Understanding the Genetic Code

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Dissecting the Genetic Code

Genetic Code Table
Genetic Code Table. Darryl Leja, NHGRI

The genetic code is the sequence of nucleotide bases in nucleic acids (DNA and RNA) that code for amino acid chains in proteins. DNA consists of the four nucleotide bases: adenine (A), guanine (G), cytosine (C) and thymine (T). RNA contains the nucleotides adenine, guanine, cytosine and uracil (U). When three continuous nucleotide bases code for an amino acid or signal the beginning or end of protein synthesis, the set is known as a codon. These triplet sets provide the instructions for the production of amino acids. Amino acids are linked together to form proteins.


RNA codons designate specific amino acids. The order of the bases in the codon sequence determines the amino acid that is to be produced. Any of the four nucleotides in RNA may occupy one of three possible codon positions. Therefore, there are 64 possible codon combinations. Sixty-one codons specify amino acids and three (UAA, UAG, UGA) serve as stop signals to designate the end of protein synthesis. The codon AUG codes for the amino acid methionine and serves as a start signal for the beginning of translation. Multiple codons may also specify the same amino acid. For example, the codons UCU, UCC, UCA, UCG, AGU, and AGC all specify serine. The RNA codon table above lists codon combinations and their designated amino acids. Reading the table, if uracil (U) is in the first codon position, adenine (A) in the second, and cytosine (C) in the third, the codon UAC specifies the amino acid tyrosine. The abbreviations and names of all 20 amino acids are listed below.

Amino Acids

Ala: Alanine               Asp: Aspartic acid    Glu: Glutamic acid    Cys: Cysteine
Phe: Phenylalanine   Gly: Glycine          His: Histidine            Ile: Isoleucine
Lys: Lysine                Leu: Leucine           Met: Methionine       Asn: Asparagine
Pro: Proline               Gln: Glutamine        Arg: Arginine            Ser: Serine
Thr: Threonine          Val: Valine               Trp: Tryptophan        Tyr: Tyrosine

Protein Production

Proteins are produced through the processes of DNA transcription and translation. The information in DNA is not directly converted into proteins, but must first be copied into RNA. DNA transcription is the process in protein synthesis that involves the transcribing of genetic information from DNA to RNA. Certain proteins called transcription factors unwind the DNA strand and allow the enzyme RNA polymerase to transcribe only a single strand of DNA into a single stranded RNA polymer called messenger RNA (mRNA). When RNA polymerase transcribes the DNA, guanine pairs with cytosine and adenine pairs with uracil.

Since transcription occurs in the nucleus of a cell, the mRNA molecule must cross the nuclear membrane to reach the cytoplasm. Once in the cytoplasm, mRNA along with ribosomes and another RNA molecule called transfer RNA, work together to translate the transcribed message into chains of amino acids. During translation, each RNA codon is read and the appropriate amino acid is added to the growing polypeptide chain. The mRNA molecule will continue to be translated until a termination or stop codon is reached.


A gene mutation is an alteration in the sequence of nucleotides in DNA. This change can affect a single nucleotide pair or larger segments of a chromosomes. Altering nucleotide sequences most often results in non-functioning proteins. This is because changes in the nucleotide sequences change the codons. If the codons are changed, the amino acids and thus the proteins that are synthesized will not be the ones coded for in the original gene sequence. Gene mutations can be generally categorized into two types: point mutations and base-pair insertions or deletions. Point mutations alter a single nucleotide. Base-pair insertions or deletions result when nucleotide bases are inserted into or deleted from the original gene sequence. Gene mutations are most commonly the result of two types of occurrences. First, environmental factors such as chemicals, radiation, and ultraviolet light from the sun can cause mutations. Secondly, mutations may also be caused by errors made during the division of the cell (mitosis and meiosis).

National Human Genome Research Institute