A codon is a three-letter code that is a combination of the four nucleotides of DNA or RNA. Every single codon codes to specific amino acids. Let us discuss.
The stop codon is a sequence of three nucleotides-trinucleotide that signals to stop the protein synthesis that takes place in the cytoplasm of the cell. There are three stop codons; UAA, UAG, and UGA. The stop codon is also known as the termination codon.
Let us discuss if AAA, TAA and ATG are stop codons, why UAA is a stop codon and many other related questions in this article.
Is AAA a stop codon?
The codons are present in the messenger RNA-mRNA that corresponds to different amino acids. Let us see if AAA is a stop codon.
AAA codon is not a stop codon but it codes for the amino acid lysine. During the translation process the transfer RNA-tRNA recognizes the AAA codon and gets attached to the specific UUU anticodon and forms lysine amino acid.
Is TAA a stop codon?
Each codon has a complementary trinucleotide called anticodon that is present in the transfer RNA and it attaches to the codon. Let us see if TAA is a stop codon.
TAA is considered as a stop codon in bacteria that are usually recognized by class I releasing factors RF1 and RF2. The frequency of TAA stop codon depends on the GC content on the nucleotide sequence.
The termination process is initiated by the releasing factors when they bind to the decoding site and release nascent peptides.
Is ATG a stop codon?
Ribosomes are made up of two subunits and they are either found freely in the cytoplasm or attached to the endoplasmic reticulum. Let us discuss if ATG is a stop codon.
ATG is not a stop codon but it is the start codon that codes for the methionine amino acid in eukaryotes and N-formylmethionine in bacteria. AUG is considered a translational initiation codon and the corresponding codon on mRNA is AUG. The start codon is usually by the 5’ untranslated regions.
Why is UAA a stop codon?
The central dogma of cells is the conversion of DNA to mRNA and the conversion of mRNA to protein. Let us know why UAA is called a stop codon.
UAA is a stop codon that is recognized by the release factors that bind to the A-site and an activated water molecule is used to hydrolyze the linkage between the polypeptide and tRNA. The UAA codon is called a nonsense codon and it is also named ochre.
Is lysine a stop codon?
Proteins are made up of a long chain of amino acids that are synthesized in the ribosomes. Let us see if lysine is a stop codon.
Lysine is not a stop codon, the corresponding codon is AAA. It is α-amino acid and a precursor for many proteins. Lysine is also coded by another codon AAG.
Is TGA always stop codon?
The amino acids which is necessary for the body’s development but are not synthesized in the body are called essential amino acids. Let us see if TGA is always a stop codon.
TGA is considered the real stop codon and its function increases when the GC nucleotide content is high in the DNA sequence of the organism. It is named the opal stop codon. The identification of the stop codon is simpler compared to the identification of the start codon.
These three codons; UAA, UAG, and UGA are recognized as universal stop codons. They do not code for amino acids and do not belong to the non-coding group of genes.