The genetic code is well described and all 64 possible codons are accounted for. How do cells insert the 'twenty first' amino acid selenocysteine?
A) It is encoded by one of the extra serine codons.
B) It is encoded by one of the extra leucine codons.
C) It is encoded by one of the extra arginine codons.
D) It is encoded by one of the stop codons.

Question

Quizplus · Accepted Answer

Selenocysteine is inserted into proteins through a process called recoding. This involves the recognition of a specific RNA structure known as a selenocysteine insertion sequence (SECIS) element, which is located in the 3' untranslated region of the mRNA. During translation, the ribosome recognizes the SECIS element and causes a selenocysteine-specific tRNA to insert the amino acid in response to a stop codon, specifically the UGA codon. This means that UGA can code for either a stop codon or selenocysteine, depending on the presence of a SECIS element in the mRNA.

The Genetic Code Is Well Described and All 64 Possible