Stages of translation
Translation: The big picture
The genetic code
- There are different codons for amino acids
- Three “stop” codons mark the polypeptide as finished
- One codon, AUG, is a “start” signal to kick off translation (it also specifies the amino acid methionine)
Codons to amino acids
Translation: Beginning, middle, and end
- Initiation ("beginning"): in this stage, the ribosome gets together with the mRNA and the first tRNA so translation can begin.
- Elongation ("middle"): in this stage, amino acids are brought to the ribosome by tRNAs and linked together to form a chain.
- Termination ("end"): in the last stage, the finished polypeptide is released to go and do its job in the cell.
- A ribosome (which comes in two pieces, large and small)
- An mRNA with instructions for the protein we'll build
- An "initiator" tRNA carrying the first amino acid in the protein, which is almost always methionine (Met)
- Complex of small ribosomal subnit and initiator tRNA (bearing methionine) binds to 5' cap of mRNA.
- Complex scans from 5' to 3' to find the start codon (AUG).
- Initiator tRNA binds to start codon.
- Large ribosomal subunit comes together with the mRNA, initiator tRNA, and small ribosomal subunit to form the initiation complex. The initiator tRNA is positioned in the P site of the assembled ribosome.
- DNA is transcribed to make an RNA inside the nucleus. The initial RNA transcript is processed into a mature mRNA before exportation to the cytosol.
- The mRNA contains just one coding sequence (specifying one polypeptide).
- DNA is transcribed to make an mRNA in the cytosol. Ribosomes can start translating the mRNA before it is even completely transcribed. No post-transcription processing steps are necessary.
- The mRNA contains three coding sequences from three different genes, each specifying its own polypeptide.