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?
There are a total of 29 hydrogen bonds between this section of DNA and its
There are 12 phosphodiester bonds present within this section of DNA.
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
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 ?
Adenine and thymine (or uracil) can form pairs and are bonded by 2 hydrogen bonds
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.
I don’t understand why item 2 has 22 phosphodiester bonds. Are we taking the complementary strand into consideration? If so, why?
It says this section of DNA, Im assuming that both strands of DNA are taken into consideration
In my opinion this question is controversial:
It starts to mention that we have “A short section of a DNA strand that codes for …”.
It means we are talking about the coding strand.
Knowing this, we’ll get:
which will not result in a correct answer for option #3.
Regarding the phosphodiester bonds, yes. The question says in #2 option “… 12 phosphodiester bonds present within this section of DNA.”, and not this section of the DNA strand like the first sentence in the question itself. So, it’s fine and correct in formulating this issue.
Alarm in my brain woke me up in the middle of the night, reminding me of my bad tRNA synthesis, because it is not made from a pre-mRNA as I have noted above. Tired head three days before IMAT.
Thank you for pointing this out, I was also confused with “DNA that codes for” and the fact that it’s not a coding strand.