Genetic code

The genetic code is a mapping that biological cells use to translate sequences of three nucleotide bases, called codons or triplets, into amino acids. Nearly all living things use the same genetic code, and all use small variations of it. The code is followed repeatedly, creating many amino acids strung together into proteins.

This process is called protein biosynthesis. First, a sub-sequence of DNA called a gene is transcribed (rewritten) into RNA. An RNA is a sequence nucleotide bases. There are four types of base: adenine, guanine, cytosine and uracil. RNA is divided into groups of three bases, called codons. Each codon represents one amino acid. There are 64 possible codons. For example, the RNA sequence UUUAAACCC contains the codons UUU, AAA and CCC, each of which specifies one amino acid. So, this RNA sequence represents a protein sequence, three amino acids long. (In DNA, the base thymine takes the place of uracil.)

The standard genetic code is shown in the following tables. Table 1 shows what amino acid each of the 4³ = 64 codons specifies. Table 2 shows what codons specify each of the 20 standard amino acids involved in translation. These are called forward and reverse codon tables, respectively. For example, the codon GAU represents the amino acid asparagine (Asp), and cysteine (Cys) is represented by UGU and by UGC.

Table 1: Codon Table

This table shows the 64 codons and the amino acid each codon codes for.

		2nd base
		U	C	A	G
1st base	U	UUU Phenylalanine UUC Phenylalanine UUA Leucine UUG Leucine	UCU Serine UCC Serine UCA Serine UCG Serine	UAU Tyrosine UAC Tyrosine UAA Ochre (Stop) UAG Amber (Stop)	UGU Cysteine UGC Cysteine UGA Opal (Stop) UGG Tryptophan
	C	CUU Leucine CUC Leucine CUA Leucine CUG Leucine	CCU Proline CCC Proline CCA Proline CCG Proline	CAU Histidine CAC Histidine CAA Glutamine CAG Glutamine	CGU Arginine CGC Arginine CGA Arginine CGG Arginine
	A	AUU Isoleucine, Start AUC Isoleucine AUA Isoleucine AUG Methionine Start 1	ACU Threonine ACC Threonine ACA Threonine ACG Threonine	AAU Asparagine AAC Asparagine AAA Lysine AAG Lysine	AGU Serine AGC Serine AGA Arginine AGG Arginine
	G	GUU Valine GUC Valine GUA Valine GUG Valine, Start	GCU Alanine GCC Alanine GCA Alanine GCG Alanine	GAU Aspartic acid GAC Aspartic acid GAA Glutamic acid GAG Glutamic acid	GGU Glycine GGC Glycine GGA Glycine GGG Glycine

¹The codon AUG both codes for methionine and serves as an initiation site: the first AUG in an mRNA's coding region is where translation into protein begins.

====Table 2: Reverse Codon Table==== This table shows the 20 amino acids used in proteins, and the codons that code for each amino acid.

Ala	GCU, GCC, GCA, GCG	Leu	UUA, UUG, CUU, CUC, CUA, CUG
Arg	CGU, CGC, CGA, CGG, AGA, AGG	Lys	AAA, AAG
Asn	AAU, AAC	Met	AUG
Asp	GAU, GAC	Phe	UUU, UUC
Cys	UGU, UGC	Pro	CCU, CCC, CCA, CCG
Gln	CAA, CAG	Ser	UCU, UCC, UCA, UCG, AGU,AGC
Glu	GAA, GAG	Thr	ACU, ACC, ACA, ACG
Gly	GGU, GGC, GGA, GGG	Trp	UGG
His	CAU, CAC	Tyr	UAU, UAC
Ile	AUU, AUC, AUA	Val	GUU, GUC, GUA, GUG
Start	AUG, GUG	Stop	UAG, UGA, UAA

In classical genetics, the stop codons were given names - UAG was amber, UGA was opal, and UAA was ocher. These names were originally the names of the specific genes in which mutation of each of these stop codons was first detected. Translation starts with a chain initiation codon (start codon). But unlike stop codons, these are not sufficient to begin the process; nearby initiation sequences are also required to induce transcription into mRNA and binding by ribosomes. The most notable start codon is AUG, which also codes for methionine. CUG and UUG, and in prokaryotes GUG and AUU also work.

It is notable that the standard genetic code contains features of basic error correction. Many codons which differ by only one base codes for the same amino acid, and most often the base that differs is the last base, which happens to be the base which is most often misread in the translation process. Furthermore, amino acids which tend to occur more frequently in proteins on average tend to have more codons which code for them.

Numerous variations of the standard genetic code are found in mitochondria, energy-burning organelles that probably evolved from symbiotic bacteria. Ciliate protozoa also show some variation in the genetic code: UAG and often UAA code for Glutamine (a variant also found in some green algae), or UGA codes for Cysteine. Another variant is found in some species of the yeast Candida, where CUG codes for Serine. In some species of bacteria and archaea, a few non-standard amino acids are substituted for standard stop codons; UGA can code for selenocysteine and UAG can code for pyrrolysine. There may be other non-standard amino acids and codon interpretations that are not known.

Despite these variations, the genetic codes used by all known forms of life on Earth are very similar. Since there are many possible genetic codes that are thought to have similar utility to the one used by Earth life, the theory of evolution suggests that the genetic code was established very early in the history of life.

External Links

• Online DNA → Amino Acid Converter by bdesham