Question

Given below are two statements: Statement (I): $20^{146}$ amino acid sequences are possible in a polypeptide, which is 146 amino acids long. Statement (II): Four triplets do not code for any amino acid. In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

Remember the three stop codons: UAA, UAG, and UGA.
Since only these three function as stop signals in the standard genetic code, only three codons are non-coding.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Polypeptide diversity is governed by the combinations of the 20 standard amino acids.
The genetic code consists of 64 triplet codons, which code for amino acids or act as translation termination signals.

Step 2: Detailed Explanation:
Let us evaluate each statement independently.
- Statement (I) states that there are \( 20^{146} \) possible sequences for a polypeptide of 146 amino acids.
For a polypeptide of length \( n \), with 20 different possible amino acids at each position, the total possible unique sequences is:
\[ \text{Total sequences} = 20^n \]
For \( n = 146 \), the value is indeed \( 20^{146} \).
Thus, Statement (I) is correct.
- Statement (II) states that four triplets do not code for any amino acid.
The genetic code contains 64 triplet codons.
Among these, 61 codons are coding codons that specify amino acids.
The remaining 3 codons are stop codons: UAA, UAG, and UGA.
These three stop codons do not code for any amino acid; instead, they signal the termination of translation.
Therefore, only three triplets (not four) do not code for any amino acid.
Thus, Statement (II) is incorrect.

Step 3: Final Answer:
Statement (I) is correct but Statement (II) is incorrect, corresponding to option 3.