Video Transcript
A tRNA contains specific regions. Which of the following does not belong to a tRNA? (A) Several loops held in shape by the complementarity of some nucleotides. (B) A ribosomal binding site in its five prime end. (C) A constant CCA sequence on its three prime end. (D) An anticodon site.
You may recall that a gene in DNA can be converted into a protein in a couple of
steps. The first step is transcription, where the gene is copied to make an mRNA
transcript. Then the sequence of nucleotides in mRNA is translated into a sequence of amino acids
to form a polypeptide which can then fold to form the protein. During translation, the mRNA binds to the ribosome. The ribosome matches three nucleotide segments of the mRNA, called codons, with
specific amino acids. These amino acids are transferred to the ribosome by a special type of RNA called
transfer RNA, or tRNA.
Let’s look at the structure of a typical tRNA molecule in more detail so we can
answer our question. tRNA is a single strand of RNA that folds upon itself to form
the characteristic cloverleaf shape that we see here. Also notice the five prime and three prime ends of the molecule indicated. Complementary nucleotides base-pair with each other by hydrogen bonding, shown here
as these black lines to create this cloverleaf shape. There is no base pairing within the loop regions, and this is why circular shapes are
formed.
There are two important regions of tRNA that we’ll discuss next. On the three prime end is the amino acid binding site. This is where the amino acid attaches and is removed during translation to join a
growing polypeptide chain. This site is characterized by a conserved CCA sequence. And on the other end of the molecule is an anticodon site. This anticodon contains three nucleotides that are complementary to specific codons
in mRNA. Here you can see an example of this, where a codon in mRNA can pair with its
complementary bases in the anticodon of tRNA. For every unique codon in mRNA, there is a unique tRNA with a matching anticodon that
carries the corresponding amino acid.
Now that we’ve learned more about the structure and function of tRNA, let’s try to
answer our question. We can see that answer choices (A), (C), and (D) are all features of tRNA molecules
that we’ve discussed. So we know that these are incorrect. Therefore, the option that isn’t part of a tRNA molecule is given by answer choice
(B): a ribosomal binding site in its five prime end.
