INI-CET 2012 — Biochemistry

Q: In humans, the genetic code is represented by a code of three nucleotides. If one amino acid is coded by more than one triplet, then this is known as?

Degeneracy. ### Explanation **1. Why "Degeneracy" is the Correct Answer:** The genetic code consists of 64 possible codons (triplets) that encode for only 20 standard amino acids. Because there are more codons than amino acids, most amino acids are specified by multiple codons. This phenomenon is termed **Degeneracy** (or Redundancy). * **Mechanism:** Degeneracy primarily occurs at the **third position** of the codon (the 3' end), often explained by the **Wobble Hypothesis**. For example, Leucine is coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This provides a protective mechanism against mutations, as a change in the third nucleotide may still result in the same amino acid (Silent Mutation). **2. Why Other Options are Incorrect:** * **Ambiguity:** This would mean one codon codes for multiple different amino acids. The genetic code is **non-ambiguous**; each specific codon always codes for only one specific amino acid. * **Frame-shift Mutation:** This occurs when the addition or deletion of nucleotides (not in multiples of three) shifts the reading frame, altering all subsequent amino acids. It is a type of mutation, not a property of the code itself. * **Mutation:** This is a general term for any permanent change in the DNA sequence. While degeneracy helps mitigate the effects of mutations, it is not a synonym for the process. **3. High-Yield Clinical Pearls for NEET-PG:** * **Universal Code:** The genetic code is nearly universal across all species. **Exception:** Human mitochondrial DNA (e.g., UGA codes for Tryptophan instead of a Stop codon). * **Non-overlapping & Comma-less:** The code is read sequentially from a fixed starting point without skipping any nucleotides. * **Initiation Codon:** **AUG** (Methionine). In prokaryotes, it codes for N-formylmethionine. * **Stop Codons (Nonsense Codons):** UAA (Ochre), UAG (Amber), UGA (Opal). These do not code for any amino acid. [Practice more Biochemistry PYQs on OnCourse]

2 Previous Year Questions with Answers & Explanations

Questions

All Subjects Biochemistry (2)Forensic Medicine (1)Microbiology (1)Pharmacology (1)Physiology (1)

Q1Easy

Same amino acid is coded by multiple codons due to:

Q2Easy

In humans, the genetic code is represented by a code of three nucleotides. If one amino acid is coded by more than one triplet, then this is known as?

INI-CET 2012 - Biochemistry INI-CET Practice Questions and MCQs

Question 1: Same amino acid is coded by multiple codons due to:

A. Degeneracy (Correct Answer)
B. Frame-shift mutation
C. Transcription
D. Mutation

Explanation: ### Explanation **Correct Option: A. Degeneracy** The genetic code consists of 64 codons, but only 20 standard amino acids. **Degeneracy** (or redundancy) refers to the phenomenon where a single amino acid is specified by two or more different codons. This occurs because the third position of the codon (the **Wobble position**) is often less specific, allowing one tRNA to recognize multiple codons. For example, Leucine is coded by six different codons. This redundancy acts as a protective mechanism against minor mutations. **Why Incorrect Options are Wrong:** * **B. Frame-shift mutation:** This is a genetic mutation caused by the insertion or deletion of nucleotides in a number not divisible by three. This shifts the reading frame, usually resulting in a completely different or non-functional protein, rather than explaining the coding mechanism. * **C. Transcription:** This is the biological process of copying a segment of DNA into RNA by the enzyme RNA polymerase. It is a step in gene expression, not a property of the genetic code itself. * **D. Mutation:** This is a general term for any change in the DNA sequence. While mutations can lead to changes in amino acids (missense) or premature stops (nonsense), they do not define the multi-codon relationship for a single amino acid. **High-Yield Clinical Pearls for NEET-PG:** * **Universal Code:** The genetic code is the same in almost all organisms, with minor exceptions (e.g., **Mitochondrial DNA**, where UGA codes for Tryptophan instead of a Stop codon). * **Non-overlapping & Commaless:** The code is read sequentially, three bases at a time, without skipping any nucleotides. * **Wobble Hypothesis:** Proposed by Francis Crick; it explains why we don't need 61 different tRNAs for 61 codons. The 5' base of the tRNA anticodon can form non-standard base pairs with the 3' base of the mRNA codon. * **Unambiguous:** While one amino acid can have many codons (Degeneracy), **one codon always codes for only one specific amino acid.**

Question 2: In humans, the genetic code is represented by a code of three nucleotides. If one amino acid is coded by more than one triplet, then this is known as?

A. Degeneracy (Correct Answer)
B. Frame-shift mutation
C. Ambiguity
D. Mutation

Explanation: ### Explanation **1. Why "Degeneracy" is the Correct Answer:** The genetic code consists of 64 possible codons (triplets) that encode for only 20 standard amino acids. Because there are more codons than amino acids, most amino acids are specified by multiple codons. This phenomenon is termed **Degeneracy** (or Redundancy). * **Mechanism:** Degeneracy primarily occurs at the **third position** of the codon (the 3' end), often explained by the **Wobble Hypothesis**. For example, Leucine is coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This provides a protective mechanism against mutations, as a change in the third nucleotide may still result in the same amino acid (Silent Mutation). **2. Why Other Options are Incorrect:** * **Ambiguity:** This would mean one codon codes for multiple different amino acids. The genetic code is **non-ambiguous**; each specific codon always codes for only one specific amino acid. * **Frame-shift Mutation:** This occurs when the addition or deletion of nucleotides (not in multiples of three) shifts the reading frame, altering all subsequent amino acids. It is a type of mutation, not a property of the code itself. * **Mutation:** This is a general term for any permanent change in the DNA sequence. While degeneracy helps mitigate the effects of mutations, it is not a synonym for the process. **3. High-Yield Clinical Pearls for NEET-PG:** * **Universal Code:** The genetic code is nearly universal across all species. **Exception:** Human mitochondrial DNA (e.g., UGA codes for Tryptophan instead of a Stop codon). * **Non-overlapping & Comma-less:** The code is read sequentially from a fixed starting point without skipping any nucleotides. * **Initiation Codon:** **AUG** (Methionine). In prokaryotes, it codes for N-formylmethionine. * **Stop Codons (Nonsense Codons):** UAA (Ochre), UAG (Amber), UGA (Opal). These do not code for any amino acid.