Mitochondrial and chloroplast codes: In some eukaryotic organelles, how does the genetic code compare to the standard bacterial code?
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AIt differs from that used in prokaryotes
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BIt is exactly the same as in prokaryotes
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CIt is partially the same but identical for all codons
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DNone of the above
Answer
Correct Answer: It differs from that used in prokaryotes
Explanation
Introduction / Context:The genetic code is nearly universal, but notable exceptions occur in organelles such as mitochondria (and some chloroplasts). Recognizing these deviations is important in genomics and interpreting organelle-encoded proteins.
Given Data / Assumptions:
- Certain mitochondrial genomes reassign codons (e.g., UGA as tryptophan in human mitochondria).
- Start and stop assignments may differ.
- Question asks if organelle codes differ from prokaryotic standard.
Concept / Approach:Organelle translation systems evolved specialized tRNAs and factors, leading to codon reassignments. Hence, their codes differ from the canonical bacterial code, even though many codons are the same.
Step-by-Step Solution:
Recall examples of codon reassignment (e.g., AUA as methionine in mitochondria).Note that “nearly universal” allows exceptions.Select the option indicating difference from prokaryotes.Verification / Alternative check:Translation tables (e.g., NCBI genetic codes) list multiple alternative codes for mitochondria and some nuclear lineages.
Why Other Options Are Wrong:
- Exactly the same: contradicted by known reassignments.
- Partially the same but identical: internally inconsistent.
- None: a clear, accurate statement exists.
Common Pitfalls:Assuming “universal” means no exceptions; in practice, exceptions are critical in sequence annotation.
Final Answer:It differs from that used in prokaryotes