SAT Subject Biology Practice Question 270: Answer and Explanation

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Question: 270

20. Scientists sequenced a portion of the aspartate transaminase enzyme from different species. The amino acid sequences are shown in the table. Each letter stands for an amino acid. Differences from the human sequence are indicated by bold letters. Missing amino acids are indicated by a dash (–).

Which genetic mutation occurred farthest back in time?

A. Insertion of a codon for serine (S)
B. Deletion of a codon for asparagine (N)
C. Conversion of a codon for lysine (K) to one for arginine (R)
D. Conversion of a codon for alanine (A) to one for cysteine (C)
E. Conversion of a codon for leucine (L) to one for methionine (M)

Correct Answer: A

Explanation:

Bacteria lack a codon, present in alfalfa, yeast, and chicken, that codes for serine (S). Because bacteria are the most distantly related to all the other species in the table, this insertion most likely occurred first in the common ancestor of plants and animals.

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