Question 38

[anon3543]

38. According to Fig. 1, the first 8 residues of the DNA template were...

I understand the answer, but I think the MCAT usually assumes sequences to be read in the 5' -> 3' direction if it is not stated, and if that were the case, the first 8 (starting at the 5' end of the template) would be read as complementary to the largest fragments (top of electrophoresis) on down. That answer is not available, so I would chose the template being read 3' to 5' as the first 8, but I was just wondering if this kind of thing will get me into trouble ever on the real MCAT... thanks!

[admin]

You are really asking if it is possible to know too much information for the MCAT (which is of course based on first year sciences). The answer is: very very rarely. I remember a question on the membrane potential (Vm) and which ions were responsible. Options included: Na and K or Na, K and Cl. They expected that students would choose the former as the right answer but I had read a 3rd year text describing the impact of Cl- on resting Vm (which was the wrong answer!). How often could that happen: very, very rarely.

This question does not, nor would the AAMC, expect that you would have memorized a standard way to read DNA segments in first year college or that it was such an important issue that it should be specifically tested in such an indirect way. Notice how much MCAT biology relies on interpreting graphs, tables, diagrams, experiments and how little bio is dependent on memorizing details especially something as subtle as the order that DNA should be read. You won't find a real MCAT question testing you on that, unless the information is explicitly presented in the passage or in the question.

[setty.nith3707]

I thought the same thing 5-3, i thought it would be assumed knowledge,,,

[vinny12936]

This is a good question, however the answer is absolutely wrong and the reasoning is simply false and misleading. DNA is always - always - read 5' to 3'. Thus, the first 8 residues of the template are: 5'-G-T-T-G-C-A-A-G-3'. None of the answer choices reflect the correct response.

Even if you don't have experience reading Sanger sequencing gels, you can figure this by either just knowing that DNA (or RNA) is always read 5' to 3' or by deducing this from the information in the passage and by understanding DNA replication, which is, by no means beyond the MCAT; knowing this is hardly "subtle."

This is also important to know because the codon-anticodon base-pairing between the mRNA and tRNA follow this convention as well. For example, the start codon 5'-A-U-G-3' has a cognate methionyl-tRNA with the anticodon of 5'-C-A-U-3'.

For clarity, the template DNA sequence from this passage would be:

5'-GTTGCAAGTCCAGCTTTGCGGTAC-3'

...while the synthesized strand is:

5'-GTACCGCAAAGCTGGACTTGCAAC-3'