Chapter 12 genetic code.docx

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University of Guelph
Molecular Biology and Genetics
MBG 2040
Dennis Baker

Chapter 12: The Genetic Code An overview: 1. The genetic code is composed of nucleotide triplets. Three nucleotides in mRNA specify one amino acid in the polypeptide product; thus, each codon contains three nucleotides. 2. The genetic code is non-overlapping. Each nucleotide in mRNA belongs to just one codon except in rare cases where genes overlap and a nucleotide sequence is read in two different reading frames. 3. The genetic code is comma-free. There are no commas or other forms of punctuation within the coding regions of mRNA molecules. During translation, the codons are read consecutively. 4. The genetic code is degenerate. All but two of the amino acids are specified by more than one codon.  Complete degeneracy: the third base can be changed to anything and will still encode same aa  Partial degeneracy: when the 3 base may be either of the two pyrimidines (U or C) or, alternatively either of the two purines (A or G).  Many single base-pair substitutions results in the substitution of one amino acid for another amino acid with very similar chemical properties, which still yields active products minimizing the effects of mutation. 5. The genetic code is ordered. Multiple codons for a given amino acid and codons for amino acids with similar chemical properties are closely related, usually differing by a single nucleotide. 6. The genetic code contains start and stop codons. Specific codons are used to initiate and to terminate polypeptide chains. 7. The genetic code is nearly universal. With minor exceptions, the codons have the same meaning in all living organisms, from viruses to humans.  In both prokaryotes and e
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