Question

One of the salient features of the genetic code is that it is nearly universal from bacteria to humans. Mention two exceptions to this rule. Why are some codes said to be degenerates?

Hint:

- Exceptions: In mitochondria, UGA = Tryptophan (instead of Stop). In Paramecium, UAA/UAG = Glutamine (instead of Stop). - Degeneracy means $61$ codons share the work of coding for just $20$ amino acids, which helps minimize the impact of genetic mutations.

Answer 1

Concept: The genetic code is the set of rules by which information encoded within genetic material ($\text{mRNA}$ triplets) is translated into proteins by living cells. While the code is highly conserved across the tree of life, it features subtle variations. Additionally, its structural organization includes specific structural redundancies designed to protect organisms from the harmful effects of point mutations.

Step 1: Analysis of Exceptions to Genetic Code Universality.
The genetic code is described as *nearly* universal because a specific codon almost always directs the insertion of the exact same amino acid, whether in a bacterium, a plant, or a human cell. However, precision genetic sequencing has uncovered notable evolutionary exceptions:
• Mitochondrial Genetic Code Deviations: Mitochondria possess their own independent genome ($\text{mtDNA}$) and translational machinery. Inside human and mammalian mitochondria, the codon {UGA, which normally functions as a translation termination (Stop) signal in the standard cytoplasmic genetic code, is translated to encode the amino acid Tryptophan. Similarly, the codon {AUA, which typically codes for Isoleucine in the cytoplasm, encodes Methionine during mitochondrial translation.
• Ciliate and Protozoan Nuclear Code Deviations: In several single-celled eukaryotic protozoans (such as Paramecium, Tetrahymena, and certain green algae), the standard stop codons {UAA and {UAG do not signal the ribosome to halt translation. Instead, these organisms read them as sense codons that direct the incorporation of the amino acid Glutamine.

Step 2: Detailed Explanation of Genetic Code Degeneracy.
The genetic code is composed of $64$ distinct triplet combinations (codons) constructed from four nitrogenous bases ($\text{A, U, G, C}$). Out of these $64$ codons, $3$ function as non-sense or Stop codons ($\text{UAA, UAG, UGA}$), leaving exactly $61$ functional triplets available to code for amino acids. However, there are only $20$ standard amino acids used to build proteins.
• Because the number of available codons ($61$) significantly exceeds the number of standard amino acids ($20$), the system features a structural redundancy termed degeneracy.
• Definition: A genetic code is said to be degenerate (or redundant) because more than one distinct codon can specify the exact same amino acid.
• Illustrative Example: The amino acid Leucine is specified by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). Similarly, Valine is coded by four codons (GUU, GUC, GUA, GUG). Only Methionine (AUG) and Tryptophan (UGG) are specified by a single codon.
• Evolutionary Significance: Degeneracy primarily occurs at the third nucleotide position of the codon triplet (often explained by Francis Crick's Wobble Hypothesis). This configuration offers a protective buffer against point mutations; if a mutation alters the third base of a codon, it frequently still codes for the exact same amino acid, preventing changes to the resulting protein structure.