The Process of Protein Synthesis
This topic covers the fundamental process of protein synthesis, explaining how the genetic information encoded in a DNA sequence of nucleotide bases is translated into a specific sequence of amino acids to build a functional protein.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Proteins are constructed from one or more polypeptide chains, which are themselves long polymers of amino acids.
- The specific sequence of amino acids in a polypeptide determines how it folds into a unique three-dimensional shape.
- This 3D structure is critical for the protein's function, for example, forming the active site of an enzyme.
- A gene is a section of DNA whose sequence of nucleotide bases dictates the sequence of amino acids in a corresponding polypeptide.
- The genetic code is read in non-overlapping groups of three bases, known as triplets. Each triplet corresponds to a specific amino acid.
Definitions
- Polypeptide
- A long, continuous, and unbranched chain of amino acids joined together by peptide bonds.
- Gene
- A specific sequence of DNA nucleotides that acts as the instruction to build a particular polypeptide or functional RNA molecule.
- Triplet
- A sequence of three consecutive nucleotide bases on a DNA or RNA molecule that codes for a specific amino acid.
Worked example
The start of a gene on a coding strand of DNA is shown below, along with a table indicating which amino acid is coded for by some DNA base triplets. DNA sequence: TAC GGT CAT | DNA Triplet | Amino Acid | |-------------|------------| | GGT | Glycine | | TAC | Tyrosine | | CAT | Histidine | | ATG | Methionine | What is the sequence of the first three amino acids in the polypeptide chain produced from this gene?
- 1
Identify the sequence of nucleotide bases from the question:
TAC GGT CAT.
- 2
Recognise that the code is read in non-overlapping triplets.
Break the sequence into its three triplets:
TAC, GGT, and CAT.
- 3
Use the provided table to translate the first triplet, TAC, into its corresponding amino acid:
Tyrosine.
- 4
Translate the second triplet, GGT, into its corresponding amino acid:
Glycine.
- 5
Translate the third triplet, CAT, into its corresponding amino acid:
Histidine.
- 6
Combine the amino acids in the correct order to determine the final sequence.
Answer: Tyrosine-Glycine-Histidine
Common mistakes
- ×Misreading diagrams of DNA strands, for instance by reading the bases on the wrong strand (e.g., template instead of coding) or in the wrong direction.
- ×Incorrectly splitting the base sequence into triplets, for example by starting from the second base instead of the first, leading to an entirely wrong 'reading frame'.
- ×Making a simple matching error when using the provided table to convert a triplet to an amino acid.
No-calculator tips
- ✓Protein synthesis questions are about careful decoding. Cover the DNA sequence with a piece of paper and reveal only three bases at a time to ensure you correctly identify each triplet.
- ✓Before answering, quickly double-check that the number of triplets you have identified matches the number of amino acids in your final polypeptide chain.
- ✓If a diagram is complex, use your pen to trace the specific DNA strand mentioned in the question to avoid being distracted by other information.