There is a 5' untranslated region (UTR) before the protein coding region, and a 3' UTR after the protein coding region. The coding sequence (the RNA region that codes for the protein) occurs in the middle. The stop codon is usually not (I'm tempted to say never) the final sequence before the poly-A-tail. The translational stop site is always the first stop codon to occur in-frame with the translational start site in the mature mRNA. However, for reasons that are still not entirely understood, in about 5% of genes the first AUG is skipped, and translation starts at one of the other AUG sequences. The translational start site is usually the first (5' most) AUG. However, only one "AUG" instance serves as the translational start site, and only one instance of the stop codon sequence serves as the translational stop site. Both the pre-mRNA and the mature-mRNA can, and usually do, contain multiple instances of all of these sequences. The start codon has the sequence "AUG", and the stop codon has the sequence "UAG", "UAA", or "UGA". The last letter of our codon is C, which means the corresponding amino acid must be in the second line of the first row.Whether or not there are multiple start and stop codons depends on what you mean by "start codon" and "stop codon". This means the amino acid could be PHE or LEU.įinally we look at the right hand side of the table “Third letter”.
The second letter of our codon is U, which is the first column across. Next, we look at the four columns at the top of the table “Second letter”. Since U is the first letter of our codon, the corresponding amino acid must be in the first row (U). We start at the left hand side of the table “First letter”. The amino acid coded for UGG is TRP (tryptophan - you don’t need to know the proper name of the amino acid ).Įxample #2: Let’s use the codon UUC as an example. The last letter of our codon is G, which means the corresponding amino acid must be in the fourth line of the first row. This means the amino acid could be CYS or TRP.įinally, we look at the right hand side of the table “Third letter”. The second letter of our codon is G, which is the fourth column across. Next we look at the four columns at the top of the table “Second letter”. Here there are four rows labelled U, C, A, and G. Methionine (Met) : Amino acid that is always at the beginning of every polypeptide chain.Įxample #1: Let’s begin with the codon UGG as an example.
#MRNA START CODON CODE#
Stop codon : Codon that does not code for any amino acid, but stops translation and releases the polypeptide chain from the ribosome.Ĭodon table : Graphically organises all 64 codons according to their corresponding amino acid.
Start codon : Codon that codes for the amino acids methionine (Met), and initiates translation. Uracil : Nitrogen base that pairs with adenine in RNA.Ĭodon : Three consecutive bases on an mRNA strand.Īnticodon : Three consecutive bases on a tRNA strand. Ribosome : Organelle located in the cytoplasm, responsible for using mRNA as the template to make a polypeptide chain. TRNA : The type of RNA that carries amino acids to the ribosome during protein synthesis. MRNA : The type of RNA that carries instructions from DNA in the nucleus to the ribosome in the cytoplasm during protein synthesis. Translation : Process that uses mRNA as a template to make a specific polypeptide chain.
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