Translation of mRNA to Protein: Initiation, Elongation & Termination Steps

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  • 0:05 The Three Steps of Translation
  • 1:24 Initiation
  • 2:29 Elongation
  • 5:15 Termination
  • 6:18 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

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.

Initiation

In initiation, mRNA is attached to tRNA, which is attached to the specified amino acid.
mRNA Translation Initiation Step

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.

Elongation

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.

Polypeptides form as amino acids are added during the elongation step.
mRNA Translation Elongation Step

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.

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