Translation, From mRNA to Polypeptide
Streptomycin, first isolated by Selman Waksman in 1943, was the first antibiotic shown to kill tuberculosis bacteria. In 1962, researchers identified its mechanism: streptomycin binds the 30S ribosomal subunit of prokaryotes and causes misreading of mRNA codons during translation. More than 50 antibiotics now work by targeting the bacterial ribosome. They are safe because bacterial ribosomes (30S and 50S subunits, together 70S) are structurally different from eukaryotic ribosomes (40S and 60S subunits, together 80S), so the antibiotics selectively block translation in bacteria without affecting human cells.
Orient yourself
Predict why the cell needs ribosomes and tRNA at all, then scan the goals and key terms.
Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
A student says, "mRNA already contains the right sequence, so it should be able to turn itself directly into a protein. Ribosomes and tRNA seem unnecessary."
Before reading on, explain why that reasoning is incomplete. If mRNA only carries the code, what else is needed to turn that code into a chain of amino acids?
Know
- The ribosome reads mRNA during translation.
- tRNA carries amino acids and pairs by anticodon-codon matching.
Understand
- How peptide bonds form during polypeptide elongation.
- Why mRNA and tRNA have different but linked roles.
Can Do
- Trace a short mRNA sequence through codon-anticodon matching.
- Explain why insulin production depends on accurate translation.
The ribosome reads in threes
Establish where translation happens and how the ribosome, built from rRNA, reads the mRNA codons.
Core Content
Site of translation · reading codons
When streptomycin, first isolated by Selman Waksman in 1943, was found to kill Mycobacterium tuberculosis, researchers could observe the effect but not yet explain the mechanism. By 1962, studies showed streptomycin binds the 30S ribosomal subunit in bacteria and causes codon misreading: the ribosome places the wrong amino acid and the resulting protein is non-functional. The specificity for bacterial ribosomes (70S = 30S + 50S subunits) versus eukaryotic ribosomes (80S = 40S + 60S subunits) explains why the drug is toxic to bacteria but not to human cells. To understand why, you need to understand what the ribosome normally does at each codon.
During translation, the mRNA attaches to a ribosome. The ribosome moves along the mRNA sequence and reads it in codons, one three-base unit at a time. This provides the framework for matching the code to the correct amino acids.
The ribosome itself is built from proteins and a third type of RNA, ribosomal RNA (rRNA). The rRNA forms the structural and catalytic core of the ribosome and is what actually catalyses the joining of amino acids, so all three RNA types have a role in polypeptide synthesis: mRNA carries the code, tRNA delivers the amino acids, and rRNA builds and runs the ribosome.
The ribosome does not create the genetic message. It interprets the message that was already copied into mRNA during transcription.
Translation: mRNA attaches to a ribosome, which reads the message in codons (three bases at a time). The ribosome interprets, but does not create, the message. Always write "ribosome reads mRNA", not "reads DNA".
Pause, copy the highlighted translation definition into your book.
Translation takes place at the _____, which reads the mRNA codons.
The adaptor molecule
See how tRNA links a codon to its amino acid through its anticodon.
tRNA function · anticodon end
We just saw that the ribosome reads mRNA in codons. That raises a question: how does the ribosome know which amino acid to use for each codon? This card answers it → tRNA molecules each carry a specific amino acid and have an anticodon that pairs with the complementary mRNA codon.
Each tRNA molecule carries a particular amino acid. On the other end of the tRNA is an anticodon, a three-base sequence that can pair with a complementary codon on the mRNA.
This is why mRNA and tRNA have different roles. The mRNA carries the code. The tRNA brings the amino acid that matches that code. Without tRNA, the cell would have no effective way to connect a codon to the correct amino acid during translation.
tRNA carries a specific amino acid at one end and has an anticodon (3 bases) at the other. The anticodon pairs with the complementary mRNA codon. mRNA = codons; tRNA = anticodons + amino acids. Do not reverse these roles.
Add the highlighted tRNA roles to your notes, include the codon/anticodon distinction.
