The big idea: During elongation the ribosome joins amino acids with peptide bonds, then moves one codon along the mRNA, again and again.
The points to remember
- Two tRNAs sit in the ribosome at once, each on its codon.
- The ribosome joins their amino acids with a peptide bond.
- The ribosome then moves one codon (three bases) along the mRNA.
- The first tRNA, now without its amino acid, leaves.
- The next tRNA arrives, and the polypeptide grows by one amino acid each time.
Remember it as: Bond, shift, release, repeat: one codon, one amino acid.
Real example: speed. A ribosome in a bacterium such as E. coli adds about 20 amino acids every second, one codon at a time.
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Many ribosomes, one mRNA: Several ribosomes can move along the same mRNA at once, one behind another.
A ribosome that started earlier has moved further along the mRNA, so it has added more amino acids. That is why the chains are different lengths: the longest chain is on the ribosome that has travelled furthest.
Not different proteins: The chains are the same polypeptide at different stages. When each ribosome reaches the end, all the chains are the same length.
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How it is asked: A diagram of ribosomes on an mRNA, with a few marks on why the chains differ, or the steps of elongation.
Outline how a ribosome adds amino acids to a growing polypeptide.
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