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 MOLECULAR GENETICS

THE CHARACTERISTICS OF THE GENETIC CODE

  1. Only 61 triplets or codons code for amino acids
    3 stop codons (aka nonsense codons or terminator codons) UUA UAG UGA.
     

  2. The code is a degenerative code
    Several codons code for the same amino acid.
    The first two letters seem to be the most important the third one tends to be interchangeable

Examples
Some suggest the code consists of 2½ letters per amino acid.
This has an effect on the number of different tRNA molecules needed for protein synthesis. The anticodons of some of these molecules can line up with several different codons. The first two are critical but the third base in the codon is “wobbly”. As few as 31 different tRNAs are needed for translation.

  1. The is no punctuation between each codon.
    The reading frame is set at the beginning of the gene. Frame shift mutations can be caused by the ADDITION or DELETION of only one or two bases. Everything downstream is misread.
     

  2. The reading of mRNA is always in the same direction 5’ to 3’ (the same way as transcription and replication).
    The polypeptide chain is constructed from the amino end to the carboxyl end.
     

  3. The code is universal for all organisms. So it is very ancient.
     

  4. Similar amino acids have similar codons.
    Example
    Aspartic acid codons GAU and GAC. Glutamic acid codons GAA and GAG. Both are acidic amino acids.
     

  5. Some amino acids are chemically altered AFTER translation.
    e.g. In collogen proline is converted to hydroxyproline
    Therefore the total number of amino acids found in proteins is greater than 20 but the total used in translation is only 20.

© Paul Billiet 2008