A section of one strand of DNA has a base sequence of:
ACG-GCT-GGT-TCC
Which of the following are correct?
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:
With that being said, let’s dissect each of the statements and test out if they are correct or not:
“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.
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.
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.
But let me raise an hypothetical situation according to the option 2. if there are possibilities of ad triplet to occur in the form AAG, so this might be -223- which adds up to 7 . Thus it might not be 8H pairs always . So how can this option 2 be true
Not all triplets have a 8H-configuration, thus not all triplets in a poly-/oligonucleotide sequence follow 8H-configuration.
This is not a rule, and the sequence given here is just an arbitrary example.
So if it is an arbitrary example this shouldn’t bring it in a conclusive answer for a option , I hope if the option had an mentioned 7/8 Hydrogen bonds this answer would have been more appropriate and comprehensible. And thankyou for responding for my query.
Thank you for the comment.
The sequence is arbitrary. The answer option is not arbitrary. It is connected to this exact example. There could be many other examples which could follow the same pattern.
You make it complicated to understand for yourself. Please review the wording of the exercise again, especially option 2 answer.
Option 2 is given as:
Please notice the word: THESE
which means: not any other triplets.
If we take your hypothetical example (or some other possible examples), then we will have a mixed 7H-8H configuration. We can also have many other only-8H-configuration examples.
This exercise asks only about these triplets and only this exact sequence and not in general because there is no general rule for H-bonds of triplets.
I do not believe that two master of sciences make an error on the same issue.
Thank you