Biology on the MCAT: How to Simplify Complex Problems

MCAT MD/PhD admissions
By Henry


Many problems on the MCAT seem quite complex upon first inspection but can actually be reframed to be more simple. This allows a test-taker to serve time as well as avoid the errors that come with repeated detailed analysis. To demonstrate this tactic, see the example below.

Example problem

Restriction enzymes recognize and cut sequences in DNA called Recognition Sequences. A table of restriction enzymes is provided below. The * between the bases signifies the location of the cut in the recognition sequence.

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When Bgl II cuts bacterial DNA containing its Recognition Sequence, two sticky ends are formed. These sticky ends are able to hybridize to the sticky ends created by the action of which of the following restriction enzymes on its own Recognition Sequence?

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While you might be tempted to start writing all the cut recognition sequences of the answer choices and find a match, this is an awful lot of work that’s unnecessary. What we need to do is solve the problem just enough to have a way to test each answer choice. Here’s how I do it. 

First, lets remember what the sticky ends referred to in the question are using the enzyme in the question. The double stranded section of DNA cut by the restriction enzyme Bgl II is
Screen Shot 2016-11-21 at 5.47.24 PM.png

Now if you look at the sticky ends above on the right side, I have bolded the sequence on the sticky end that must be hybridized to in order to answer this question. This sequence will bind its complement, and as such we are looking for the complement of the complement, which is in fact the bolded sequence. As such, we look for GATC after the cut symbol in the table above (this is the test I referred to above). Perusing the table, we find a match in Bam HI. The other answer choices can be discounted for not having that sequence, and as such, are unable to form hydrogen bonds with the sticky end resulting from a Bgl II cut.

Take Away

From exploring one example, we find that the real question being asked is to find that 4-base sequence, which is a relatively simple task. Therefore, when faced with seemingly complex problems like the one above, try doing one example , and using that example to come up with a simple test to delineate between the answer choices. If you do this when possible, you will realize that the seemingly complex problems are often in fact quiet manageable.

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