Translation is the process by which ribosomes read mRNA codons and assemble amino acids into a polypeptide. It proceeds in three stages: initiation, elongation, and termination. Mutations are changes to the DNA sequence that can alter codons and therefore the resulting protein, with effects ranging from silent (no change) to devastating (loss of function).
Difficulty: Introductory Prerequisites: Genetic Code, Transcription and RNA Processing study notes (companion document). You should already know what codons, anticodons, and mRNA are.
Translation is the final step in gene expression: it takes the mRNA message and builds the protein. This is where the genetic code is physically decoded. If transcription is photocopying a recipe, translation is cooking the dish. Mutation sits alongside this process because any error introduced at the DNA level can ripple through transcription and translation to change the protein, and understanding how mutations affect the reading frame or the amino acid sequence is a staple exam topic.
Ribosome
The molecular machine that reads mRNA and assembles amino acids into a polypeptide. It consists of a large subunit and a small subunit, made of ribosomal RNA (rRNA) and proteins. Think of it as the workbench where the protein is built.
Small ribosomal subunit
The subunit that binds mRNA first and scans for the start codon (AUG). It positions the mRNA so codons can be read correctly.
Large ribosomal subunit
The subunit that joins after initiation and catalyses peptide bond formation between amino acids. It contains the A, P, and E sites.
A site (aminoacyl site)
The ribosomal site where a new aminoacyl-tRNA (charged tRNA) binds, delivering the next amino acid to be added to the chain.
P site (peptidyl site)
The ribosomal site that holds the tRNA carrying the growing polypeptide chain.
E site (exit site)
The ribosomal site from which a now-empty tRNA is released after its amino acid has been transferred to the growing chain.
Polypeptide
A chain of amino acids linked by peptide bonds. Once translation is complete and the polypeptide folds into its functional shape, it is called a protein.
Peptide bond
The covalent bond formed between the carboxyl group of one amino acid and the amino group of the next. This is the bond that links amino acids together in a polypeptide.
Release factor
A protein that recognises a stop codon in the A site and triggers the release of the completed polypeptide from the ribosome. In simple terms, it is the signal that says "the protein is finished."
Amino acid activation (charging)
The process by which aminoacyl-tRNA synthetase attaches the correct amino acid to its tRNA, using ATP. The resulting molecule is called an aminoacyl-tRNA, or "charged" tRNA.
Point mutation
A change in a single nucleotide in the DNA sequence. Depending on where it falls, it may alter one codon.
Silent (synonymous) mutation
A point mutation that changes a codon but still codes for the same amino acid, thanks to the degeneracy of the genetic code. The protein is unaffected.
Missense mutation
A point mutation that changes a codon so it codes for a different amino acid. The effect on the protein ranges from negligible to severe, depending on the amino acid substituted.
Nonsense mutation
A point mutation that converts an amino-acid codon into a stop codon, causing translation to end prematurely. The resulting protein is truncated and usually non-functional.
Frameshift mutation
An insertion or deletion of nucleotides (not in multiples of three) that shifts the reading frame of every codon downstream. Nearly always produces a non-functional protein.
The small ribosomal subunit binds to the 5' end of the mRNA, recognising the 5' cap.
It scans along the mRNA in the 5' → 3' direction until it finds the start codon, AUG.
A special initiator tRNA, carrying methionine (Met), base-pairs its anticodon (UAC) with the AUG start codon. This tRNA occupies the P site.
The large ribosomal subunit then joins, completing the functional ribosome. Initiation factors (proteins that assist assembly) are released.
The result: a fully assembled ribosome sitting on the mRNA with the initiator tRNA in the P site and an empty A site ready to receive the next charged tRNA.
Elongation is a repeating cycle of three steps:
Codon recognition: A charged tRNA (aminoacyl-tRNA) enters the A site. Its anticodon must base-pair correctly with the mRNA codon exposed there. If it does not match, it is rejected.
