T HE D NA CODE The Key to Protein Synthesis

The Question of DNA DNA stores information to build proteins in sequences of nucleotides - DNA nucleotides contain one of 4 nitrogen bases A T C G - there are 20 different amino acids used to build protein Problem : How to code for 20 amino acids with only 4 nitrogen bases? Solution : Use groups of 3 nucleotides to code for each amino acid

Why are 3 nucleotides required? Using one nucleotide can only code for 4 amino acids Using pairs of nucleotides produces 16 combinations – can code for 16 amino acids Using groups of three nucleotides produces 64 different combinations – can code for all 20 amino acids - several different groups can each code for the same amino acid

T HE DNA C ODE The DNA code is: - universal to all living things -the groups of nucleotides code for the same amino acid in all living things 3 DNA nucleotides = Triplet - one triplet = one amino acid Examples – TCA = Serine CTG = Aspartic Acid

T HE C ODE CONTINUED 3 mRNA nucleotides = Codon - codon is the complement of a triplet - codon codes for the same amino acid as the triplet it is complementary to Example : DNA triplet = CTG = Aspartic Acid mRNA codon = GAC = Aspartic Acid

Triplet Codon Amino Acid Transcription Translation DNA mRNA Amino Triplet Codon Acid CTG GAC Aspartic Acid CGC GCG Alanine

T HE C ODON C HART – F ROM C ODON TO A MINO A CID Codon = UGC Cysteine Codon = CAC Histidine Codon = AAA Lysine Codon = GCG Alanine