polypeptide synthesis notes A2 BIOLOGY AQA

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Polypeptide synthesis ­ translation
Synthesising the polypeptide
When mRNA has passed out nuclear pore it determines synthesis of the polypeptide.
How a polypeptide is made:
1. Ribosome becomes attached to the starting codon at one end of mRNA molecule
2. tRNA molecule with complimentary anticodon sequence moves to ribosome and pairs
with starting codon sequence of mRNA. tRNA carries an AA.
3. tRNA molecule with complementary anticodon pairs with next codon on mRNA. This tRNA
molecule carries another AA.
4. Ribosome moves along mRNA, bringing together two tRNA molecules at one time, each
pairing up with the corresponding 2 codons on mRNA.
5. By enzyme and ATP, the 2 AA on tRNA are joined by peptide bond
6. Ribosome moves on to third codon of mRNA, linking the AA on second and third tRNA
molecules
7. First tRNA is released from its AA and is free to collect another AA, from the AA pool in
the cell
8. Process continues until a complete polypeptide chain is built up
9. Synthesis of a polypeptide continues until a ribosome reaches a stop codon, where the
ribosome, mRNA and last tRNA molecules separate and chain is complete
Assembling a protein
When a quaternary structure is
made, 1+ polypeptide joined.
What happens to the polypeptide
next depends upon the protein
being made, usually involving the
following:
o Polypeptide is
coiled/folded = 2nd ary structure
o Secondary structure is
folded = tertiary structure
o Different polypeptide
chains, along with introns, are
linked to form quaternary structure

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