Translation is the process cells use to read an mRNA message and build a protein from amino acids. It matters because proteins carry out most cell functions, including catalyzing reactions, forming structures, and sending signals. The ribosome is the molecular machine that coordinates this process by matching mRNA codons with tRNA anticodons.
A correct translation produces a polypeptide with the amino acid sequence specified by the gene.
During translation, the ribosome moves along the mRNA in groups of three nucleotides called codons. Each tRNA carries a specific amino acid and has an anticodon that pairs with a matching codon on the mRNA. Translation begins at a start codon, continues as amino acids are linked by peptide bonds, and ends when a stop codon enters the ribosome.
The final polypeptide may then fold and be modified to become a functional protein.
Understanding Biology: Translation in Detail
Before a ribosome can add any amino acid, each tRNA must be loaded with the right one. Enzymes called aminoacyl tRNA synthetases do this job. Each enzyme recognises a particular amino acid and the tRNA molecules meant to carry it.
It uses energy from ATP to attach the amino acid to the tRNA. This charging step is a major accuracy check.
Some synthetases can remove a wrongly attached amino acid before translation continues. If the wrong amino acid is loaded, the anticodon may still match the mRNA correctly, but the growing protein will contain the wrong building block.
The ribosome has three working spaces for tRNA molecules. The A site accepts the next charged tRNA. The P site holds the tRNA carrying the growing chain.
The E site is where an empty tRNA leaves. When a matching tRNA enters the A site, the ribosome checks the codon anticodon pairing. It then forms a peptide bond and transfers the growing chain onto the new amino acid.
The ribosome shifts forward by one codon, using energy supplied by helper proteins and GTP. This repeated movement keeps the message in order and makes the chain grow one amino acid at a time.
Keeping the correct reading frame is essential. The cell begins at a start signal and groups every following nucleotide into sets of three. Adding or removing one or two nucleotides from a gene can shift this frame.
Such a frameshift changes nearly every codon after the mutation, often creating an early stop signal. Adding or removing three nucleotides does not shift the frame, though it adds or removes one amino acid. A base change can have different effects.
It may leave the amino acid unchanged because several codons can mean the same amino acid. It may replace one amino acid, or it may create a stop signal. These outcomes help explain why some DNA mutations have little effect while others cause serious disease.
Many ribosomes can read one mRNA molecule at the same time. This group is called a polysome. It allows a cell to make many copies of a needed protein quickly.
Translation is closely controlled because cells do not waste energy making every protein at all times. For example, cells may increase production of enzymes after food is available or make protective proteins during stress. Some antibiotics work by blocking bacterial ribosomes.
Bacterial ribosomes differ enough from human ribosomes for certain medicines to target bacteria more strongly. When studying translation, follow the direction of the mRNA, identify the reading frame, and remember that a codon chart uses mRNA bases. DNA uses thymine, while mRNA uses uracil.
Key Facts
- Translation converts the nucleotide sequence of mRNA into the amino acid sequence of a polypeptide.
- A codon is a 3-nucleotide mRNA sequence, and most codons specify one amino acid.
- The start codon is usually AUG, which codes for methionine.
- Stop codons are UAA, UAG, and UGA, and they do not code for amino acids.
- The number of codons in a coding sequence is total nucleotides ÷ 3.
- Peptide bonds form between amino acids as the ribosome moves from the 5' end to the 3' end of the mRNA.
Vocabulary
- Translation
- Translation is the cellular process that uses an mRNA sequence to assemble amino acids into a polypeptide.
- Ribosome
- A ribosome is a complex of rRNA and proteins that reads mRNA and catalyzes peptide bond formation.
- Codon
- A codon is a three-nucleotide sequence on mRNA that specifies an amino acid or a stop signal.
- Anticodon
- An anticodon is a three-nucleotide sequence on tRNA that base-pairs with a complementary mRNA codon.
- Polypeptide
- A polypeptide is a chain of amino acids linked by peptide bonds that can fold into a protein.
Common Mistakes to Avoid
- Reading the mRNA in the wrong direction is incorrect because ribosomes translate mRNA from 5' to 3'. This changes the codons and can produce a completely different amino acid sequence.
- Treating each nucleotide as one amino acid is incorrect because translation reads mRNA in codons of three nucleotides. One codon usually corresponds to one amino acid.
- Matching codons to identical anticodons is incorrect because codon and anticodon sequences are complementary, not the same. For example, the mRNA codon AUG pairs with the tRNA anticodon UAC.
- Assuming stop codons add amino acids is incorrect because stop codons recruit release factors instead of tRNAs. This causes the completed polypeptide to detach from the ribosome.
Practice Questions
- 1 An mRNA coding region contains 300 nucleotides, including one stop codon at the end. How many codons are present, and how many amino acids are added to the polypeptide?
- 2 Translate the mRNA sequence 5'-AUG GCU UUU UGA-3'. Use AUG = methionine, GCU = alanine, UUU = phenylalanine, and UGA = stop.
- 3 A mutation changes the start codon AUG to AUA near the beginning of an mRNA. Explain how this could affect translation and the final protein.