In a disorder called gyrate atrophy, cells in the retina...

In a disorder called gyrate atrophy, cells in the retina begin to degenerate in late adolescence, causing night blindness that progresses to total blindness. The cause is a mutation in the gene that encodes an enzyme, ornithine aminotransferase (OAT). Researchers sequenced the $O A T$ gene for five patients with the following results: Patient A: A change in codon 209 of UAU to UAA Patient B: A change in codon 299 of UAC to UAG Patient C: A change in codon 426 of CGA to UGA Patient D: A two-nucleotide deletion at codons 64 and 65 that results in a UGA codon at position 79 Patient E: Exon 6, including 1071 nucleotides, is entirely deleted. a. Which patient(s) have a frameshift mutation? b. How many amino acids is patient E missing? c. Which patient(s) will produce a shortened protein?

In a disorder called gyrate atrophy, cells in the retina begin to degenerate in late adolescence, causing night blindness that progresses to total blindness. The cause is a mutation in the gene that encodes an enzyme, ornithine aminotransferase (OAT). Researchers sequenced the $O A T$ gene for five patients with the following results:
Patient A: A change in codon 209 of UAU to UAA
Patient B: A change in codon 299 of UAC to UAG
Patient C: A change in codon 426 of CGA to UGA
Patient D: A two-nucleotide deletion at codons 64 and 65 that results in a UGA codon at position 79
Patient E: Exon 6, including 1071 nucleotides, is entirely deleted.
a. Which patient(s) have a frameshift mutation?
b. How many amino acids is patient E missing?
c. Which patient(s) will produce a shortened protein?

Key Concepts

Frameshift Mutation

A frameshift mutation occurs when nucleotides are inserted or deleted from the DNA sequence in numbers not divisible by three, leading to a shift in the triplet reading frame. This usually results in a completely different translation from the mutation point onward, often causing a premature stop codon and a truncated, nonfunctional protein.

Nonsense Mutation

A nonsense mutation is a point mutation in which a single nucleotide change converts a codon encoding an amino acid into a stop codon. This premature stop signal causes the translation machinery to halt protein synthesis, resulting in a shortened protein that typically lacks normal function.

In-frame Deletion

An in-frame deletion is a type of mutation where a deletion of nucleotides occurs in multiples of three, so that the reading frame is maintained. Although the overall sequence downstream is unchanged, the resulting protein is missing a segment of amino acids, potentially affecting its structure and function.

Exon Deletion

Exon deletion involves the loss of an entire exon from the gene. When the exon’s length is a multiple of three nucleotides, its deletion is in frame. This produces a protein that is shorter than the normal version, missing the series of amino acids normally encoded by that exon. This type of mutation may disrupt protein domains or functional sites critical for normal activity.

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