#Sequence of mrna code#
In rare instances, start codons in the standard code may also include GUG or UUG these codons normally represent valine and leucine, respectively, but as start codons they are translated as methionine or formylmethionine. In the standard code, the sequence AUG-read as methionine-can serve as a start codon and, along with sequences such as an initiation factor, initiates translation.
Three sequences, UAG, UGA, and UAA, known as stop codons, do not code for an amino acid but instead signal the release of the nascent polypeptide from the ribosome. There are 64 different codons in the genetic code and the below tables most specify an amino acid. Different tables with alternate codons are used depending on the source of the genetic code, such as from a cell nucleus, mitochondrion, plastid, or hydrogenosome. The DNA codons in such tables occur on the sense DNA strand and are arranged in a 5′-to-3′ direction. It can also be represented in a DNA codon table. In this context, the standard genetic code is referred to as translation table 1.
The mRNA sequence is determined by the sequence of genomic DNA. The standard genetic code is traditionally represented as an RNA codon table, because when proteins are made in a cell by ribosomes, it is messenger RNA (mRNA) that directs protein synthesis. The standard RNA codon table organized in a wheelĪ codon table can be used to translate a genetic code into a sequence of amino acids.
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