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

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 :