Translation, the second part of the central dogma of molecular biology, describes how the genetic code is used to make amino acid chains.
In this lesson, explore the mechanics involved in polypeptide synthesis. Learn the three major steps of translation as you watch tRNA, mRNA, and ribosomes go to work.
The Three Steps of Translation
Translation is the second step in the central dogma that describes how the genetic code is converted into amino acids. We’ve talked about how the mRNA codes are recognized by tRNA and how the amino acids are linked together by peptide bonds.
A chain of amino acids is also called a polypeptide. Polypeptides are assembled inside the ribosomes, which are tiny organelles on the rough ER of a cell.Now that we’re learning more about the mechanics of translation, we’re going to have to start putting the pieces together.
We already understand the role of the ribosome and the amino acids in the process of translation, but how does polypeptide assembly actually occur? There are three important steps to the process of translation.There’s a beginning step, called initiation, a middle step, called elongation, and a final step, called termination. These three words may sound familiar to you. The same terms are used in transcription to describe the steps involved in making the mRNA strand. But, here in translation, we’re making a polypeptide strand. In either case, we’re making a long molecule out of a chain of smaller subunits.
So, whether we’re referring to transcription or translation, the three terms accurately describe the mechanics of the process. Let’s walk through each step, one at a time.
We’ll start with initiation. During initiation, the mRNA, the tRNA, and the first amino acid all come together within the ribosome. The mRNA strand remains continuous, but the true initiation point is the start codon, AUG.
Remember that the start codon is the set of three nucleotides that begins the coded sequence of a gene. Remember also that the start codon specifies the amino acid methionine. So, methionine is the name of the amino acid that is brought into the ribosome first.And, how did methionine get itself to the ribosome? By attaching to the tRNA that contains the right anticodon.
The anticodon for AUG is UAC. We know that because of the rules of complementary base pairing. The tRNA with the anticodon UAC will automatically match to the codon AUG, bringing the methionine along for the ride.
So, there you have it – mRNA is attached to tRNA, and tRNA is attached to methionine. That’s initiation.
The next step makes up the bulk of translation. It’s called elongation, and it’s the addition of amino acids by the formation of peptide bonds. Elongation is just what it sounds like: a chain of amino acids grows longer and longer as more amino acids are added on. This will eventually create the polypeptide.
Now that we’ve begun with the start codon, the mRNA shifts a little through the ribosome so that the next codon is up for grabs. Let’s say the next codon is UAU. So, now we need a tRNA that has the matching anticodon, AUA. Oh, look! Here’s a tRNA with the right anticodon, and it’s brought along a tyrosine.
Tyrosine is the amino acid that is specified by the codon UAU. The tRNA attaches to the mRNA in the ribosome and lines up tyrosine right next to the waiting methionine. A peptide bond forms between the two amino acids.Then, the first tRNA leaves everyone else behind and floats off to find more work to do. Poor methionine! Now it’s just drifting around like a lonely kite in the wind! That tRNA left methionine hanging by only one anchor: its peptide bond with tyrosine. The tyrosine is still attached to its own tRNA, which, in turn, is clinging to the mRNA inside the ribosome. Already we can see the beginnings of a polypeptide elongating outward.
Should we walk through that process one more time? Let’s keep everything just as we have it here and move on to add our third amino acid. mRNA shifts over again, and now the third codon is ready for a match. What’s that codon? CAC. Here comes a tRNA with the matching anticodon, GUG.
It’s also brought us a histidine, since CAC codes for histidine. The tRNA’s anticodon matches up with the mRNA’s codon, putting the histidine in perfect position for making a peptide bond with tyrosine.So, now we have methionine, tyrosine, and histidine all connected. We won’t be needing tyrosine’s tRNA anymore, so that tRNA detaches and floats away, just like the first one did in the beginning. Now we have an even longer kite; methionine and tyrosine are drifting around with only their peptide bonds to hold them down to the ribosome.But, the histidine is still connected to its own tRNA, and it’ll stay that way until it has the next amino acid to latch onto. You can see how this chain of amino acids would grow longer as each new codon is translated.
The addition process and peptide bond formation continues over and over again until the chain is about one hundred amino acids long.
The chain finally ends when a stop codon moves into the ribosome. This is the final step of translation, called termination. Termination begins with the arrival of one of the three stop codons: UAA, UAG, or UGA. When any of these enters the ribosome, the last amino acid cuts off its anchor to the last tRNA. The tRNA and ribosome are no longer needed. The gene has been successfully translated, and now we have a completed polypeptide.
The polypeptide is finally free – free to drift happily all over the cytoplasm, untethered by covalent bonds! This final step, termination, is the end of polypeptide synthesis, signaled by a stop codon entering the ribosome.
So, does that mean we’ve finally built a protein? Well, no, but we have built a polypeptide. It often takes more than one polypeptide to build a protein. Remember that protein synthesis is a separate process from translation. So, just because you’ve made a polypeptide doesn’t mean you’ve made a protein.
Now that we’ve worked through all the details of translation, we can give it a more specific definition.
We can say that: translation is the synthesis of a polypeptide using the genetic code found in mRNA. It still fits into our earliest outline of the central dogma. Translation is the mechanism by which the information in RNA is transformed into a protein.
In this lesson, we learned the three steps of translation. Let’s go back and make sure we’ve got all of them nice and clear.Initiation of translation occurs when mRNA, tRNA, and an amino acid meet up inside the ribosome. Once translation has begun, it continues down the line as mRNA shifts along through the ribosome. Each new codon matches with a new tRNA anticodon, bringing in a new amino acid to lengthen the chain.
During elongation, amino acids are continually added to the line, forming a long chain bound together by peptide bonds.Once a stop codon reaches the ribosome, translation stops, or terminates. At termination, the polypeptide is freed from the ribosome, and tRNAs stop bringing the amino acids in. All of the components come apart from one another, and translation is done.
The result is a brand new free-floating polypeptide.
Following this video, you’ll be able to describe each of the three steps of translation: initiation, elongation and termination.