Video Transcript
Let us consider this sequence of
three amino acids: methionine, tryptophan, histidine. Use the provided codon wheel to
determine which of these RNA sequences corresponds to this polypeptide. (A) 5 prime-AUGUGGCAC-3 prime, (B)
3 prime-AUGUGGCAU-5 prime, (C) 5 prime-GUAGGUCAC-3 prime, (D) 3 prime-GUAGGUCAC-5 prime.
In order for a gene in DNA to be
converted into a protein, it needs to go through two steps. First, the DNA sequence is
converted into mRNA during a process called transcription. Then, the sequence of nucleotides
in mRNA is converted into a sequence of amino acids, or a polypeptide, during
translation. This can then fold into a
protein. Let’s look at how a sequence of
mRNA can be translated into amino acids.
A group of three nucleotides in
mRNA is called a codon. In this six-nucleotide sequence of
mRNA, there’s two codons. The specific sequence in a codon
corresponds to a specific amino acid. We can use the provided codon wheel
to figure this out. When reading a sequence of mRNA, we
start from the 5 prime end and go towards the 3 prime end. In the codon wheel, we start from
the center and work our way out. The codon CCA gives us the amino
acid proline, and the codon GUA gives us the amino acid valine.
Now that we know how to use a codon
wheel, let’s take another look at the question. Here we are given a sequence of
three amino acids: methionine, tryptophan, and histidine. We need to work backwards to
determine the mRNA sequence. Let’s start with methionine. Methionine is given by the codon
AUG. Tryptophan is given by the codon
UGG. And histidine is given by two
possible codons: CAC or CAU.
If we look at our answers, options
(C) and (D) are incorrect because they don’t have the right sequence. Answer (B) is written in the 3
prime to 5 prime direction so this is incorrect. Therefore, the correct answer is
(A) because it has the correct sequence and the correct direction, 5
prime-AUGUGGCAC-3 prime.
