IMAT 2016 Q31 [DNA Plasmid]

The diagram represents a whole DNA plasmid that has been cut open using a single restriction enzyme:


What are the bases in positions 1, 2, 3, 4 and 5 respectively?

A. T T G C T
B. U C G U U
C. U G C U U
D. U U G C T
E. T C G T T

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The correct answer is A.

Plasmids are small circular DNA molecules. They are found mainly inside bacteria but can occur in some eukaryotes.

Their purpose is to provide the bacteria with genetic advantages, and they can be transferred to other prokaryotes.

Because it is a DNA molecule, it is made of nucleotides. 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 all of this, we can determine that the positions 1 to 5 are complementary to the first 5 nucleotides on the left.

Therefore, the bases are :



hello! i have one question
if this is an opened version of a circular DNA, doesn’t it mean that the parts we have opened should complement each other when we close the circle
meaning, 12345 should go in the opposite direction as when we close the circle back, 1 touches to A 2 touches to G 3 touches to C and on
this has confused me a lot


I thought the same thing, that’s why I chose E). I’m just as confused.


I hope someone clears it soon, I am pretty in confusion here :frowning:

I think it is becase restriction enzyme cut in at very specific sequence. i.e. it made a cut on AACGA. Therefore, it has to be exactly the same base positions on the other end, AACGA. It doesnt work the other way around because of its specificity i.e. AACGA is not the same as AGCAA.

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I hope this can help you guys


you are a saver! thank you