Translation and the Genetic Code
Decoding Information into Protein
The Genetic Code
The genetic code maps mRNA codons to amino acids. It is redundant but nearly universal, with start and stop codons defining reading frames and translation boundaries. Codon usage can influence translation efficiency and accuracy.
Reading Frame
A shift in reading frame can change every downstream codon, often producing a completely different and usually nonfunctional protein.
Ribosome Function
The ribosome is a ribonucleoprotein machine that aligns mRNA and tRNAs, catalyzes peptide bond formation, and moves along the transcript through repeated elongation cycles. tRNAs bring amino acids according to codon-anticodon pairing.
Translation Stages
| Stage | Main Events |
|---|---|
| Initiation | Start codon recognition and assembly of the initiation complex |
| Elongation | Aminoacyl-tRNA entry, peptide bond formation, translocation |
| Termination | Stop codon recognition and polypeptide release |
Which codon usually serves as the start codon in standard translation?
AUG typically encodes methionine and initiates translation.
Correct answer: AUG
Why is the genetic code described as redundant?
Multiple codons can specify the same amino acid, which helps buffer some mutations.
Correct answer: Because most amino acids are encoded by more than one codon.
Key Translation Players
mRNA
- Template read by ribosome
- Contains codons
tRNA
- Carries amino acids
- Uses anticodons to match codons
Translation Cycle
-
1
Step 1: Initiation factors assemble the ribosome at the start codon.
-
2
Step 2: The first tRNA binds and the large subunit joins.
-
3
Step 3: Aminoacyl-tRNAs enter the A site during elongation.
-
4
Step 4: Peptide bonds form and the ribosome translocates.
-
5
Step 5: A stop codon triggers release and ribosome recycling.
What is the most direct consequence of a nonsense mutation?
A nonsense mutation introduces a stop codon too early, truncating translation.
Correct answer: A premature stop codon
How can codon usage bias affect protein expression?
tRNA availability and translational kinetics influence expression levels.
Correct answer: Preferred codons can be translated more efficiently, affecting the amount of protein produced.