Video Transcript
Which of the following describes how a polypeptide chain is formed? (A) mRNA catalyzes the formation of polypeptide bonds. (B) The amino acids are joined with hydrogen bonds. (C) The ribosome moves along the DNA and joins the mRNA together. (D) The ribosome moves along the mRNA strand and joins the amino acids together. (E) tRNA catalyzes the reaction between amino acids to form bonds.
Polypeptide chains are a linear sequence of amino acids bonded together by peptide bonds, another primary structure of proteins. So understanding the process of protein synthesis is key knowledge necessary for selecting the correct solution option here, especially the second step of protein synthesis called translation. But let’s take a quick look at an overview of the entire process before focusing on a more detailed view of translation.
Protein synthesis starts in the nucleus of the cell because the information needed to produce a specific polypeptide chain is coded by a sequence of nucleotides called a gene in DNA. DNA has four types of nucleotides depending on the type of base which can be T for thymine, A for adenine, C for cytosine, or G for guanine. The gene is used to produce a messenger molecule called mRNA that carries a copy of the information in the gene to where the protein is assembled. This process is called transcription because the information coded by the gene is rewritten into another sequence of nucleotides.
Keep in mind that the gene in the diagram here is much shorter than an actual gene which, in humans, are hundreds to tens of thousands of nucleotides long. So, transcription is the production of an mRNA molecule composed of a complementary nucleotide sequence to the DNA gene with the base uracil replacing thymine. And the mRNA is then used as a genetic code-bearing messenger that travels out of the nucleus to a ribosome which first associates with the nucleotide sequence AUG which is called the start codon. Codons are sequences of three nucleotides that code for an amino acid.
And the start codon, AUG, codes for the amino acid called methionine represented here with the abbreviation M. Amino acids are transported to the ribosome by a transfer or tRNA molecule that recognizes the AUG codon on mRNA with its complementary base sequence UAC. You can look up an mRNA codon chart to translate the genetic code in mRNA codons to the amino acid each represents. And since we can think of nucleotide sequences as a kind of a different language than that of amino acids, we can call the step of protein or polypeptide synthesis translation.
We’re almost at a point where we can solve our problem, except we still need to see how additional amino acids are added to our methionine here. As mentioned earlier, a polypeptide chain is a sequence of many amino acids held together by peptide bonds, so let’s take a little look at the next steps of translation to help us choose the correct option. Next, another tRNA complementary to the second codon GUC and loaded with a specific amino acid, valine or V, enters the active site of the ribosome. Now, the active site of the ribosome catalyzes the formation of peptide bond between the two amino acids. As the peptide bond forms, the ribosome moves along the mRNA strand to read the next codon, while the first tRNA releases the methionine amino acid.
Another tRNA with a complementary sequence to the mRNA codon GAU now enters the active site of the ribosome, bringing the amino acid aspartic acid, which has the abbreviation D. And the process of translation continues as the ribosome catalyzes the formation of another peptide bond, this time, joining aspartic acid to valine, while continuing to move along the mRNA molecule. Additionally, the ribosome releases the first tRNA that had carried methionine, and the amino acid valine is released by its tRNA. The ribosome will continue by accepting the next complementary tRNA, but we have enough information to correctly select our answer, so let’s take a second look at the options.
Option (A) states that mRNA catalyzes the formation of polypeptide bonds, but we know that’s not true. It’s the active site of the ribosome that catalyzes the formation of peptide bonds, so we can eliminate option (A). Option (B) states that the amino acids are joined by hydrogen bonds. But, again, we know that’s not true; amino acids must be joined by peptide bonds, so we can eliminate option (B). Option (C) says the ribosome moves along the DNA and joins the mRNA together, which clearly isn’t true, so we can eliminate option (C). Option (D) states that the ribosome moves along the mRNA strand and joins the amino acids together, and that’s just what we saw. The ribosome moves along the mRNA strand and joins the amino acids together.
Finally, option (E) states that tRNA catalyzes the reaction between amino acids to form bonds. But as we’ve seen, it’s the active site of the ribosome that catalyzes the reaction between the amino acids to form bonds, so we can also eliminate option (E).
Therefore, the correct option is option (D): the ribosome moves along the mRNA strand and joins the amino acids together.
