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Hello! Last lesson followed RNA polymerase as it made an RNA transcript from a DNA template. Once a mature messenger RNA (mRNA) is available to a ribosome, the question changes: how can a sequence of RNA bases specify a chain of amino acids?
This lesson follows that change in three stages—initiation, elongation and termination. By the end, you should be able to explain not just what translation produces, but how the mRNA sequence determines the order of amino acids in a polypeptide. Allow about 35–40 minutes for the video, selected reading and lesson.
A message written in codons
Translation uses the information in mRNA to build a polypeptide: a chain of amino acids joined by peptide bonds. It takes place at a ribosome, which holds the mRNA and helps link the amino acids. Transfer RNA (tRNA) molecules bring those amino acids to the ribosome.
Watch the translation portion of Protein Synthesis (Updated) by Amoeba Sisters for a visual overview. You can skip its earlier transcription recap, which you covered last lesson.
Protein Synthesis (Updated) by Amoeba Sisters introduces how ribosomes and tRNAs use an mRNA message to build an amino acid chain.
Watch translation overview, beginning as mRNA reaches the cytoplasm and ending with the chain specified by its sequence. Focus on codons, matching tRNA anticodons, peptide bonds and the stop signal. When the video says a tRNA “drops off” an amino acid, use the more precise account below: the amino acid is joined to the growing chain before the empty tRNA leaves.
The ribosome reads mRNA from its end toward its end, in groups of three bases called codons. Each amino-acid codon specifies an amino acid according to the genetic code. The codon AUG ordinarily starts translation and specifies methionine; UAA, UAG and UGA are stop codons, which do not specify amino acids.
A tRNA makes the code usable. One part of it carries a particular amino acid; another part has a three-base anticodon that can pair with a complementary mRNA codon. For example, the mRNA codon pairs with the tRNA anticodon . The two sequences run in opposite directions, just as complementary nucleic-acid strands do.
Read the selected beginning of Khan Academy’s Stages of translation. It gives the codon–anticodon relationship before the article moves into the stages of the process.
Stages of translation (article)
Khan Academy’s Stages of translation explains how three-base mRNA codons are associated with amino acids and how tRNAs connect the two.
Under “Translation: The big picture,” begin in “The genetic code” at the paragraph opening “In an mRNA, the instructions for building a polypeptide.” Read codons and tRNAs through the paragraph in “Codons to amino acids” ending “The other end of the tRNA carries the amino acid that's specified by the codon.” Notice that the codon is on the mRNA and the anticodon is on the tRNA. You do not need to memorise the full codon chart for this lesson.
The start codon establishes the reading frame: the way bases are grouped into successive triplets. Beginning at a different base would change the groups being read, even if the mRNA’s bases themselves had not changed.
Initiation: assembling at the start codon
Translation begins when the small ribosomal subunit, mRNA and initiator tRNA come together at an AUG start codon. The initiator tRNA’s anticodon pairs with AUG, positioning its methionine at the beginning of the chain. The large ribosomal subunit joins to form the working ribosome.
Read the short “Initiation” passage in Stages of translation to identify the components of this assembly.
Stages of translation (article)
The initiation passage identifies what must come together before the first amino acid can be joined to another.
In “Translation: Beginning, middle, and end,” go to the subsection “Initiation.” Starting at “In order for translation to start,” read initiation ingredients through the paragraph ending “the molecular setup needed to start making a new protein.” Stop there; the later detailed comparison of organisms is not needed to explain the core mechanism.
A working ribosome has places for tRNAs called the A, P and E sites. The initiator tRNA begins in the P site, holding methionine. The adjacent A site is where the next amino-acid-carrying tRNA will bind. In the figure below, the first two panels show the initiator tRNA pairing with AUG and the next tRNA arriving.
Elongation: making the chain grow
The ribosome now repeats a cycle for each amino-acid codon. First, a tRNA with an anticodon complementary to the codon exposed in the A site binds there. Because that tRNA carries the amino acid associated with the codon, matching the RNA bases helps put the appropriate amino acid next in line.
The ribosome then catalyses a peptide bond. The growing chain, initially just methionine on the P-site tRNA, is transferred onto the amino acid carried by the A-site tRNA. This is the key chemical step: the amino acids are not simply placed beside one another; they become joined in a polypeptide.
Finally, the ribosome shifts one codon along the mRNA toward the end. The tRNA carrying the growing chain moves into the P site. The now-empty tRNA moves to the E site and exits, leaving the A site available for another incoming tRNA. Repeating this cycle lengthens the chain in the order specified by the mRNA codons.
Read Khan Academy’s account of one cycle, then its brief description of how that cycle repeats. The article contains further chemical detail between these passages; you can skip that detail for this learning outcome.
Stages of translation (article)
These two passages connect codon recognition and peptide-bond formation to the ribosome’s movement to the next codon.
In “Elongation,” begin at “Our first, methionine-carrying tRNA starts out” and read the first cycle through the sentence ending “the tRNA in the A site.” Then, after the material on the ends of a polypeptide, resume at “Once the peptide bond is formed” and read the next codon through “so the whole cycle can repeat.” Track what happens to the chain and to the empty tRNA separately.
The lower panel of the “Step-by-step translation” image makes those roles easier to separate: the growing chain is held on a tRNA in the P site, while the empty tRNA is at the E site. An incoming amino-acid-carrying tRNA can bind at A. These letters name positions in the ribosome, not types of amino acid.
Termination: releasing the product
The cycle does not continue past a stop codon. When UAA, UAG or UGA reaches the A site, there is no tRNA carrying a matching “stop amino acid.” Instead, a release factor recognises the stop signal. The completed polypeptide is released, and the ribosomal subunits and other components can separate and be reused.
Be precise about the product: translation produces a polypeptide with a particular amino acid sequence. That chain may later fold or undergo other changes before it functions as a protein. A stop codon ends translation; it is not the signal that ended transcription in the previous lesson.
From an mRNA sequence to an HSC explanation
Last lesson’s worked transcription example produced this RNA sequence:
Starting at AUG groups the bases into four codons. Using the genetic code shown in the Khan Academy reading gives:
| mRNA codon | Meaning during translation |
|---|---|
| AUG | Start; adds methionine |
| AUC | Adds isoleucine |
| UCG | Adds serine |
| UAA | Stop; adds no amino acid |
The newly made chain is therefore methionine–isoleucine–serine, not a four-amino-acid chain. The order matters: it reflects the order of the codons read from to .
For an HSC explain response, connect the sequence to the mechanism rather than merely listing the stages. For example:
During translation, a ribosome assembles on an mRNA at an AUG start codon, where an initiator tRNA brings methionine. As the ribosome reads successive mRNA codons from to , tRNAs with complementary anticodons bring the corresponding amino acids. The ribosome catalyses peptide bonds between these amino acids and moves one codon at a time, producing a polypeptide in the order specified by the mRNA. When a stop codon is reached, a release factor causes the polypeptide to be released.
That answer makes the essential causal link explicit: codon order determines which tRNAs bind in sequence, and thus the order of amino acids in the polypeptide.
Key takeaways
Translation converts the information in mRNA codons into the amino acid sequence of a polypeptide. Initiation positions the ribosome and methionine-carrying tRNA at AUG. During elongation, matching tRNAs arrive, peptide bonds form and the ribosome advances one codon at a time. A stop codon brings termination and release of the chain.
Next, you’ll shift from how genetic information is used to how it is inherited, predicting outcomes of monohybrid crosses with Punnett squares and probability.