IMAT 2017 Q27 [Unmutated Nucleic Acids]

Which one of the following statements about unmutated nucleic acids is correct?

A. rRNA has anticodons which bind to tRNA.

B. Prokaryotic DNA is a single strand which forms a loop.

C. tRNA is made up of one phosphate-sugar backbone and may have adenosine and thymine bases.

D. mRNA is made up of a single nucleotide with a codon of uracil, cytosine and guanine bases.

E. Bases in DNA may form hydrogen bonds with uracil bases.

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The correct answer is E.

First, what is a nucleic acid ?

The 2 main types of nucleic acids are the DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Their purpose is to :

  1. Carry the genetic information
  2. Express genes and regulate cellular activities

Now, let’s review each item individually :

  • Transfer RNA or tRNA is important during protein synthesis : it serves as the adaptor between the messenger RNA and the chain of amino-acids that is being synthesized. In other words, it carries the amino-acids into the ribosome to enable protein synthesis. It contains an anticodon, which is made of 3 nucleotides, and serves to ensure proper protein synthesis. The anticodon is complementary to the messenger RNA or mRNA, thus making item A incorrect.

  • The difference between prokaryotes and eukaryotes is the fact that their DNA is not surrounded by a nuclear membrane. In other words, the have no nuclei : instead, they have a structure called the nucleoid. Single strand DNA is found in DNA viruses but NOT prokaryotes (like bacterias and archeas). While they do contain circular DNA, it is a double-stranded.

  • tRNA is made up of a single strand of RNA. Because it is made of RNA, it does NOT contain thymine but uracil instead : thymine does NOT occur in RNA.

  • mRNA is made up of a single-stranded molecule of RNA. It is relatively short (~2kb) and made up of four nucleotides : adenine, uracil, guanine and cytosine, thus making item D false.

  • Uracil is the demethylated form of thymine and bonds to adenine. Adenine being common to both RNA and DNA structures, it is possible for a DNA strand to form hydrogen bonds with uracil bases.