Which molecule carries a specific amino acid and has an anticodon?
Codon meets anticodon
See how antiparallel codon-anticodon pairing fixes the order of amino acids.
Matching the code · sequence determines sequence
We just saw that tRNA brings specific amino acids and its anticodon pairs with an mRNA codon. That raises a question: how does this matching step actually determine the order of amino acids in the final protein? This card answers it → the codon sequence on mRNA directly dictates the amino acid sequence in the polypeptide.
At the ribosome, a tRNA anticodon pairs with a complementary mRNA codon. Like the two strands of DNA, the codon and anticodon pair in an antiparallel way: they run in opposite directions, so a 5′-GCA-3′ codon is read by a 3′-CGU-5′ anticodon. This matching ensures that the amino acid brought by that tRNA is placed in the correct position in the growing chain.
The order of codons on the mRNA therefore determines the order of amino acids in the polypeptide. If the mRNA sequence changes, the sequence of amino acids produced may also change. The genetic code is also degenerate: most amino acids are specified by more than one codon (for example, several codons all code for leucine). Because of this, some point mutations change a codon to another codon for the same amino acid, so the protein is unchanged.
mRNA
- Contains codons
- Carries transferable coded information
- Read by the ribosome
tRNA
- Contains anticodons
- Carries specific amino acids
- Matches codons during translation
At the ribosome, the tRNA anticodon pairs with the complementary mRNA codon, placing the correct amino acid in position. The order of codons determines the order of amino acids. Change the mRNA sequence → the amino acid sequence (and protein function) may change.
Pause, write the highlighted codon-to-amino-acid sequence rule into your book.
mRNA carries the anticodons and tRNA carries the codons.
Translation occurs at ribosomes and uses tRNA molecules to deliver amino acids according to the mRNA codon sequence.
The start codon AUG codes for the amino acid tryptophan.
Start, elongate, stop
See how translation begins at the start codon, joins amino acids with peptide bonds, and ends at a stop codon.
Assembly · polypeptide elongation
We just saw that codon-anticodon matching positions each amino acid in the correct order. That raises a question: how are those amino acids actually joined into a chain? This card answers it → the ribosome catalyses peptide bond formation between adjacent amino acids, building the polypeptide step by step.
Translation does not begin just anywhere on the mRNA. It starts at the start codon, AUG, which also codes for the amino acid methionine, so the first amino acid placed is usually methionine. Once the correct amino acids are positioned by codon-anticodon matching, the ribosome helps join adjacent amino acids with peptide bonds. As this process repeats, the amino acid chain lengthens. This is called polypeptide elongation.
Elongation continues until the ribosome reaches a stop codon (UAA, UAG or UGA). No tRNA pairs with a stop codon; instead a release factor triggers termination, the finished polypeptide is released and the ribosome lets go of the mRNA.
The importance of this process is substantial. Enzymes, structural proteins, transport proteins and signalling proteins all depend on accurate amino acid sequences. Insulin is one real example of a protein product that depends on correct translation.
The ribosome joins adjacent amino acids with peptide bonds; repeating this builds the chain (polypeptide elongation). All functional proteins, enzymes, structural, transport, signalling, depend on correct sequences. Insulin is a real example requiring accurate translation.
Add the highlighted peptide bond and polypeptide elongation point to your notes.
What type of bond joins adjacent amino acids in a polypeptide?
Put it together and apply
Read the full model, decode the anticodons, then reason about insulin and clear the misconceptions.
Model · ribosome, tRNA and peptide bonds
We just saw that peptide bonds join amino acids into a growing polypeptide chain. That raises a question: what does the full translation process look like as an integrated sequence? This card answers it → a four-step model showing mRNA, ribosome, tRNA and peptide bonds working together.
Translation steps: 1, mRNA binds to a ribosome and reading begins at the start codon AUG (methionine); 2, tRNA anticodons pair with complementary mRNA codons; 3, the ribosome (rRNA) joins amino acids with peptide bonds; 4, the polypeptide elongates as more codons are translated; 5, at a stop codon a release factor ends translation (termination) and the polypeptide is released.
