IMAT 2013 Q36 [DNA Sequence]

A section of one strand of DNA has a base sequence of:

ACG-­GCT-­GGT-­TCC

Which of the following are correct?

  1. The other DNA strand would include a CGA triplet.
  2. If adenine always binds with 2 H bonds to its complementary base and guanine with 3 H bonds, then each of these triplets will have a total of 8 H bonds.
  3. The mRNA sequence transcribed from this DNA sequence would contain 3 uracil bases.

A. 1 only
B. 2 and 3 only
C. 3 only
D. 2 only
E. 1 and 2 only

The answer to this question is E, only statments #1 and #2 are correct.

Since this question relies heavily on the concept of base pairing, it would be useful to be reminded of a few concepts regarding base pairing:

  • The nitrogenous bases that are always paired together in DNA are [Adenine & Thymine] and [Guanine & Cytosine].
  • There are 2 hydrogen bonds between [Adenine & Thymine], whereas there are 3 hydrogen bonds between [Guanine & Cytosine].
  • When DNA is being transcribed by RNA polymerase, uracil is used instead of thymine.

With that being said, let’s dissect each of the statements and test out if they are correct or not:

  1. The other DNA strand would include a CGA triplet.” : given the strand that was written in this question, we can decipher the other DNA strand. Using base pairing, the other strand would be TGC-CGA-CCA-AGG. The presence of the codon “CGA” makes this statement correct.

  2. If adenine always binds with 2 H bonds to its complementary base and guanine with 3 H bonds, then each of these triplets will have a total of 8 H bonds. A quick way to tackle this question is to look at the letters as numbers, according to the number of hydrogen bounds that they share. Thus, regard A & T as 2 (because they have 2 H bonds), while G & C can be regarded as 3. Therefore, the DNA strand given will look like this: 233-332-332-233. If you add the individual numbers within each triplet, you will consistently get a total of 8 H bonds, which makes this statement true.

  3. The mRNA sequence transcribed from this DNA sequence would contain 3 uracil bases. If you were to transcribe the DNA sequence provided knowing that uracil is used in place of thymine in the process of transcription, the mRNA sequence would look like this: UGC-CGA-CCA-AGG. As you can see, there is only 1 uracil base in this mRNA sequence since there is only 1 adenine in the DNA sequence provided in this question.

It makes sense therefore, for choice E to be the correct answer.