Combinatorics of the code: With four RNA bases taken three at a time, how many distinct codons exist in the standard genetic code?
Biochemistry
Genetic Code and Regulation
Difficulty: Easy
Choose an option
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A3
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B20
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C64
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DAn infinite number
Answer
Correct Answer: 64
Explanation
Introduction / Context:Codons are triplets of nucleotides formed from four bases (A, U, G, C). Counting possible combinations is a basic question linking combinatorics and molecular biology.
Given Data / Assumptions:
- Four symbols (A, U, G, C).
- Ordered triplets (position matters).
- Repetition allowed.
Concept / Approach:The number of ordered triplets with repetition from a set of four is 4^3. This yields the total number of possible codons recognized by the translation machinery.
Step-by-Step Solution:
Compute combinations with order and repetition: 4^3.Calculate: 4^3 = 4 * 4 * 4 = 64.Interpretation: 61 sense codons for amino acids plus 3 stop codons in the standard code.Verification / Alternative check:Genetic code tables list 64 entries, confirming the combinatorial calculation.
Why Other Options Are Wrong:
- 3: confuses number of bases with number of codons.
- 20: number of standard amino acids, not codons.
- Infinite: the triplet code yields a finite set of combinations.
Common Pitfalls:Mixing up amino acid count with codon count; forgetting that multiple codons can encode the same amino acid (degeneracy).
Final Answer:64