusive, therefore the 50 divisions before death does not apply to Theory II. And it is possible that the speed of the cell divisions, not the number of divisions, would/could determine the longevity of life. So, obviously I was wrong. What clued you in to thinking they way you did and combine the theories and not assume what I did? Any general advice regarding questions like this on the MCAT? Thanks, I really appreciate the response(s)

 

[techfan]

I'm wrong, refer below.

 

[kraskadva]

The two theories are set up as mutually exclusive. That's the point. You get answer D by choosing the one that would support Theory II and detract from Theory I.

The cell has already divided 30 times. So...
If Theory I is true, then the Vit. E won't matter and the cell will divide 20 more times and then die.
If Theory II is true, than reduction of damage will increase the cell's lifespan (i.e. number of divisions) and only D supports this (30+40=70 divisions)
Ans A & B would not prove or disprove either case since both give total divisions <50.

 

[tshank]

@kraskadva I will pose the same question to you that I posed to techfan, with a more concise modification. How/why did you think or go to the assumption that the cell's lifespan is the same as the number of divisions. Sometimes cells have longer G phases than others, and I don't see anything in the passage that says the G phase can't be expanded rather than the number of cell cycles.

 

[kraskadva]

@kraskadva I will pose the same question to you that I posed to techfan, with a more concise modification. How/why did you think or go to the assumption that the cell's lifespan is the same as the number of divisions. Sometimes cells have longer G phases than others, and I don't see anything in the passage that says the G phase can't be expanded rather than the number of cell cycles.

Because that's how a cell's lifespan is defined- by divisions. Doesn't matter how long G0 is.

For example, your brain cells are in senescence (old age) and will never exit G0 again (barring nasty things like cancer which throws all this for a loop).
However, they will still continue to live and maintain as long as you do, but they will never (normally) divide again, so we call them "old" (senescent) cells.
That's what makes stem cells different- they have 'unlimited' divisions, but each daughter cell they spin off has a limited lifespan.

Strictly according to the passage though, there is no definition of lifespan, since that's what the 2 theories are trying to define. Theory I wants to define it in # of divisions; Theory II in terms of calendar time, which can be affected by damage. The point of the experiment is to collect evidence for one over the other. So you can't make overarching generalizations that take from both theories, since they're opposed to each other.


Going back and looking at your op and the passage, I think I see the problem though....
In the passage Evidence for Theory 1 is given in terms of cell division while evidence for Theory II is given in terms of mouse lifespan. These are apples and oranges, which then allows you to bring in the experimental questions that actually is apples to apples.

(In reality, the mouse can live longer because the cells have less damage and can sit in G0 longer, which increases the overall lifespan of the organism. This is not related to the cell's lifespan, which is measured in divisions, not calendar time. )

I hope this makes sense...I just woke up.

 

[tshank]

@kraskadva , your bed-head brain works alright by me. Thanks for the explanation. Your last point in parenthesis seems to be the key point for clarification, that the lifespan of a cell is different than the lifespan of the organism, and they don't necessarily cause one or the other to be short or longer, but they can.

factual takeaways:
-cell lifespan is measured in # of divisions
-lifespan of organism is different than lifespan of cell
-some cells are senescent (old, non-replicating)

passage takeaways:
-recognize the clear differences/distinction between each other, there are two proven theories for the same result. Some facts confirm or deny one or the other.

Thanks a bunch!