Question

Given below are two statements, one is labeled as Assertion (A) and other one labeled as Reason (R).
Assertion (A) : The genetic code is not universal.
Reason (R) : In mammalian mitochondria, the codon 'UGA' specifies the amino acid 'Trp'
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:

Mitochondria contain their own circular DNA and protein synthesis machinery.
Due to their evolutionary history, they have developed several minor variations in the genetic code, such as reassigning the standard stop codon UGA to code for Tryptophan (Trp).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
For many years, the genetic code was considered completely universal, meaning that a given codon specifies the exact same amino acid in all living organisms, from bacteria to humans.
However, research has revealed several exceptions to this rule, particularly in mitochondrial genomes and some unicellular organisms.

Step 2: Detailed Explanation:
- Assertion (A) is true:
While the standard genetic code is highly conserved across the tree of life, it is not completely universal.
There are several documented exceptions where specific codons have been reassigned to code for different amino acids or stop signals.
- Reason (R) is true:
In the standard (canonical) genetic code used in the cytoplasm of eukaryotic cells, the codon UGA is a termination (stop) codon that signals the end of translation.
However, in mammalian mitochondria, the genetic machinery has evolved an altered coding system.
In mammalian mitochondria, the UGA codon does not function as a stop codon; instead, it specifically codes for the amino acid Tryptophan (Trp).
Other mitochondrial deviations include AUA coding for Methionine (instead of Isoleucine) and AGA/AGG acting as stop codons (instead of Arginine).
Because these mitochondrial alterations are direct, concrete examples of codon reassignment, the fact that UGA specifies Trp in mitochondria (R) is the correct explanation for why the genetic code is not completely universal (A).

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A).