Molecular genetics: Which stop codon is reassigned to encode the 21st amino acid, selenocysteine, when the specialized SECIS machinery is present?

Introduction / Context:
In the standard genetic code, three triplets signal termination (UAA, UAG, UGA). Biology makes regulated exceptions for rare amino acids. Selenocysteine (Sec), considered the 21st amino acid, is co-translationally inserted at certain stop codons under special sequence and protein factor control.

Given Data / Assumptions:

• The cell has the SECIS element (selenocysteine insertion sequence) and associated protein factors.
• We are asked which stop codon is recoded to Sec under these conditions.

Concept / Approach:

Selenocysteine incorporation requires a SECIS element in the mRNA, a specialized tRNA^Sec, and factors that redirect a stop signal into an amino acid insertion event. Of the three stops, UGA is the one reassigned to encode Sec in bacteria and eukaryotes (with differences in SECIS placement and protein factors).

Step-by-Step Solution:

Verification / Alternative check:

Known selenoproteins (e.g., glutathione peroxidases, thioredoxin reductases) contain in-frame UGA codons in their genes, which are read as Sec only with SECIS and selenocysteine-specific factors. Without them, translation terminates at UGA as a stop.

Why Other Options Are Wrong:

• UAA and UAG: remain classic stops; UAG is reassigned for pyrrolysine in specific organisms, not for selenocysteine.
• AGA: not a stop codon; it encodes arginine in the universal code.

Common Pitfalls:

• Confusing Sec (UGA) with Pyl (UAG).
• Thinking reassignment is universal; it is context-dependent and requires SECIS and specialized factors.

Final Answer:

UGA