Question

Which one of the following is not true of the genetic code?

• A. Changes in the universal genetic code have occurred in some species
• B. Modifications in the anticodon do not affect the pattern of wobble pairing
• C. Some of the codons cause termination of translation
• D. In Euplotes crassus, UGA codon specifies both selenocysteine and cysteine

Hint:

The wobble hypothesis, proposed by Francis Crick, states that the base at the 5' end of the anticodon is less spatially constrained, allowing for non-standard base pairing.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The genetic code is a set of rules used by cells to translate nucleotide sequences into proteins. While largely universal, some evolutionary variations and biochemical modifications exist.

Step 2: Detailed Explanation:
- (A) is true because non-canonical genetic codes exist in mitochondrial genomes and in some protozoans and archaea.
- (C) is true because the codons UAA, UAG, and UGA serve as stop codons that terminate translation under normal conditions.
- (D) is true because in the ciliate Euplotes crassus, the UGA codon has dual coding capacity, encoding either selenocysteine or cysteine depending on the ribosomal context and mRNA structural elements.
- (B) is false because chemical modifications at the wobble position (the 5' position of the anticodon, base 34 of tRNA) are crucial for regulating wobble-pairing specificity.
For example, modifying adenosine to inosine (I) at this position allows the tRNA to pair with U, C, or A, directly altering wobble pairing.

Step 3: Final Answer:
The statement that modifications in the anticodon do not affect wobble pairing is not true.