Difficulty: Intermediate to Advanced | Prerequisites: Parts 1 and 2 of these notes (gene expression, genetic code, transcription, RNA processing)
This section covers the second half of gene expression: how the ribosome reads mRNA and builds a polypeptide (translation), what happens to the polypeptide afterwards (folding, modification, targeting), and how mutations in DNA can alter the resulting protein. Translation is where the genetic code meets amino acid chemistry, and mutations are where things go wrong. This material is tested heavily and connects to genetics, evolution, and disease.
Transfer RNA (tRNA) molecules act as translators, matching mRNA codons to the correct amino acids. Ribosomes coordinate the process in three stages: initiation, elongation, and termination. After translation, polypeptides may be modified and targeted to specific locations. Mutations in DNA (substitutions, insertions, deletions) can alter the protein product, with consequences ranging from silent to catastrophic.
Transfer RNA (tRNA)
A small RNA molecule (about 80 nucleotides) that carries a specific amino acid at its 3' end and has an anticodon at the opposite end. Think of it as the translator that reads an mRNA codon and delivers the matching amino acid.
Anticodon
The three-nucleotide sequence on a tRNA that base-pairs with a complementary mRNA codon. Written 3' to 5' to align with the codon's 5' to 3' direction.
Aminoacyl-tRNA synthetase
A family of 20 enzymes, one for each amino acid, that attach the correct amino acid to its matching tRNA. The active site fits only one specific amino acid-tRNA combination, ensuring accuracy.
Charged tRNA (aminoacyl tRNA)
A tRNA molecule with its amino acid covalently attached, ready to deliver that amino acid to the ribosome. The attachment is driven by ATP hydrolysis.
Wobble
Flexible base pairing at the third position of a codon (and the first position of the anticodon), allowing a single tRNA to recognise more than one codon. This is why bacteria need only about 45 tRNAs for 61 sense codons.
Ribosome
A molecular machine made of ribosomal RNA (rRNA) and proteins that carries out translation. It has a large and a small subunit that join only when attached to an mRNA molecule.
Ribosomal RNA (rRNA)
The most abundant type of cellular RNA. It forms the structural and catalytic core of the ribosome, including the A and P sites and the interface between subunits.
P site (peptidyl-tRNA site)
The ribosomal binding site that holds the tRNA carrying the growing polypeptide chain.
A site (aminoacyl-tRNA site)
The ribosomal binding site that holds the tRNA carrying the next amino acid to be added to the chain.
E site (exit site)
The ribosomal site from which empty (discharged) tRNAs leave the ribosome.
Exit tunnel
A channel through the large ribosomal subunit through which the growing polypeptide passes and is eventually released.
Initiation factors
Proteins that help assemble the translation initiation complex (mRNA + initiator tRNA + small subunit + large subunit). GTP hydrolysis provides the energy.
Elongation factors
Proteins that assist each cycle of amino acid addition during translation. GTP is hydrolysed during codon recognition and translocation.
Release factor
A protein shaped like an aminoacyl tRNA that binds to a stop codon in the A site, triggering release of the finished polypeptide. It causes addition of a water molecule instead of an amino acid.
Polyribosome (polysome)
A string of ribosomes translating the same mRNA simultaneously, enabling rapid production of many copies of a polypeptide.
Signal peptide
A short sequence (about 20 amino acids) at the N-terminus of a polypeptide that targets the protein to the endoplasmic reticulum for secretion or membrane insertion.
Signal-recognition particle (SRP)
A protein-RNA complex that recognises the signal peptide as it emerges from the ribosome and escorts the ribosome to the ER membrane.
Post-translational modification
Chemical changes to a polypeptide after translation, such as addition of sugars, lipids, or phosphate groups, cleavage of the polypeptide, or removal of the initial methionine.
Point mutation
A change in a single nucleotide pair of a gene. If it occurs in a gamete, it can be inherited.
Silent mutation
A nucleotide substitution that changes a codon but still codes for the same amino acid (thanks to redundancy). No effect on the protein or phenotype.
Missense mutation
A nucleotide substitution that changes one amino acid to a different one in the polypeptide. The effect ranges from negligible to severe, depending on the amino acid and its location.
Nonsense mutation
A nucleotide substitution that changes an amino acid codon into a stop codon, causing premature termination of translation. Usually produces a nonfunctional protein.
Frameshift mutation
An insertion or deletion of nucleotides (not a multiple of three) that shifts the reading frame of all downstream codons. Almost always produces a nonfunctional protein.
