Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): An eukaryotic polypeptide chain containing 100 amino acid residues is coded by a gene containing more than 300 nucleotide bases.
Reason (R): Eukaryotic gene designated for a protein contain both the coding and noncoding part.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Prokaryotic genes are continuous (no introns).
Eukaryotic genes are discontinuous, containing non-coding introns that can make the gene thousands of base pairs longer than the final coding mRNA sequence.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
In genetics, the relationship between the nucleotide sequence of a gene and the amino acid sequence of the corresponding protein is governed by the genetic code.
However, gene structure differs significantly between prokaryotes and eukaryotes.

Step 2: Detailed Explanation:
- Assertion (A) evaluation: Under the triplet codon rule, 1 codon (3 nucleotides) codes for 1 amino acid. Thus, 100 amino acids would require at least 300 coding nucleotides. In eukaryotes, the actual gene length is significantly longer than 300 nucleotides. This makes Assertion (A) true.
- Reason (R) evaluation: Eukaryotic genes are "split genes" containing both coding regions (exons) and non-coding regions (introns and untranslated regions, UTRs). During transcription, the entire gene is transcribed, and introns are subsequently spliced out to form the mature mRNA. This makes Reason (R) true.
- Explanation check: The presence of these extensive non-coding introns is the direct reason why the physical gene length in eukaryotic chromosomes is much larger than the simple triplet calculation of 300 bases. Thus, (R) is the correct explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are true and (R) is the correct explanation of (A). This corresponds to option (A).