Aminoacyl-tRNA synthetase is an enyme that catalyzes the attachment of amino acids to tRNA molecules. Cells produce:

Question

BlackTom AI · Accepted Answer

Each amino acid has its own specific aminoacyl-tRNA synthetase that recognizes that amino acid and all tRNA molecules that accept it.

Relevant knowledge points:
- There are 20 standard amino acids used in protein synthesis in cells. 
- Aminoacyl-tRNA synthetases catalyze the attachment (charging) of a specific amino acid to its corresponding tRNA(s). 
- The genetic code is degenerate: 61 sense codons encode 20 amino acids, so many amino acids have multiple tRNAs (isoacceptors). Despite multiple tRNAs per amino acid, charging specificity is at the amino-acid level, not at each codon.

Step-by-step reasoning:
1. Count the standard amino acids: 20. Each requires an enzyme that recognizes that amino acid and its set of cognate tRNAs. 
2. Although there are 64 total codons (4^3 = 64), 3 are stop codons and 61 are sense codons that map to amino acids. These 61 codons correspond to the same 20 amino acids via redundancy (wobble). 
   - Calculation: total codons = 4^3 = 64; stop codons = 3; sense codons = 64 − 3 = 61.
3. Because one synthetase can charge multiple isoacceptor tRNAs for the same amino acid, you do not need one synthetase per codon or per tRNA. Therefore the required number of distinct synthetases equals the number of amino acids: 20.

Why the correct answer is correct:
- Each distinct amino acid chemistry requires a distinct active site in its synthetase to ensure fidelity. Thus 20 different synthetases are produced to recognize the 20 amino acids.

Why other choices are incorrect:
- "1 type" is impossible because one enzyme could not selectively attach 20 chemically different amino acids.
- "64 types" would imply a separate enzyme for every codon, ignoring degeneracy and wobble; unnecessary and not how biology organizes charging.
- "128 types" is unrelated and exceeds the number of codons or amino acids.

Conclusion: Cells produce 20 types of aminoacyl-tRNA synthetases, which matches the provided correct answer.

Aminoacyl-tRNA synthetase is an enyme that catalyzes the attachment of amino acids to tRNA molecules. Cells produce:单项选择题

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