Question

Cystic fibrosis is caused by a mutation in a gene involved in plasma membranes. The image shows the amino acid sequences coded by a normal and a mutated gene.

Normal Sequence: VALINE-HISTIDINE-LEUCINE-THREONINE-PROLINE-GLUTAMIC ACID-GLUTAMIC ACID

Mutated Sequence: VALINE-HISTIDINE-LEUCINE-THREONINE--ALANINE-TRYPTOPHAN


Analyze the sequences and select the option which best describes the change in the DNA sequence of the gene coding for the mutated protein.
Responses

A base pair is added in the DNA sequence
A base pair is added in the DNA sequence


A base pair is deleted in the DNA sequence
A base pair is deleted in the DNA sequence

A base pair is translocated in the DNA sequence.

Answer 1

To analyze the difference between the normal and mutated amino acid sequences, we need to look closely at what changes occurred.

The normal amino acid sequence is:

**Normal Sequence:** VALINE-HISTIDINE-LEUCINE-THREONINE-PROLINE-GLUTAMIC ACID-GLUTAMIC ACID

The mutated amino acid sequence is:

**Mutated Sequence:** VALINE-HISTIDINE-LEUCINE-THREONINE-ALANINE-TRYPTOPHAN

Comparing both sequences:

1. The first four amino acids are identical: VALINE, HISTIDINE, LEUCINE, and THREONINE.
2. The fifth amino acid differs: PROLINE (in the normal sequence) is replaced by ALANINE (in the mutated sequence).
3. The last two amino acids are also different: the normal sequence has two GLUTAMIC ACIDS at the end, while the mutated sequence features TRYTPTOPHAN at the end.

This change indicates that where there should have been a PROLINE followed by two GLUTAMIC ACIDS, there are instead an ALANINE followed by a TRYPTOPHAN.

This suggests that there has been a DNA mutation where bases were likely deleted or replaced, as it changed the identity of three consecutive amino acids (the PROLINE and the two GLUTAMIC ACIDS have been changed to an ALANINE and a TRYPTOPHAN).

### Conclusion:
The most likely scenario is that a base pair was deleted or altered, leading to this change in aforementioned amino acids. Since this mutated sequence has two fewer amino acids compared to the normal one and seems to fit the criteria of substitution, the best choice provided is:

**A base pair is deleted in the DNA sequence.**