Question Video: Using a Codon Wheel to Determine a Sequence of Amino Acids Biology

A sequence of DNA is transcribed into an RNA sequence. This RNA sequence reads 5′-GCUUUCACGCAC-3′. Use the codon wheel provided to determine the sequence of amino acids. [A] Arg, Ser, Thr, Pro [B] Ser, Leu, Ala, His [C] Ala, Phe, Thr, His [D] Ser, Leu, Ala, Gln [E] Ala, Leu, Thr, Gln


Video Transcript

A sequence of DNA is transcribed into an RNA sequence. This RNA sequence reads from five prime to three prime GCUUUCACGCAC. Use the codon wheel provided to determine the sequence of amino acids. Arginine, serine, threonine, proline. Serine, leucine, alanine, histidine. Alanine, phenylalanine, threonine, histidine. Serine, leucine, alanine, glutamine. Or alanine, leucine, threonine, glutamine.

This question is asking us about how to translate a sequence of mRNA into the corresponding amino acids. Before we can answer this, let’s clear the answer choices and review some key points.

Let’s say our cell here needs to produce insulin. The gene for insulin is located here in pink in the cell’s DNA. In order for this cell to produce insulin or any other protein, it must go through two processes called transcription and translation. During transcription, the gene for the protein is transcribed or copied to produce what’s called messenger RNA or mRNA. This messenger RNA is a message for the cell that tells it that it needs to make the protein, or insulin in this case.

Transcription is the process of converting a section of double-stranded DNA or our insulin gene shown here in pink into a single-stranded mRNA molecule. Like DNA, the sequence for mRNA is written in the five-prime to three-prime direction and contains four different nucleotides or bases: adenine or A for short, guanine, cytosine, but instead of thymine, which is in DNA, RNA uses uracil or U for short.

After transcription, the sequence in the mRNA molecule can be translated into its corresponding amino acids. This step is called translation, and it forms a polypeptide with each of these colored circles representing a different amino acid. This polypeptide can then fold to form its corresponding protein, or insulin in this example.

Now that we’ve covered transcription and translation, let’s turn our attention to this mRNA sequence in the question and describe how this specific sequence can be translated into amino acids.

An mRNA sequence is translated in groups of three nucleotides called a codon. A codon is a sequence of three nucleotides that code for an amino acid. Codons are always read in a way that they’re not overlapping. So, in this sequence, this is the first codon, this is the second codon, this is the third codon, and here’s the last codon. So, this mRNA sequence has four codons.

Now, in order to translate the sequence of nucleotides into its corresponding amino acid, we need to use a codon wheel, like the one shown on the left. To use the codon wheel, we start from the inside. This corresponds to the five-prime end of the codon. And we work our way outwards to the three-prime end of the codon. So for this codon, we’ll be working from the five-prime to the three-prime end.

So, with the codon GCU, we start with G, then we move over to C, and finally we land on U. So, the codon GCU corresponds to the amino acid alanine. Then, for the next codon UUC, we do the same thing. So, the first nucleotide is U, then U again, and then C. This corresponds to the amino acid phenylalanine. Then, for the codon ACG, this corresponds to threonine. And finally, the codon CAC corresponds to the amino acid histidine. Therefore, the corresponding amino acid sequence for the given mRNA sequence is alanine, phenylalanine, threonine, and histidine.

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