SAT Subject Biology Practice Question 358: Answer and Explanation

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Question: 358

8. Most bacteria can be grown in the laboratory on agar plates containing glucose as their only carbon source. Some bacteria require additional substances, such as amino acids, to be added to the growth medium. Such bacteria are termed auxotrophs. These bacteria are denoted by the amino acid they require followed by a "-" in superscript (e.g., arg-). Bacteria that do not require that particular amino acid can be indicated by a "+" in superscript.

Different strains of bacteria were grown on several plates containing a variety of nutrients. Figure 1 shows the colonies (numbered) that grew on each plate. The supplements in each plate are indicated.

In a second experiment, Colony 1 was mixed with soft agar and spread over a plate so that an even lawn of bacteria grew. Bacterial lawns appear cloudy on agar plates. A single drop of an unknown organism was placed in the center of the

A. arg+, leu+, pro+
B. arg+, leu-, pro+
C. arg+, leu+, pro-
D. arg-, leu-, pro+
E. arg-, leu-, pro-

Correct Answer: C

Explanation:

C The only plate that Colony 3 cannot grow on is Plate C, which lacks proline. Thus Colony 3 requires proline to grow and is a proline auxotroph (pro-). This eliminates choices A, B, and D. Choice E is incorrect because Colony 3 does not require arginine or leucine to grow; it can grow just fine in the absence of these amino acids, as is indicated on plates A and B.

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