Concepts of Biology · Molecular Biology
Translation
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Translation Reading an mRNA sequence to build a polypeptide chain Full entry → is the second step of gene expression: the cell reads a messenger RNA (mRNA) and assembles a chain of amino acids — a polypeptide — which folds into a working protein. It is the "read the recipe and cook the dish" step of the central dogma (DNA → RNA → protein). Translation happens in the cytoplasm on ribosomes, with transfer RNAs (tRNAs) acting as adapters that match the three-letter codons of the mRNA to specific amino acids. It runs in three stages — initiation, elongation, and termination — and the rules linking codons to amino acids are called the Genetic code The rules linking codons to amino acids Full entry →. Because proteins do most of the work in cells, translation is where genetic information finally becomes biological function.
Why this matters
- Proteins do the work: Enzymes, antibodies, hemoglobin, collagen, and insulin are all products of translation; defects here show up as defective or missing proteins.
- Genotype to phenotype: A single nucleotide change can alter a Codon Three-nucleotide group on mRNA specifying one amino acid Full entry →, change one amino acid, and change a protein's behavior — the sickle-cell example below is the classic case.
- Medicine: Antibiotics such as tetracyclines and erythromycin block translation on bacterial ribosomes, which differ enough from human ribosomes to be targeted selectively.
- Biotechnology: Cells can be engineered to translate almost any gene — how human insulin and many vaccines and therapeutics are produced.
- Exams: Expect codon-reading questions, tRNA Adapter RNA carrying a specific amino acid Full entry → pairing, ribosome-site questions, and mutation-effect questions.
The college version
Core Concepts
The genetic code: three bases per amino acid
The mRNA is read in non-overlapping groups of three nucleotides called codons. With four bases there are 64 possible codons: 61 code for amino acids and 3 are stop codons (UAA, UAG, UGA). AUG is the start codon; it codes for methionine and sets the Reading frame The grouping of mRNA bases into consecutive codons Full entry → — the grouping of bases into codons — for the rest of the message. The code is degenerate (redundant): most amino acids are specified by more than one codon, which buffers some mutations. It is also nearly universal across life — strong evidence of shared ancestry and the reason a human gene can be expressed in bacteria.
tRNA: the adapter molecules
Transfer RNAs are small RNA molecules folded into a cloverleaf shape. One end carries a three-base Anticodon The three bases on a tRNA that pair with a codon Full entry → that pairs with an mRNA codon; the other end carries the specific amino acid for that codon. The enzyme Aminoacyl-tRNA synthetase Enzyme attaching each amino acid to its matching tRNA Full entry → "charges" each tRNA by attaching its correct amino acid — the charging step is where the code is actually read, so its accuracy is critical. There are fewer tRNA types than codons, partly because of wobble: the third base of a codon can pair loosely with the first base of the anticodon, so one tRNA often recognizes more than one codon.
Ribosomes: the protein factories
Ribosomes are made of rRNA and proteins, with a small subunit (holds the mRNA) and a large subunit (forms peptide bonds). The peptide-bond catalysis is performed by the rRNA, making the Ribosome The rRNA-and-protein machine that assembles proteins Full entry → a ribozyme (an RNA enzyme). Each ribosome has three tRNA sites: the A site (aminoacyl — a charged tRNA enters), the P site (peptidyl — holds the growing chain), and the E site (exit — empty tRNAs leave).
The three stages of translation
- Initiation: The small subunit binds the mRNA near the start codon (in eukaryotes the 5′ cap helps find it); the initiator tRNA carrying methionine pairs with AUG; the large subunit joins.
- Elongation: A charged tRNA whose anticodon matches the next codon enters the A site. A peptide bond forms between the amino acids in the P and A sites, transferring the growing chain to the A-site tRNA. The ribosome translocates one codon down the mRNA (using GTP energy), moving the chain-carrying tRNA to the P site and the empty tRNA to the E site. The cycle repeats.
- Termination: When a stop codon enters the A site, no tRNA matches it. A release factor binds instead, water releases the polypeptide, and the subunits separate.
After translation: folding and finishing
The polypeptide begins folding as it is made, often with help from chaperone proteins. Many proteins are further modified, and signal sequences direct them to destinations (membrane, organelles, secretion). Ribosomes on the rough endoplasmic reticulum make proteins destined for secretion, such as hormones and antibodies. One mRNA is usually translated by many ribosomes at once (polyribosomes), allowing rapid production of many protein copies.
How It Works / Step-by-Step Process
- Initiation: Small subunit + initiator tRNA bind the mRNA at the start codon; the large subunit joins.
- Elongation: Charged tRNAs enter the A site one at a time; peptide bonds form; the ribosome translocates (GTP-powered); the cycle repeats.
- Termination: A stop codon reaches the A site; a release factor releases the polypeptide; subunits separate.