Mutagen
A physical or chemical agent that interacts with DNA and causes mutations. Examples include X-rays, UV light, and certain chemicals.
tRNA is the molecule that translates between the nucleic acid language (codons) and the amino acid language (protein)
Each tRNA is about 80 nucleotides, single-stranded, but folds into an L-shaped 3D structure through internal base pairing
The 3' end carries the amino acid (attachment site)
The anticodon loop at the opposite end base-pairs with a complementary mRNA codon
Anticodons are written 3' to 5' so they align correctly with codons written 5' to 3'
Example: mRNA codon 5'-GGC-3' (glycine) pairs with tRNA anticodon 3'-CCG-5'
Accurate translation requires two recognition events:
The correct amino acid must be attached to the correct tRNA (done by aminoacyl-tRNA synthetases, 20 types, one per amino acid, using ATP)
The tRNA anticodon must pair with the correct mRNA codon
The resulting charged (aminoacyl) tRNA is released and delivers its amino acid to the ribosome
Wobble: the base-pairing rules at the third codon position are relaxed. A U at the 5' end of the anticodon can pair with A or G at the 3' end of the codon. This allows fewer than 61 tRNAs to service all 61 sense codons (bacteria have roughly 45 tRNAs).
Each ribosome has a large subunit and a small subunit, made of rRNA and proteins
The two subunits join only when attached to an mRNA molecule
Eukaryotic ribosomal subunits are assembled in the nucleus (rRNA genes transcribed, assembled with imported proteins, then exported)
About one-third of a ribosome's mass is protein; the rest is rRNA (three molecules in bacteria, four in eukaryotes)
rRNA is the most abundant type of cellular RNA
The ribosome has one mRNA binding site and three tRNA binding sites:
P site: holds the tRNA carrying the growing polypeptide
A site: holds the incoming aminoacyl tRNA (the next amino acid)
E site: where discharged tRNAs exit
The rRNA of the large subunit catalyses peptide bond formation (the ribosome is a ribozyme)
The growing polypeptide passes through the exit tunnel in the large subunit
Eukaryotic and bacterial ribosomes differ slightly in size, which is medically significant: certain antibiotics target bacterial ribosomes without affecting eukaryotic ones
The start codon AUG sets the reading frame
The small ribosomal subunit binds to both the mRNA and a special initiator tRNA carrying methionine
In bacteria: the small subunit binds the mRNA at a specific sequence upstream of AUG, and the initiator tRNA can join in either order
In eukaryotes: the small subunit (with initiator tRNA already bound) attaches to the 5' cap and scans downstream until it reaches AUG
The large subunit then joins, completing the translation initiation complex
Initiation factors and GTP hydrolysis are required
At the end of initiation, the initiator tRNA sits in the P site; the A site is empty and ready
Each amino acid is added in a three-step cycle:
Codon recognition: an aminoacyl tRNA with the correct anticodon enters the A site (requires GTP hydrolysis for accuracy)
Peptide bond formation: the rRNA of the large subunit catalyses the transfer of the polypeptide from the P-site tRNA to the amino acid on the A-site tRNA
Translocation: the ribosome shifts one codon along the mRNA (requires GTP hydrolysis). The tRNA that was in the A site moves to the P site; the empty tRNA in the P site moves to the E site and is released
The polypeptide grows from the amino end (N-terminus, methionine) to the carboxyl end (C-terminus)
Translation stops when a stop codon (UAA, UAG, or UGA) reaches the A site
A release factor (shaped like a tRNA) binds the stop codon
The release factor triggers addition of a water molecule instead of an amino acid, breaking the bond between the polypeptide and the tRNA in the P site
The polypeptide is released through the exit tunnel
The ribosomal subunits, mRNA, and other components dissociate (requires hydrolysis of two GTPs)
Translation alone does not always produce a functional protein
Post-translational modifications may include:
Addition of sugars, lipids, or phosphate groups
Removal of the initial methionine or other amino acids
Cleavage of the polypeptide into two or more pieces
The gene determines primary structure (amino acid sequence), which determines 3D folding
Targeting polypeptides to specific locations:
Free ribosomes (in the cytosol) generally make proteins that function in the cytosol
Bound ribosomes (on the ER) make proteins destined for the endomembrane system or for secretion
All translation starts on free ribosomes; the growing polypeptide's signal peptide (about 20 amino acids at the N-terminus) cues the ribosome to attach to the ER
The signal-recognition particle (SRP) recognises the signal peptide and escorts the ribosome to an ER receptor, part of a translocation complex
Polyribosomes (polysomes): multiple ribosomes translate the same mRNA simultaneously, producing many copies of a polypeptide quickly
Mutations are the ultimate source of genetic diversity and new genes
Large-scale mutations: chromosomal rearrangements affecting long DNA segments
Point mutations: changes in a single nucleotide pair
Classic example: sickle-cell disease results from a single nucleotide substitution in the beta-globin gene. The mRNA codon changes from GAG (glutamic acid) to GUG (valine), altering the haemoglobin protein.
Nucleotide-pair substitutions:
Silent mutation: the new codon still codes for the same amino acid (no phenotypic effect)
Missense mutation: the new codon codes for a different amino acid. Effect ranges from none to severe.
Nonsense mutation: the new codon is a stop codon, causing premature termination. Usually produces a nonfunctional, truncated protein.
Insertions and deletions:
These are additions or losses of nucleotide pairs
If the number inserted or deleted is not a multiple of three, a frameshift mutation results, altering the reading frame of every downstream codon
Frameshifts are usually more damaging than substitutions because they affect the entire downstream protein sequence
Mutagens (X-rays, UV light, chemicals) interact with DNA and cause mutations
Errors during DNA replication or recombination also cause mutations spontaneously
The modern definition of a gene: a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule
Students often think the ribosome "reads" the mRNA. The ribosome provides the platform, but it is the tRNA that actually reads (base-pairs with) the codon. The ribosome facilitates the process and catalyses peptide bond formation.
