Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
Amino acid sequences of cytochrome e and ribulose 5-phosphate epimerase from
40 organisms were chosen and phylogenetic trees were obtained for each of these
two protein families.
Determine the correctness or otherwise of the following Assertion [a] and the
Reason [r].
Assertion [a]: The two trees will not be identical
Reason [r]: The nature and frequency of mutations in the two families are
different
Options:
(A) Both [a] and [r] are true and [r] is the correct reason for [a].
(B) Both [a] and [r] are true but [r] is not the correct reason for [a].
(C) Both [a] and [r] are false.
(D) [a] is false but [r] is true.
6 ---
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2020 Q.25)**
**1. Governing Concept**:
- **Topic**: Molecular Evolution & Molecular Clock Theory
- Evaluating Assertion and Reason:
- **Assertion [a]**: Phylogenetic trees constructed from cytochrome $c$ and ribulose 5-phosphate epimerase across 40 organisms will not be strictly identical $\rightarrow$ **TRUE**. Different proteins experience different selective constraints, horizontal gene transfers, and lineage-specific substitution rates.
- **Reason [r]**: The nature, functional constraints, and frequency of amino acid substitutions (evolutionary rates) in the two protein families are different $\rightarrow$ **TRUE**.
- Cytochrome $c$ is a highly conserved mitochondrial electron carrier with strict functional constraints, while metabolic epimerases exhibit different evolutionary pressures. The divergent evolutionary rates and site-specific mutation dynamics directly explain why the resulting topologies differ.
- Thus, both [a] and [r] are true, and [r] is the correct reason for [a].
**2. Option Analysis**:
- **Option (A) is CORRECT**: Both statements are scientifically true, and the difference in mutation rates and selection pressures directly accounts for topological non-identity.
- **Option (B) is INCORRECT**: [r] directly explains [a].
- **Option (C) is INCORRECT**: Both statements are true.
- **Option (D) is INCORRECT**: [a] is true.
**3. Final Answer**:
- **Correct Choice**: **A**
Practice This in Real CBT Exam Interface
Test yourself with the official TCS-iON palette, virtual calculator, and immediate performance analytics.