IMAT 2017 Q34 [DNA Strand]

A short section of the DNA strand that codes for a protein has the sequence:
CAT TGG GCA TCG

Which of the following statements about this section of the sequence is/are correct?

  1. There are a total of 29 hydrogen bonds between this section of DNA and its
    complementary strand.

  2. There are 12 phosphodiester bonds present within this section of DNA.

  3. The unmutated tRNA molecules used to translate this sequence contain a total of 3 uracil bases.

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

1 Like

The correct answer is E.

Let’s review what nucleotides and bases are.

First, there are 4 types of nucleotides (or bases) : adenine, cytosine, thymine and guanine. Each of them are represented by a letter, respectively A, C, T, G.

These bases form pairs and are the building blocks of nucleic acids, such as DNA or RNA. In the case of RNA, thymine is replaced by uracil (U), through a process of demethylation.

Which bases can form pairs ?

  1. Adenine and thymine (or uracil) can form pairs and are bonded by 2 hydrogen bonds

  2. Guanine and cytosine can form pairs and are bonded by 3 hydrogen bonds

Knowing this, we can determine wether the first item is correct or not :

  • There are 5 A and T bases which will form 5*2 = 10 hydrogen bonds with the complementary strand : CAT TGG GCA TCG

  • There are 7 C and G bases, which will form 7*3 = 21 hydrogen bonds with the complementary strand : CAT TGG GCA TCG

  • Therefore, there are 10 + 21 = 31 hydrogen bonds between this section of DNA and its complementary strand, making item 1 incorrect.

Now, onto item 2 : what is a phosphodiester bond ?

  • It is the element that ensures the integrity of nucleic acid strands. It links the 3’ carbon atom of a molecule A and the 5’ carbon atom of a molecule B.

  • How to calculate the number of phosphodiester bonds in a DNA molecule ? You can use the formula 2n-2, where n s the number of base pairs.

  • There are 12 base pairs, so n = 12 and therefore we can establish that there are (2*12)-2 = 22 phosphodiester bonds. Thus, item 2 is incorrect.

We can now study item 3 :

  • Like we’ve said previously, in the case of RNA, thymine is replaced by uracil (U), through a process of demethylation.

  • There are 3 thymine bases in the DNA strand : CAT TGG GCA TCG

  • Each of these bases will then be demethylated into 3 uracil bases in the tRNA strand, thus making item 3 correct.