Students often confuse the A, P, and E sites. Remember: A = aminoacyl (incoming), P = peptidyl (growing chain), E = exit (empty tRNA leaves).
Students often think all mutations are harmful. Silent mutations have no effect. Some missense mutations are neutral or even beneficial. Mutations are the raw material for evolution.
Students often think insertions and deletions always cause frameshifts. If the number of nucleotides inserted or deleted is a multiple of three, the reading frame is preserved (though amino acids are added or removed).
⚠️ Know the three stages of translation (initiation, elongation, termination) and the role of GTP at each stage.
⚠️ Be able to explain what happens at each ribosomal site (A, P, E) during the elongation cycle.
⚠️ Understand the sickle-cell disease example as a case study in point mutation: one nucleotide change, one amino acid change, a completely different protein behaviour.
⚠️ Be able to classify a mutation as silent, missense, nonsense, or frameshift given a DNA or mRNA sequence change.
⚠️ The signal peptide/SRP pathway for targeting proteins to the ER is a common exam diagram question.
⚠️ Know the modern definition of a gene: a region of DNA that can be expressed to produce a polypeptide or a functional RNA molecule.
Fill in the blank: The tRNA binding site that holds the growing polypeptide chain is the ______ site. (P site)
True or False: A release factor is a special tRNA that recognises stop codons. (False. A release factor is a protein shaped like a tRNA, not an actual tRNA.)
Fill in the blank: A mutation that changes an amino acid codon to a stop codon is called a ______ mutation. (nonsense)
True or False: Wobble allows one tRNA to recognise multiple codons for the same amino acid. (True.)
Fill in the blank: The ______ recognises the signal peptide and escorts the ribosome to the ER. (signal-recognition particle / SRP)
Q: Describe the three steps of the elongation cycle in translation.
A: (1) Codon recognition: an aminoacyl tRNA with the matching anticodon enters the A site; GTP is hydrolysed to improve accuracy. (2) Peptide bond formation: the rRNA of the large subunit catalyses transfer of the polypeptide chain from the P-site tRNA onto the amino acid of the A-site tRNA. (3) Translocation: the ribosome shifts one codon along the mRNA (powered by GTP hydrolysis); the A-site tRNA moves to the P site, the P-site tRNA moves to the E site and exits.
Q: A point mutation changes the DNA template strand from 3'-CTC-5' to 3'-CAC-5'. What type of mutation is this, and what is the effect on the protein?
A: The original mRNA codon is GAG (glutamic acid); the mutant codon is GUG (valine). This is a missense mutation, because one amino acid is replaced by a different one. This is the exact mutation responsible for sickle-cell disease in the beta-globin gene.
Q: Why are frameshift mutations usually more damaging than missense mutations?
A: A frameshift shifts the reading frame of every codon downstream of the insertion or deletion. This means nearly every amino acid from that point onwards will be wrong, and the ribosome will likely encounter a premature stop codon, producing a truncated, nonfunctional protein. A missense mutation changes only one amino acid, so the rest of the protein may still function.
Q: Explain how a polypeptide destined for secretion is targeted to the ER.
A: Translation begins on a free ribosome in the cytosol. As the N-terminal signal peptide (about 20 amino acids) emerges from the ribosome, it is recognised by the signal-recognition particle (SRP). The SRP escorts the ribosome to an SRP receptor on the ER membrane. The ribosome docks at a translocation complex, and the growing polypeptide is threaded into the ER lumen as translation continues. The signal peptide is cleaved off inside the ER.
Q: What is the modern definition of a gene, and why was the earlier "one gene-one polypeptide" definition insufficient?
A: A gene is a region of DNA that can be expressed to produce a final functional product that is either a polypeptide or an RNA molecule. The earlier definition was insufficient because some genes produce functional RNAs (such as rRNA and tRNA) that are never translated, and because alternative splicing allows a single gene to produce multiple different polypeptides.
Translation connects back to protein structure (Ch. 5), since the amino acid sequence determines folding and function. Mutations connect to genetics and inheritance (Ch. 14, 15) and to evolution (Ch. 23), where mutations provide the raw variation that natural selection acts upon. The signal peptide pathway connects to the endomembrane system (Ch. 6). Gene regulation (Ch. 18) controls when and where these processes occur in different cell types.
translation, tRNA, transfer RNA, anticodon, aminoacyl-tRNA synthetase, charged tRNA, wobble, ribosome, rRNA, ribosomal RNA, P site, A site, E site, exit tunnel, initiation factors, elongation factors, release factor, polyribosome, polysome, signal peptide, SRP, signal-recognition particle, post-translational modification, free ribosome, bound ribosome, point mutation, silent mutation, missense mutation, nonsense mutation, frameshift mutation, insertion, deletion, substitution, mutagen, sickle-cell disease, beta-globin, genetic disorder, central dogma, gene definition