Effects of Three-Base Deletions in Coding Regions: What Cannot Occur?

A deletion of three consecutive bases (one codon) in the coding region of a gene is classified as an in-frame deletion because it removes one amino acid without altering the downstream reading frame. Understanding the consequences of such deletions is important for interpreting their effects on protein structure and function.

What Happens When Three Bases Are Deleted?

• Deletion of a single amino acid without other changes:
  Since three bases correspond to one codon, their deletion removes exactly one amino acid from the protein sequence without affecting the rest of the sequence. This is a common and expected outcome of in-frame deletions.

• Replacement of two adjacent amino acids by a single amino acid:
  This would require a frameshift or more complex mutation, not a simple three-base deletion. A three-base deletion cannot directly cause two amino acids to be replaced by one.

• Replacement of a single amino acid by another without other changes:
  This is typically caused by a point mutation (single nucleotide substitution), not a deletion. A three-base deletion removes an amino acid rather than substituting it.

• Production of a truncated protein:
  Truncation generally results from frameshift mutations or introduction of premature stop codons. A three-base deletion, being in-frame, does not cause frameshifts and usually does not lead to truncation unless it removes a stop codon or affects splicing.

Correct Answer

(3) replacement of a single amino acid by another without any other change in the protein sequence

This cannot result from a deletion of three consecutive bases because deletions remove amino acids rather than substitute them.

Additional Insights

• In-frame deletions maintain the reading frame and thus preserve the downstream amino acid sequence.

• Frameshift mutations, caused by deletions or insertions not divisible by three, alter the reading frame and often result in truncated, nonfunctional proteins.

• The phenotypic impact of in-frame deletions depends on the functional importance of the deleted amino acid.

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Conclusion

A deletion of three consecutive bases in a gene’s coding region typically results in the loss of a single amino acid without affecting the rest of the protein sequence. It cannot cause the replacement of one amino acid by another without other changes. This distinction is crucial for understanding mutation impacts on protein structure and function.

Correct answer: (3) replacement of a single amino acid by another without any other change in sequence protein.