Translation is the process of converting an RNA sequence into a protein.

The Genetic Code

  • how to code for 20 amino acids with only four different bases?
    • a triplet code would allow 64 different amino acids
  • Tsugita and Frankel-Conrat used point mutations in TMV to show that the code was nonoverlapping
  • Crick used acridine mutagens (proflavin) to create deletions and insertions
    • no gaps in the code
    • a specific start point
    • as three insertions or deletions could produce wild-type activity -the code must be triplet
  • Nirenberg and Matthaei - in vitro translation of poly U = polyphenylalanine
  • Khorana - used poly(UC) to show that the code has an odd number of bases (he got poly(serine-leucine))
  • Nirenberg showed that a trinucleotide could bind an aminoacyl-tRNA in vitro - he and Khorana then broke the code
    • the code is degenerate
      • there is more than one codon for an amino acid (up to six different codons for some amino acids) except for methionine and tryptophan
    • there are special start and stop codons
      • AUG codes for methionine and is also the start codon
      • UAA, UAG and UGA don't code for amino acids and are the stop codons (a special protein, releasing factor, binds to them and stops translation)
  • The complete sequence of the phage MS2 and of the proteins coded for by the MS2 genes confimed the genetic code
  • The code is almost universal, with only a few minor variations, particulary in mitochondria


  • tRNA is the translator
  • aminoacyl synthetases puts amino acids on one end of the tRNA
    • if the amino acid is changed after it is linked to the tRNA the new amino acid will be inserted as if it were the original amino acid
      • cysteine-tRNACys treated with nickel hydride gives alanine-tRNACys
      • proteins made with this tRNA have alanine where cysteine should be
  • each tRNA has an anticodon that can base pair with the codon
  • code is degenerate
    • Crick's wobble hypothesis - the first base base of the anticodon can form unusual base pairs (G to U for instance), also unusual bases such as inosine can pair with more than one nucleotide
  • some amino acids have more than one tRNA
    • only about 32 different tRNA's
  • the aminoacyl synthetases recognize various characteristics of the tRNA molecule
  • three codons have no tRNA - the stop codons
    • the stop codons are recognized by protein releasing factors that terminate translation
    • mutations that create premature stops can be suppressed by a mutation in a tRNA that creates a new tRNA that can recognize the stop codon
      • UAG = amber, UGA = opal, UAA = ochre
  • Try and answer this question about translation at the Biology Project


  • Ribosomes - the site of protein synthesis
    • two subparticles - 30S and 50S in prokaryotes, 40S and 60S in eukaryotes
      • 30S = 16SrRNA + 21 proteins
      • 50S = 23SrRNA + 5S rRNA + 34 proteins
    • 30S + 50S gives a 70S ribosome; in eukaryotes 40S + 60S gives an 80S ribosome
    • self-assembly
  • multiple ribosomes can traslate from one mRNA at the same time (polysomes)
  • initiation
    • initiation factors + 30S + mRNA + GTP + fMet-tRNA
    • requires ribosome binding sites on the mRNA (Shine-Dalgarno sequence 5'-AGGAGGU-3'
    • an AUG (sometimes GUG) downstream of the Shine-Dalgarno sequence is the first codon used
  • elongation
    • mRNA is read 5' to 3'
    • aminoacyl tRNA binds at the A site of the ribosome
    • peptidyl transferase makes a peptide bond between the amino acid at the P site and the one in the A site, releasing the peptide from the P site
    • the ribosome translocates 3 nucleotides moving the peptide chain to the P site using one GTP for energy
  • termination
    • a protein release factor recognizes a stop codon in the A site and releases the peptide chain from the tRNA at the P site

Variations in Transcription and Translation in Prokaryotes and Eukaryotes

  • Eukaryotic mRNA is heavily processed, unlike prokaryotic mRNA
    • the initial transcript is called hnRNA (heterogenous RNA) because it was originally this unknown RNA found with a large range of sizes only in the nucleus
    • the hnRNa is has sequences removed from each end and then a 5' cap (methyl guanasine added 5' to 5') and a poly A tail (a string of about 300 As added to the 3' end of the RNA) are added
    • introns are then spliced out - various stretches of RNA sequence are cut out of the transcript and then the remaining exons are spliced back together, the introns are presumabely degraded
    • the mature mRNA is then exprted to the cytoplasm for translation
    • Here is a more complete description of the discovery of introns, from the "DNA from the Beginning" site
    • Try answering these quesions about eukaryotic transcription from the Biology Project: question1, question2, question3, question4, question5, question6, question7




Pribnow's Box


RNA Polymerase

One Type

Three Types

Pol I - rRNA
Pol II - mRNA
Pol III - tRNA

Product of Transcription

mRNA, no cap or tail

hnRNA which is modified with Methyl Guanosine Cap & Poly A Tail


Very rare

Very common


Starts while the mRNA is still being synthesized (no nucleus)

Only in the cytoplasm on processed mRNAs




Start Codon

AUG (f-met)

AUG (met)

Initiation Sequences

Shine-Delgarno sequences

Weak consensus sequence


Polycistronic possible

need cap for initiation

Origin of Life

This document is maintained by: Jeff Bell
Last Update: Friday, March 1, 2002