SAT Subject Biology Practice Question 361: Answer and Explanation

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Question: 361

11. 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. No, Colony 4 is an arginine auxotroph (arg-).
B. No, Colony 4 cannot grow in the presence of leucine.
C. Yes, Colony 4's genotype is leu-, pro-.
D. Yes, Colony 4 requires only glucose to grow.
E. The data available are insufficient to make a prediction.

Correct Answer: A

Explanation:

A It really helps to predict an answer BEFORE you look at the answer choices. Sometimes looking at the choices first can confuse your thinking and lead you to a trap, but if you have an idea of the correct answer before you look at the choices, you will be less tempted. Colony 4 cannot grow in the absence of arginine as is evidenced by Plate B. Thus, because the liquid medium does not contain arginine, no bacterial growth would be observed.

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