Peptide bond formation: The large subunit catalyses the transfer of the growing polypeptide from the tRNA in the P site onto the amino acid carried by the tRNA in the A site. A new peptide bond is formed, and the chain is now one amino acid longer.
Translocation: The ribosome shifts one codon (three nucleotides) along the mRNA in the 5' → 3' direction. The now-empty tRNA moves from the P site to the E site and is released. The tRNA carrying the polypeptide moves from the A site to the P site. The A site is now empty and ready for the next charged tRNA.
This cycle repeats for every codon until a stop codon is reached. The ribosome adds roughly 15 to 20 amino acids per second in eukaryotes.
The ribosome reaches a stop codon (UAA, UAG, or UGA) in the A site.
No tRNA has an anticodon for stop codons. Instead, a release factor protein enters the A site.
The release factor triggers hydrolysis of the bond between the polypeptide and the final tRNA in the P site. The completed polypeptide is released.
The ribosome dissociates into its large and small subunits. The mRNA is freed. All components (subunits, tRNAs, mRNA) can be recycled for future rounds of translation.
After release, the polypeptide folds into its three-dimensional shape (sometimes with help from chaperone proteins) and may undergo further post-translational modifications such as phosphorylation or glycosylation.
Mutations are permanent changes to the DNA nucleotide sequence. They can arise from errors during replication, exposure to mutagens (UV light, chemicals), or failures in DNA repair.
Point mutations (substitutions)
A single nucleotide is replaced by another. Three outcomes are possible:
Silent mutation: The new codon still codes for the same amino acid (thanks to degeneracy). No effect on the protein.
Missense mutation: The new codon codes for a different amino acid. The effect depends on how chemically different the substituted amino acid is and where it falls in the protein. Sickle-cell disease is caused by a single missense mutation in the haemoglobin gene (GAG → GUG, glutamic acid → valine).
Nonsense mutation: The new codon is a stop codon. Translation terminates early, producing a truncated, usually non-functional protein.
Insertions and deletions (indels)
If the number of nucleotides inserted or deleted is not a multiple of three, the reading frame shifts for every codon downstream. This is a frameshift mutation and almost always destroys protein function.
If the insertion or deletion is a multiple of three, the reading frame is preserved, but the protein gains or loses one or more amino acids. The effect depends on the location.
Chromosomal mutations
Larger-scale changes such as duplications, inversions, translocations, and deletions of whole chromosome segments. These are beyond the scope of this set of notes but are worth knowing exist.
No formulas, but you should be able to sketch or label these from memory:
Ribosome diagram: Show the large and small subunits, the mRNA strand running through, the three tRNA binding sites (E, P, A from left to right), and a growing polypeptide chain emerging from the top.
Translation stages: Three panels showing initiation (small subunit + initiator tRNA on AUG), elongation (ribosome moving along mRNA, multiple tRNAs cycling through), and termination (release factor in A site at stop codon, polypeptide released).
Mutation effects chart: Given a short mRNA sequence, show how a substitution produces a silent, missense, or nonsense outcome, and how a single-base insertion shifts the reading frame.
Antibiotics like tetracycline and chloramphenicol work by targeting bacterial ribosomes and blocking translation, which is why they kill bacteria without harming human cells (human ribosomes are structurally different). Understanding mutations is the foundation of medical genetics: sickle-cell disease, cystic fibrosis, and many cancers trace back to specific point mutations or frameshifts in particular genes.
Students often think the ribosome "reads" DNA directly. It does not. The ribosome reads mRNA, which was transcribed from DNA in the nucleus.
The A, P, and E sites are often memorised in the wrong order. Remember: a new tRNA Arrives at the A site, the Peptide chain grows at the P site, and empty tRNAs Exit from the E site.
A frameshift mutation does not just change one amino acid. It changes every amino acid downstream of the insertion or deletion, which is why it is usually far more damaging than a point substitution.
