Elongation at Higher Level: The same statement as SL. At HL the same cycle is described with the ribosome's A, P and E sites, on the initiation page.
Practise this as you read
- Outline how amino acids are added to a polypeptide.
- Explain why chains on one mRNA differ in length.
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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: puromycin. The antibiotic puromycin looks like the end of a tRNA carrying an amino acid. The ribosome joins it to the chain, but nothing can be added after it, so elongation stops early.
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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.
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.
An electron micrograph shows several ribosomes on one mRNA molecule, each with a polypeptide chain of a different length. Explain why the chains are different lengths.
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