Q.43 Selenocysteine is an amino acid, which is:
1. present in only prokaryotes
2. coded by UGA
3. derived from cysteine
4. is present in many eukaryotic proteins
Selenocysteine, the 21st amino acid, is encoded by the UGA codon (normally a stop signal) and derived from cysteine by replacing sulfur with selenium. The correct answer for this multiple-choice question is option 2, as it uniquely identifies its genetic code assignment.
Correct Answer
Option 2: coded by UGA
Selenocysteine (Sec) is incorporated via a specialized mechanism where UGA, typically a termination codon, encodes Sec in the presence of a SECIS element in the mRNA 3′ UTR. This recoding requires unique elongation factors (SelB in prokaryotes, EFsec in eukaryotes) and occurs in selenoproteins like glutathione peroxidases and thioredoxin reductases.
Term Explanations
Selenocysteine Structure
Sec mirrors cysteine but substitutes selenium (Se) for sulfur (S), yielding a selenol (-SeH) group with lower pKa (~5.2 vs. cysteine’s 8.3), enhancing nucleophilicity for redox catalysis in enzymes.
Incorporation Mechanism
Unlike standard amino acids, Sec-tRNASec is synthesized on its tRNA and inserted at UGA via ribosomal recoding, enabling its presence across domains of life in ~25 human selenoproteins.
Options Analysis
| Option | Statement | Correct? | Reason |
|---|---|---|---|
| 1 | Present in only prokaryotes | No | Found in Archaea, Bacteria, and Eukarya; ~25 eukaryotic selenoproteins exist |
| 2 | Coded by UGA | Yes | UGA recoded as Sec codon with SECIS; defines its “21st amino acid” status |
| 3 | Derived from cysteine | Partially true but incomplete | Structural analog (Se replaces S), but question seeks defining trait; not the best single answer |
| 4 | Present in many eukaryotic proteins | No | Rare; only in specific selenoproteins (e.g., GPx family), not broadly distributed |
Exam Tip
Memorize Sec as UGA-recoded, cysteine-derived, redox-active in selenoproteins. Distinguish from pyrrolysine (UAG-recoded, 22nd AA). Practice codon-expansion questions for NEET molecular biology.