Students sometimes assume all mutations are harmful. Silent mutations have no effect on the protein, and some missense mutations are neutral or even beneficial (this is how evolution works).
⚠️ Know the three stages of translation (initiation, elongation, termination) and what happens at each.
⚠️ Be able to label the A, P, and E sites on a ribosome diagram and describe the role of each.
⚠️ Given a DNA or mRNA sequence and a mutation, determine whether it is silent, missense, nonsense, or frameshift, and predict its effect on the protein.
⚠️ Understand why frameshift mutations are typically more damaging than point substitutions.
⚠️ Know the role of release factors and how termination differs from initiation and elongation.
True or false: The small ribosomal subunit binds to the mRNA before the large subunit joins. (True.)
Fill in the blank: During elongation, a new charged tRNA enters the ______ site. (A site.)
True or false: A release factor is a special tRNA that recognises stop codons. (False, it is a protein, not a tRNA.)
Fill in the blank: A mutation that changes an amino-acid codon into a stop codon is called a ______ mutation. (Nonsense.)
True or false: An insertion of three nucleotides causes a frameshift. (False, three is a multiple of three, so the reading frame is preserved, though one extra amino acid is added.)
Q: Describe the three stages of translation and the key event that defines each.
A: Initiation: the small subunit, mRNA, and initiator tRNA (carrying Met) assemble at the start codon; the large subunit joins. Elongation: charged tRNAs enter the A site, peptide bonds form, and the ribosome translocates along the mRNA codon by codon. Termination: a stop codon is reached, a release factor enters the A site, the polypeptide is released, and the ribosome disassembles.
Q: The mRNA sequence reads AUG-GCA-UAC-UGA. What polypeptide is produced, and how many amino acids does it contain?
A: Met-Ala-Tyr (three amino acids). AUG = Met, GCA = Ala, UAC = Tyr, UGA = stop. Translation begins at AUG and ends at the stop codon.
Q: A single adenine is inserted after the second nucleotide of the second codon in the sequence AUG-GCA-UAC-UGA. What type of mutation is this, and what happens to the protein?
A: This is a frameshift mutation. The insertion shifts the reading frame for every codon downstream of the insertion point. The resulting amino acid sequence from the second codon onward will be completely different, and the original stop codon will likely be read out of frame. The protein is almost certainly non-functional.
Q: Explain why a point mutation at the third position of a codon is less likely to affect the protein than one at the first or second position.
A: Because of wobble base pairing and the degeneracy of the genetic code. Most of the redundancy in the code is concentrated at the third (wobble) position. A change there often produces a synonymous codon that still codes for the same amino acid, resulting in a silent mutation.
Q: Why do antibiotics that target bacterial ribosomes not harm human cells?
A: Bacterial ribosomes (70S, made of 30S and 50S subunits) are structurally different from eukaryotic ribosomes (80S, made of 40S and 60S subunits). The antibiotic binds specifically to the bacterial ribosome and blocks its function without affecting the human version.
Translation follows directly from transcription and RNA processing (covered in the companion study notes). Protein folding and post-translational modifications, covered later in most courses, pick up where translation leaves off. Mutation connects to evolution and natural selection: mutations generate the genetic variation that selection acts on. In genetics courses, mutation analysis is also the basis for understanding genetic diseases and for interpreting DNA sequencing data.
Translation, protein synthesis, ribosome, large subunit, small subunit, A site, P site, E site, aminoacyl site, peptidyl site, exit site, polypeptide, peptide bond, release factor, initiation, elongation, termination, start codon, stop codon, AUG, UAA, UAG, UGA, aminoacyl-tRNA, charged tRNA, amino acid activation, mutation, point mutation, substitution, silent mutation, synonymous mutation, missense mutation, nonsense mutation, frameshift mutation, insertion, deletion, indel, reading frame, sickle-cell disease, mutagen, DNA repair, protein folding, post-translational modification, intro to biology, BIO