Question

In some organisms, a normal stop codon, UGA, encodes for the amino acid?

1) leucine
2) arginine
3) isoleucine
4) selenocysteine
5) cysteine

Answer 1

The stop codon UGA typically signals the end of protein synthesis, but in some organisms, it can encode for the amino acid selenocysteine via a specific stem-loop structure in mRNA.

Step-by-step explanation:

In the genetic code, the stop codon UGA typically signals the end of protein synthesis. However, there are exceptions where UGA can encode for an amino acid. In some organisms, UGA encodes for the amino acid selenocysteine, not normally found in the standard set of amino acids used for protein synthesis. This occurs through a special mechanism where a stem-loop structure known as the SECIS element, located in the mRNA's 3' untranslated region, allows the UGA codon to be read as selenocysteine instead of as a stop signal.

Some organisms have a unique way of using the UGA codon as a stop codon to encode for the amino acid selenocysteine. Normally, UGA is a stop codon that does not code for any amino acid. However, in these organisms, a stem-loop structure called the selenocysteine insertion sequence (SECIS element) allows UGA to code for selenocysteine. This is an exception to the universal genetic code.