Pause, write the four-step translation sequence into your book in shorthand form.
Translation links the mRNA code to amino acids through ribosomes and tRNA.
Activities
Decode and Match
If the mRNA sequence contains the codons A U G G A A, write two complementary tRNA anticodons that could pair with them. Then state which molecule carries the amino acids.
Insulin Translation Reasoning
Explain why an incorrect amino acid sequence during translation could affect the function of insulin as a protein product.
Core idea
- Translation uses the mRNA code to assemble amino acids into a polypeptide.
Mechanism / process
- Ribosomes read mRNA codons, tRNA anticodons pair with them, and peptide bonds join amino acids during elongation.
Common mistake
- Do not mix up codons with anticodons or say that mRNA carries amino acids.
Exam sentence starter
- "Translation is important because it converts the coded information in mRNA into..."
Practise independently
Attempt every response in your own words first. The model answers are for checking, not for copying.
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Outline the roles of mRNA, tRNA and the ribosome in translation.
AnalyseBand 4(4 marks) 2. Explain how codon-anticodon matching and peptide bond formation lead to polypeptide elongation.
EvaluateBand 5–6(5 marks) 3. Evaluate the statement: "Correct translation is essential for producing a functional protein such as insulin."
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1, Decode and Match
mRNA codons: A U G G A A
Possible tRNA anticodons: U A C C U U
Molecule carrying amino acids: tRNA.
Activity 2, Insulin Translation Reasoning
If translation produces the wrong amino acid sequence, the resulting insulin polypeptide may fold or function incorrectly. That can reduce or prevent its normal biological role as a protein hormone.
Short Answer Model Responses
Q1 (3 marks): mRNA carries the codon sequence that contains the genetic message [1]. tRNA carries specific amino acids and has anticodons that pair with mRNA codons [1]. The ribosome reads the mRNA and coordinates amino acid joining during translation [1].
Q2 (4 marks): During translation, a tRNA anticodon pairs with a complementary mRNA codon at the ribosome [1]. This brings the correct amino acid into position [1]. The ribosome then helps form a peptide bond between adjacent amino acids [1]. Repeated matching and bond formation lengthen the amino acid chain, causing polypeptide elongation [1].
Q3 (5 marks): The statement is correct because translation determines the amino acid sequence of a protein [1]. Ribosomes read the mRNA codons and tRNA brings the matching amino acids [1]. Peptide bonds then link those amino acids into a polypeptide [1]. If this sequence is incorrect, the resulting protein may not fold or function properly [1]. Therefore correct translation is essential for producing a functional protein such as insulin [1].
Retrieve and reflect
Use the review session first, then compare how your thinking has changed since the opening prediction.
Ribosome
Reads mRNA and coordinates protein assembly.
tRNA
Brings amino acids and carries anticodons.
Peptide bond
Joins adjacent amino acids in a growing chain.
Exam trap
mRNA carries codons; tRNA brings amino acids.
Rapid-fire questions on ribosomes, tRNA, codon-anticodon matching and peptide bonds. Beat the boss to bank a tier, gold (perfect + fast), silver (80%+), or bronze (cleared).
Streptomycin's mechanism, binding the 30S subunit of the bacterial 70S ribosome to cause codon misreading, while leaving the eukaryotic 80S ribosome unaffected, is explicable only through the translation mechanism this lesson covers. Normal translation requires the ribosome to hold the mRNA and provide two sites where tRNA anticodons match incoming codons precisely; streptomycin distorts the 30S small subunit so that tRNA anticodon matching is no longer accurate, incorrect amino acids are inserted, and non-functional proteins accumulate. The fact that 50+ antibiotics now target bacterial translation (each at a different ribosome site) demonstrates how central and precisely choreographed the ribosome-tRNA-mRNA interaction is. mRNA alone cannot assemble a polypeptide, the ribosome reads, tRNA delivers, and peptide bonds form sequentially.