Correct option is A
The codon UGA is used to code for selenocysteine in Escherichia coli. Selenocysteine, often referred to as the 21st amino acid, is incorporated into proteins during translation, and it is encoded by the UGA codon under specific circumstances. Normally, UGA is a stop codon, but in the presence of certain machinery such as the selenocysteine insertion sequence (SECIS) element in the mRNA, UGA is reinterpreted as coding for selenocysteine. This makes selenocysteine unique as it is the only amino acid to be incorporated in response to a stop codon under these conditions.
Information Booster
- Selenocysteine is a naturally occurring amino acid derived from serine that contains selenium in place of sulfur.
- UGA is recognized as a stop codon in general translation, but in the case of selenocysteine, it is not a stop codon due to the presence of the SECIS element, which allows its translation.
- SECISThe element is a specific sequence found in the 3' untranslated region of the mRNA that facilitates the incorporation of selenocysteine by directing the translation machinery to bypass the stop signal of UGA.
- Selenocysteine plays a critical role in the function of certain enzymes, particularly those involved in redox reactions, such as selenoproteins.
- Selenocysteine incorporation is essential for the proper functioning of antioxidant defense systems in cells.
- Unlike selenocysteine, all other standard amino acids are coded by only one of the 64 codons without any ambiguity or special sequence context.
Additional Information
- UAAUAGUAAUAGincorrect and are known as stop codons in the standard genetic code and are not used for the incorporation of selenocysteine. Therefore, and are choices in this question.
- UCCincorrect is a codon that codes for the amino acid serine and does not code for selenocysteine, making it as well.
