Genetic code is a sequence of the DNA bases, adenine (A), thymine (T), guanine (G), and cytosine (C), found in the chromosomes of an organism.
A codon is a set of three bases that translates to an amino acid. Additionally, there are two special types of codons: start and stop. A start codon signals the start of an open reading frame, wherein codons should be translated into amino acids. A stop codon signals the end of an open reading frame, ceasing the translation of codons to amino acids.
For example, take the reading frame
ATGTTTCCCAAAGGGTAG; the first three bases
ATG are the start codon, which also translates to methionine (M). The next four codons
GGG translate to phenylalanine (F), proline (P), lysine (L), and glycine (G), respectively. The final three bases TAG are the stop codon. Therefore the sequence of amino acids translated from this open reading frame is
A table of all amino acids and their corresponding codons is given below:
|Amino acid||Code||DNA codons|
|Alanine||A||GCT, GCC, GCA, GCG|
|Arginine||R||CGT, CGC, CGA, CGG, AGA, AGG|
|Aspartic acid||D||GAT, GAC|
|Glutamic acid||E||GAA, GAG|
|Glycine||G||GGT, GGC, GGA, GGG|
|Isoleucine||I||ATT, ATC, ATA|
|Leucine||L||CTT, CTC, CTA, CTG, TTA, TTG|
|Proline||P||CCT, CCC, CCA, CCG|
|Serine||S||TCT, TCC, TCA, TCG, AGT, AGC|
|Threonine||T||ACT, ACC, ACA, ACG|
|Valine||V||GTT, GTC, GTA, GTG|
|Stop||*||TAA, TGA, TAG|
The sequence of amino acids translated from an open reading frame is known as a protein. A protein's amino acid sequence determines its shape, and its shape determines its function. The function of a protein gives rise to traits.
The function of a protein is measured by how it differs from the consensus sequence of its ideal form. A consensus sequence is a series of weak lowercase amino acids, and strong uppercase amino acids. If a weak amino acid is substituted, the protein's effectiveness will be lowered. Strong amino acids are essential to the function of a protein, and, if substituted, will cause the protein to be defective.
For example, take the ideal protein
MppVHpppLIHpppp; note that all instances of proline (P) are weak, meaning they are nonessential to the function of the protein. If an instance of proline is swapped with serine (S), such as in the protein
MPPVHPPPLIHPPPS, the effectiveness of the protein may be reduced, but the protein will continue to function. All other amino acids in this protein are strong, meaning that if even one is substituted, such as histidine (H) for tryptophan (W) in the protein
MPPVHPPPLIWPPPP, the protein will no longer function.