Amino Acids

Step 1: Understanding selenocysteine.
Selenocysteine is often referred to as the 21st amino acid and is used by some organisms in protein synthesis. It is chemically similar to cysteine, but with selenium replacing sulfur.
Step 2: Analyzing the options.
(1) Cysteine: Correct — Selenocysteine is biosynthesized from cysteine by replacing sulfur with selenium.
(2) Serine: This is incorrect. Serine is involved in the synthesis of other amino acids but is not directly involved in the synthesis of selenocysteine.
(3) Threonine: This is incorrect. Threonine is a separate amino acid and not a precursor for selenocysteine.
(4) Proline: This is incorrect. Proline does not play a direct role in the biosynthesis of selenocysteine.
Step 3: Conclusion.
The correct answer is (1) Cysteine, as selenocysteine is biosynthesized from cysteine by substituting selenium for sulfur.