2.6.2 Translation

• Translation is the synthesis of polypeptides on ribosomes
• This stage of protein synthesis occurs in the cytoplasm of the cell
• After leaving the nucleus, the mRNA molecule attaches to a ribosome
• A ribosome is a complex structure that is made of a large and small subunit
  • Ribosomes are themselves made of proteins and RNA (called ribosomal RNA or rRNA)
• There are binding sites on the subunits for the various other molecules involved in translation

A ribosome is built of large and small subunits, ribosomal RNA and an area on the surface that catalyses the formation of peptide bonds in a newly-synthesised protein

• The amino acid sequence of polypeptides is determined by mRNA according to the genetic code
• mRNA varies in length, depending on the length of the gene, but is around 2,000 nucleotides long, on average (in mammals)
• Only certain genes are transcribed in a particular cell, depending on the function of that cell
  • The gene for rhodopsin (a light-sensitive protein in the eye) is transcribed to mRNA in retina cells, but not transcribed in other body cells where rhodopsin is not required; that would be a waste of cellular energy

• Codons of three bases on mRNA correspond to one amino acid in a polypeptide
• The four nucleotide bases in mRNA are not enough to code for 20 separate amino acids
• Pairs of nucleotides would only give 16 combinations (4² = 16), which is still not enough
• Triplets of nucleotides would yield 64 combinations (4³ = 64), which is more than enough
• Different triplets code for the same amino acid, giving some protection against mutation
  • A triplet is a sequence of three DNA bases that codes for a specific amino acid
  • A codon is a sequence of three mRNA bases that codes for a specific amino acid
  • A codon is transcribed from the triplet and is complementary to it

• An anticodon is a sequence of three transfer RNA (tRNA) bases that are complementary to a codon
  • The transfer RNA carries the appropriate amino acid to the ribosome
  • The amino acid can then be condensed onto the growing polypeptide chain

• Certain codons carry the command to stop translation when the polypeptide chain is complete ('Stop codons')

mRNA Codons and Amino Acids Table

Step 1: Work out the antisense sequence using A-T and C-G base pairing rules

AAG CTC GTA ATG CGG

Step 2: Work out the mRNA codons, complementary to the antisense strand

UUC GAG CAU UAC GCC

Step 3: Use the mRNA Codons and Amino Acids Table (above) to work out the first amino acid

First base in codon = U, second base = U, third base = C

So we're looking in the top-left box of the table; this amino acid is Phe

Step 4: Repeat for the remaining 4 codons

GAG = Glu

CAU = His

UAC = Tyr

GCC = Ala

Answer: The final sequence of amino acids is Phe-Glu-His-Tyr-Ala

• Translation depends on complementary base pairing between codons on mRNA and anticodons on tRNA
• In the cytoplasm, there are free molecules of tRNA (transfer RNA)
• The tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome
• The triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet (codon) on the mRNA molecule
• Two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side
• A peptide bond is then formed (by condensation) between the two amino acids
  • The formation of a peptide bond between amino acids is an anabolic reaction
  • It requires energy, in the form of ATP
  • The ATP needed for translation is provided by the mitochondria within the cell
• This process continues until a ‘stop’ codon on the mRNA molecule is reached – this acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete
• This amino acid chain then diffuses away from the ribosome and forms the final polypeptide

The translation stage of protein synthesis – tRNA molecules bind with their specific amino acids

The translation stage of protein synthesis – an amino acid chain is formed

An polypeptide forms as peptide bonds are added in sequence