Translation begins when a small ribosomal subunit binds to mRNA and a large ribosomal subunitThe larger part of a ribosome that helps form peptide bonds during translation.. The large ribosomal subunit has a three sites in it. tRNAs arrive at the conveniently named A site. They will then move to the middle site called the P site (for peptide, I think, but I couldn’t find confirmation of that). tRNAs will then move to the E site before they exit the ribosome. Be sure that you realize that the movementA fundamental property of life involving motion of the body or its parts. through this picture will be right to left.
Let’s start. We have the first codon on our mRNA as AUG. The ribosome reads this codon and kinda calls out: I got AUG, anyone match with AUG? Somewhere out in the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol., there is a tRNA that has the anticodon UAC and it says: oh! Me! I can match that! The tRNA comes over to the ribosome, and we get started with the first amino acidThe building blocks of proteins, consisting of an amino group, carboxyl group, and side chain. in the protein chain. Just as a hint…all amino acidA substance that releases hydrogen ions (H⁺) in solution. sequences start with MET and all mRNA sequences start with AUG. The first tRNA will always have the UAC anticodon. Now, upon folding of the protein, that MET amino acid might get clipped off, but every single mRNA transcript starts with AUG.
A second tRNA will come over and enter at the A site of the ribosome. The anticodon on the picture below is GUC and so a tRNA with CAG comes over to the P site. The amino acid from the second tRNA gets bonded to the amino acid from the first tRNA via a peptide covalent bondStrong chemical bonds formed by the sharing of electrons between atoms.. Once all three of these bays in the ribosome are full, now we start moving tRNAs through like a car wash. The very first tRNA, sans amino acid, moves into the E site, the second tRNA, with both amino acids attached moves into the P site and a third tRNA binds to the A site. This static picture represents this really fluid process of the movement of the tRNA and the growing length of the amino acid chain as more and more tRNAs cycle through.
Once we have this in motion, surely it will end. Yes. There are three codons on mRNA that will stop this process: UAA UAG UGA. When the ribosome calls out these codons on the mRNA, no tRNAs respond. They are all like, “We don’t know who you mean. No one in our group goes by those names.” So, the ribosome processes the last tRNAs out and stops or terminates translationThe process of converting mRNA into a protein.. It’s important to note that no proteinsLarge molecules made of amino acids with various functions in the body. are added to the growing amino acid chain after a stop codonA codon that signals the end of translation (UAA, UAG, UGA). is read. At that point, the amino acid chain is complete.
In looking at this process of translation, it occurs to students that there seems to be a very specific match from codon on DNA to anticodon on mRNA and then to amino acid in the protein. Yes. In factA statement based on direct observation that is repeatedly confirmed., this relationship is set in stone and reflected in this strange grid here. This is the genetic codeThe set of rules by which DNA sequences are translated into proteins.. It is a list of mRNA codons and the amino acid to which they match.
You may notice that thymine is not present. This picture is restricted to representing the nucleotidesThe building blocks of nucleic acids. that are present in RNAs. Actually, these are going to represent nucleotides on our mRNA that we made in transcription. What this picture does is connect each codon, or three-nucleotide sequence of mRNA with a specific amino acid.
Let’s do an example. Let’s say that on the mRNA, there is a sequence of three nucleotides, a codon, of A U G. Let’s start at the middle ring of this diagram and choose A in the red, on the bottom left of the inner circle. Now let’s move outward one circle and choose U, in green , almost at the bottom of the circle. Let’s move outward one more ring and choose G in blue. The amino acid that corresponds to AUG is Met or methionine. In our class, it is only necessary that you use the 3-letter abbreviation.
What you might notice is the fidelity and infidelity of codons and amino acids. It’s not an equal relationship. Every single codon matches to only one amino acid. This means that every anticodon matches only one amino acid and therefore every tRNA can attach to only one amino acid. AUU will always attach to isoleucine. Nothing else. The opposite is not true for amino acids. They don’t commit to one codon. A few match with two, a few more match with three and there are even some amino acids that will match with 4 different codons. These qualities of the genetic code are called ambiguity and redundancy.
List of terms
- large ribosomal subunit
- movement
- cytoplasm
- amino acid
- acid
- covalent bond
- translation
- proteins
- stop codon
- fact
- genetic code
- nucleotides