Translation is the process of reading the base sequence of mRNA and converting it into an amino acid sequence (protein). The genetic code (codons), tRNA, and ribosomes work together.
Basic Knowledge
Genetic code (codons): Three consecutive bases in mRNA form a codon. The 4³=64 codons correspond to 20 amino acids and 3 stop codons (UAA, UAG, and UGA). Degeneracy means that multiple codons code for the same amino acid. The start codon is AUG (methionine).
tRNA (transfer RNA): tRNA has an anticodon (three bases complementary to a codon) and its corresponding amino acid. Aminoacyl-tRNA synthetase attaches an amino acid to tRNA (using ATP).
Stages of translation: ① Initiation: The small ribosomal subunit binds to the 5' end of mRNA and finds the start codon (AUG). The large subunit binds, and fMet-tRNA is positioned at the P site. ② Elongation: The next aminoacyl-tRNA enters the A site → peptidyl transferase (rRNA, a ribozyme) forms a peptide bond → translocation (A→P→E sites). ③ Termination: A release factor binds to the stop codon → the polypeptide chain is released.
Polyribosomes (polysomes): Multiple ribosomes simultaneously bind to one mRNA and synthesize proteins in parallel, increasing translation efficiency.
Post-translational modifications: These include phosphorylation, glycosylation, cleavage, and disulfide-bond formation. Chaperones are required for protein folding.
State the amino acid sequence encoded by mRNA sequence 5'-AUG-UUU-GAA-UAA-3' and explain the role of each codon.
Answer: AUG = start codon (methionine), UUU = phenylalanine, GAA = glutamic acid, and UAA = stop codon (does not code for an amino acid). The amino acid sequence consists of three residues: methionine-phenylalanine-glutamic acid. When the stop codon is reached, the polypeptide is released.
- Codon = three mRNA bases; 64 types; degeneracy exists
- Start codon = AUG (methionine)
- Stop codons = UAA, UAG, and UGA
- tRNA = anticodon+amino acid; aminoacyl-tRNA synthetase attaches them
- Ribosomal A, P, and E sites function in elongation and translocation
- Peptidyl transferase = peptide-bond formation (rRNA functions as a ribozyme)
- Polysome = 1 mRNA+multiple ribosomes increases efficiency
Points to Note
① The anticodon is located on tRNA and binds complementarily and antiparallel to the mRNA codon. ② The genetic code is degenerate but not ambiguous (one codon always corresponds to one amino acid). ③ Peptidyl transferase activity is a ribozyme activity performed by ribosomal RNA (rRNA), not by a protein component.
Practice
- Explain why all 20 amino acids can be encoded even though there are only 64 codons, using degeneracy.
- State the functions of the A, P, and E sites of the ribosome during elongation.
- Explain why polyribosomes increase translation efficiency.