Start codon

From Infogalactic: the planetary knowledge core
Jump to: navigation, search

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 often preceded by a 5' untranslated region (5' UTR). In prokaryotes this includes the ribosome binding site.

Alternative start codons

Alternative start codons are different from the standard AUG codon and are found in both prokaryotes (bacteria) and eukaryotes. 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.[1]

Bioinformatics programs usually allow for alternate start codons when searching for protein coding genes.[citation needed]

Eukaryotes

Alternate start codons (non AUG) are very rare in eukaryotic genomes. However, naturally occurring non-AUG start codons have been reported for some cellular mRNAs.[2] Seven out of the nine possible single-nucleotide substitutions at the AUG start codon of dihydrofolate reductase were functional as translation start sites in mammalian cells.[3] In addition to the canonical Met-tRNA Met and AUG codon pathway, mammalian cells can initiate translation with leucine using a specific leucyl-tRNA that decodes the codon CUG.[4][5]

Mitochondrial genomes (and prokaryotes) use alternate start codons more significantly (AUA and AUU in humans and mainly GUG and UUG in prokaryotes).[citation needed]

Prokaryotes

E. coli uses 83% AUG (3542/4284), 14% (612) GUG, 3% (103) UUG [6] and one or two others (e.g., an AUU and possibly a CUG).[7][8]

Well-known coding regions that do not have AUG initiation codons are those of lacI (GUG)[9][10] and lacA (UUG)[11] in the E. coli lac operon.

Standard genetic code

nonpolar polar basic acidic (stop codon)
Standard genetic code
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[A] (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.[12]

See also

External links

  • The Genetic Codes. Compiled by Andrzej (Anjay) Elzanowski and Jim Ostell, National Center for Biotechnology Information (NCBI), Bethesda, Maryland, U.S.A.[1]

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  9. ↑ E.coli lactose operon with lacI, lacZ, lacY and lacA genes GenBank: J01636.1
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  11. ↑ NCBI Sequence Viewer v2.0
  12. ↑ Lua error in package.lua at line 80: module 'strict' not found.
Retrieved from "https://infogalactic.com/w/index.php?title=Start_codon&oldid=2204798"