- Folding and finishing: The polypeptide folds (often with chaperones) and may be modified or directed to its destination.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Codon | Anticodon | Codon is on the mRNA; anticodon is on the tRNA and pairs with it |
| Transcription | Translation | Transcription makes RNA from DNA; translation makes protein from RNA |
| AUG (start) | Internal methionine codons | AUG is the start codon and also codes for methionine anywhere in a message |
| Degenerate code | Ambiguous code | Degenerate = several codons per amino acid, but each codon still means exactly one amino acid |
| A site | P site | A site is where incoming charged tRNAs land; P site holds the growing chain |
| Silent mutation | Missense / nonsense | Silent changes codon but not amino acid; missense changes one amino acid; nonsense creates a premature stop |
| Ribosome | Spliceosome | Ribosomes make proteins; spliceosomes process pre-mRNA in the nucleus |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The mRNA is a string of three-letter words, each word naming one Lego block. The ribosome is a reading machine: it reads each word and, with the help of little adapter molecules (the tRNAs), snaps on the matching Lego block. When it reaches a stop word, it lets go of the finished chain, which folds itself into a working shape — the protein.
Worked example
Take a fictional mRNA: AUG GCU UUU AAA UGA (start, alanine, phenylalanine, lysine, stop), translated as Met–Ala–Phe–Lys. Now suppose a mutation changes the third codon from UUU to UCU. The message reads AUG GCU UCU AAA UGA and the peptide becomes Met–Ala–Ser–Lys: one amino acid swapped, and the protein's shape and function may change with it. If the mutation changed UUU to UUC instead, the peptide would be unchanged, because both UUU and UUC code for phenylalanine — a silent change made possible by the code's degeneracy. If a mutation turned a sense codon into a stop codon (AAA → UAA), translation would stop early, producing a shortened, usually nonfunctional protein — a nonsense mutation. This is the molecular story behind sickle-cell disease: one base change in the β-globin gene swaps glutamic acid for valine at position 6 of the hemoglobin β chain, and that single swapped amino acid makes hemoglobin distort red blood cells.
Key takeaways
- Translation: mRNA sequence → amino acid sequence, on ribosomes in the cytoplasm.
- AUG = start codon (methionine); UAA, UAG, UGA = stop codons.
- Codon (mRNA) pairs with anticodon (tRNA); tRNAs are charged by aminoacyl-tRNA synthetase.
- Ribosome sites: A (entry), P (growing chain), E (exit).
- Peptide bonds form between amino acids; rRNA catalyzes the reaction (ribozyme).
- The genetic code is degenerate (redundant) and nearly universal.
- Elongation requires GTP; a release factor ends translation at stop codons.
- Mutations that change codons cause silent, missense, or nonsense effects.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What are the start codon and the three stop codons?
Show answer
AUG is the start codon (methionine); UAA, UAG, and UGA are the stop codons.
A tRNA has the anticodon 3′-AAG-5′. Which mRNA codon does it recognize?
Show answer
The codon is 5′-UUC-3′; the anticodon pairs antiparallel to it (3′-AAG-5′ pairs with 5′-UUC-3′).
What happens at the A, P, and E sites during elongation?
Show answer
A charged tRNA enters the A site; a peptide bond forms, transferring the growing chain to the A-site tRNA; translocation moves the chain-carrying tRNA to the P site and the empty tRNA to the E site, where it exits.
Why is the genetic code described as degenerate, and why is that useful?
Show answer
Degenerate means most amino acids are specified by more than one codon while each codon still means one amino acid. The redundancy buffers some mutations (e.g., UUU→UUC still codes for phenylalanine).
What is a nonsense mutation, and why is it usually more damaging than a silent mutation?
Show answer
A nonsense mutation turns a sense codon into a stop codon, truncating the protein early; a silent mutation leaves the amino acid unchanged. Truncated proteins usually lose function entirely.
Why can tetracycline and erythromycin block bacterial protein synthesis without stopping human protein synthesis?
Show answer
Bacterial and eukaryotic ribosomes differ structurally (rRNA and protein components), so these drugs bind bacterial ribosomes and block their translation with much less effect on human ribosomes — though side effects and resistance vary.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Translation
- Reading an mRNA sequence to build a polypeptide chain
- Codon
- Three-nucleotide group on mRNA specifying one amino acid
- Anticodon
- The three bases on a tRNA that pair with a codon
- Genetic code
- The rules linking codons to amino acids
- Start / stop codons
- AUG (methionine) / UAA, UAG, UGA
- tRNA
- Adapter RNA carrying a specific amino acid
- Aminoacyl-tRNA synthetase
- Enzyme attaching each amino acid to its matching tRNA
- Ribosome
- The rRNA-and-protein machine that assembles proteins
- Reading frame
- The grouping of mRNA bases into consecutive codons
Sources & references
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