Start Codon
The Central dogma of molecular biology describes the process of translation of a gene to a protein. Specific sequences of DNA act as a template to synthesize mRNA.
The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and a modified Met (fMet) in prokaryotes. The most common start codon is AUG.
The start codon is almost always preceded by an untranslated region 5' UTR. In prokaryotes this includes the ribosome binding site.
Alternate start codons (non ATG) are very rare in eukaryotic genomes. Mitochondrial genomes and prokaryotes use alternate start codons more significantly (mainly GUG and UUG). For example E. coli uses 83% ATG (AUG) (3542/4284), 14% (612) GTG (GUG), 3% (103) TTG (UUG) and one or two others (e.g., an ATT and possibly a CTG). Bioinformatics programs usually allow for alternate start codons when searching for protein coding genes.
Note that these alternate start codons are still translated as Met when they are at the start of a protein (even if the codon encodes a different amino acid otherwise). This is because a separate transfer RNA (tRNA) is used for initiation.
Well-known coding regions that do not have ATG initiation codons are those of lacI (GTG) and lacA (TTG) in the E. coli lac operon.
| nonpolar | polar | basic | acidic | (stop codon) |
| 1st base |
2nd base | 3rd base |
|||||||
|---|---|---|---|---|---|---|---|---|---|
| U | C | A | G | ||||||
| U | UUU | (Phe/F) Phenylalanine | UCU | (Ser/S) Serine | UAU | (Tyr/Y) Tyrosine | UGU | (Cys/C) Cysteine | U |
| UUC | UCC | UAC | UGC | C | |||||
| UUA | (Leu/L) Leucine | UCA | UAA | Stop (Ochre) | UGA | Stop (Opal) | A | ||
| UUG | UCG | UAG | Stop (Amber) | UGG | (Trp/W) Tryptophan | G | |||
| C | CUU | CCU | (Pro/P) Proline | CAU | (His/H) Histidine | CGU | (Arg/R) Arginine | U | |
| CUC | CCC | CAC | CGC | C | |||||
| CUA | CCA | CAA | (Gln/Q) Glutamine | CGA | A | ||||
| CUG | CCG | CAG | CGG | G | |||||
| A | AUU | (Ile/I) Isoleucine | ACU | (Thr/T) Threonine | AAU | (Asn/N) Asparagine | AGU | (Ser/S) Serine | U |
| AUC | ACC | AAC | AGC | C | |||||
| AUA | ACA | AAA | (Lys/K) Lysine | AGA | (Arg/R) Arginine | A | |||
| AUG | (Met/M) Methionine | ACG | AAG | AGG | G | ||||
| G | GUU | (Val/V) Valine | GCU | (Ala/A) Alanine | GAU | (Asp/D) Aspartic acid | GGU | (Gly/G) Glycine | U |
| GUC | GCC | GAC | GGC | C | |||||
| GUA | GCA | GAA | (Glu/E) Glutamic acid | GGA | A | ||||
| GUG | GCG | GAG | GGG | G | |||||
- A The 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.
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