85. UGA is NOT a stop codon in:
1. Plant nuclear encoded proteins
2. Plant mitochondria
3. Yeast nuclear encoded proteins
4. Yeast mitochondria
UGA Codon and Its Role in Protein Synthesis
In the genetic code, the UGA codon is typically one of the three stop codons that signals the end of translation. However, interestingly, in certain organisms and organelles, UGA does not function as a stop codon. Instead, it codes for a specific amino acid, selenocysteine, in a process known as recoding.
UGA as a Stop Codon in Most Organisms
In most organisms, UGA is one of the three nonsense codons (the others being UAA and UAG) that terminate translation and signal the release of the newly synthesized polypeptide. However, in some species, particularly in certain mitochondria and chloroplasts, the role of UGA is different.
UGA and Its Recoding in Organisms
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Plant Mitochondria: In plant mitochondria, the UGA codon is not used as a stop codon. Instead, UGA is recognized as coding for selenocysteine, an amino acid that is incorporated into proteins during translation. This phenomenon is part of a special case of codon recoding, where certain codons that are normally stop codons are redefined to code for amino acids.
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Yeast Mitochondria: Similarly, in yeast mitochondria, UGA is also not a stop codon. Instead, it is recoded to incorporate selenocysteine into mitochondrial proteins.
The Key Difference in Other Organisms
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Plant Nuclear Encoded Proteins: In contrast, in plant nuclear encoded proteins, UGA remains a stop codon, as it typically does in yeast nuclear encoded proteins. Both plant nuclear genomes and yeast nuclear genomes adhere to the standard genetic code where UGA signals the end of translation.
Why UGA Is Not a Stop Codon in Some Organisms
The recoding of UGA to incorporate selenocysteine in mitochondria and plastids is a fascinating aspect of molecular biology. This recoding process involves a specialized machinery that includes a selenocysteine insertion sequence (SECIS) in the mRNA, which ensures that selenocysteine is added in place of what would normally be a termination signal.
This process is crucial for the function of certain enzymes that require selenocysteine as a cofactor, such as those involved in antioxidant defense and redox regulation. The incorporation of selenocysteine in these organelles highlights the flexibility of the genetic code and its adaptation to the specific needs of different cellular environments.
Conclusion
In summary, UGA is not a stop codon in the mitochondria of plants and yeast, where it is used to code for the amino acid selenocysteine instead. This phenomenon is an example of codon recoding, which allows for more flexibility in protein synthesis, especially in mitochondria and plastids.
Correct Answer: Yeast Mitochondria
This article explains how the UGA codon behaves differently in some organisms, particularly in mitochondria, and how it plays a critical role in protein synthesis by coding for selenocysteine instead of acting as a stop codon.
