Process of Protein Synthesis – transcription and translation

Transcription – DNA base sequence to mRNA base sequence
• The ‘code’ for the protein is carried by one of the DNA strands in the gene.
• An enzyme separates the two DNA strands at the gene locus exposing the gene sequence.
• A complementary copy - mRNA - is made of the gene sequence –
o new nucleotides form a complementary RNA strand
o using the DNA gene sequence strand as a ‘master’
o the enzyme RNA polymerase links the new nucleotides forming mRNA.
• Uracil (U) is the complementary base to adenine in RNA, thymine (T) is not found in RNA.
• The complementary RNA copy is called messenger RNA (mRNA).
• The mRNA separates from the DNA strand and passes from the nucleus to the cytoplasm.
Translation – mRNA base sequence to amino acid sequence.
• A ribosome binds to the start point of the mRNA.
• The ribosome will ‘decode’ the mRNA in sets of three bases (a codon).
• Each codon specifies a particular amino acid.
• The sequence of bases on the mRNA determines the sequence of amino acids in the protein – any change = a mutation
• Two codons of the mRNA are exposed in turn.
• Two complementary tRNA molecules attach to these two mRNA triplets.
• The amino acids of the tRNA bond together (peptide bond – condensation reaction)
• The leading tRNA detaches from its amino acid and from the mRNA.
• The ribosome ‘moves’ to the next codon and another complementary tRNA attaches.
• The newly arrived complementary tRNA then adds a new amino acid.
• The process repeats, codon by codon, to the end of the mRNA (until a stop codon is reached).
• The amino acid sequence is now complete.
• The polypeptide (amino acid chain) folds giving the protein its normal functional shape.
• Primary structure = amino-acid sequence (determined by DNA sequence)
• Secondary structure = many H- bonds making

• Alpha helix (very common) or
• Beta-pleated sheet (rare – butterfly wings and silk)
• Thus affected by pH, temperature

• Tertiary structure = disulphide bridges and further H-bonds – forms active sites
• Quaternary structure (rare) – only haemoglobin (Van der Waal’s forces)