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Translation - the basics
• Synthesis of protein on an mRNA template
• Sequence of amino-acids in protein (primary
structure) determined by sequence of bases in
• Uses a triplet code: 3 bases (a codon) code for 1
amino acid
• Requires a complex assembly of proteins and
rRNAs (the ribosome) plus many other factors
Ingredients for translation
• mRNA provides the sequence of codons that determines
protein sequence
• Ribosomes are complex protein+RNA structures which
catalyse protein synthesis
• tRNA - small RNA molecules that bring together codons
on the mRNA and corresponding amino acids
• Aminoacyl tRNA synthetase enzymes join tRNA and
amino acid
• Protein factors for initiation, elongation, and termination of
protein synthesis
3 stages of the translation process
• Initiation - mRNA, small subunit of ribosome and tRNA
charged with 1st amino-acid (methionine) come together,
then joined by ribosome large subunit and tRNA with 2nd
amino acid
• Elongation - next aminoacylated tRNA brought into
sequence; peptide bond formed to previous amino-acid;
move ribosome along mRNA to next position
• Termination - when a stop-codon is reached, there is no
corresponding tRNA; release factors cut protein from last
tRNA and ribosome is recycled for translation of another
The genetic code
• Codons with 3 bases are enough to code for all 20
amino acids (43 = 64 but 42 = only 16)
• Code has no “punctuation” so it’s vital to start
“reading” at exactly correct place
• Code is universal - same in all organisms (with
very minor exceptions)
• Code is degenerate - more than 1 codon for most
amino acids (“synonymous” codons)
• 3rd base in codon can often mutate without
changing the amino acid (“silent” mutation)