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26.13: Genetic Code

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  • The Human Genome Project started in 1990 with the lofty goal of sequencing the complete set of human DNA. This project was completed in April 2003, ahead of schedule and under the budget set for it (a rare occurrence for a government project). With this knowledge, we can now identify genetic disorders quickly and personalize treatment for many diseases. However, much work still remains to fully understand the connections between specific DNA sequences and specific diseases.

    The Genetic Code

    Each particular organism contains many protein molecules that are specific to that organism. The particular base sequence of DNA is responsible for the production of all of the different proteins that are present in each and every living thing that has ever inhabited the Earth. How does that work? Cells use the unique sequence of DNA bases to decide which proteins to synthesize. A gene is a segment of DNA that carries a code for making a specific polypeptide chain. The cell essentially decodes the DNA in order to make whatever peptides and proteins are needed by that organism.

    The genetic code works as a series of three-letter codes. Each sequence of three letters, called a triplet, corresponds to one of the twenty common amino acids. The triplets are read by the cell, one after the other, in the process of protein synthesis. The table below shows all of the possible triplets and the amino acids that result from each three-letter code.

    Table 26.13.1: DNA Triplet Codes for Amino Acids

    AAA Lys GAA Glu TAA Stop CAA Gln
    AAG Lys GAG Glu TAG Stop CAG Gln
    AAT Asn GAT Asp TAT Tyr CAT His
    AAC Asn GAC Asp TAC Tyr CAC His
    AGA Arg GGA Gly TGA Stop CGA Arg
    AGG Arg GGG Gly TGG Trp CGG Arg
    AGT Ser GGT Gly TGT Cys CGT Arg
    AGC Ser GGC Gly TGC Cys CGC Arg
    ATA Ile GTA Val TTA Leu CTA Leu
    ATG Met GTG Val TTG Leu CTG Leu
    ATT Ile GTT Val TTT Phe CTT Leu
    ATC Ile GTC Val TTC Phe CTC Leu
    ACA Thr GCA Ala TCA Ser CCA Pro
    ACG Thr GCG Ala TCG Ser CCG Pro
    ACT Thr GCT Ala TCT Ser CCT Pro
    ACC Thr GCC Ala TCC Ser CCC Pro

    The DNA code word GCA corresponds to the amino acid arginine, while the DNA code word TCG corresponds to the amino acid serine. Most amino acids are represented by more than one possible triplet code, but each triplet code yields only one particular amino acid. Three of the DNA code words (TAA, TAG, and TGA) are stop or termination code words. The translation of a DNA base sequence begins with a start code word and runs until a stop code word is reached.

    Even with only four different bases, the number of possible nucleotide sequences in a DNA chain is virtually limitless. The particular DNA sequence of a particular organism constitutes the genetic blueprint for that organism. This genetic blueprint is found in the nucleus of each cell of the organism and is passed on from parents to offspring. The incredible diversity of life on Earth stems from the differences in the genetic code of every living thing.


    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.