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Question 5 of 5

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SAT Subject Test

Thymine is used by animal cells primarily for the synthesis of DNA. A group of sea urchin eggs was fertilized in sea water containing radioactive thymine. Following fertilization, samples of embryos were removed at regular intervals and the radioactivity in the nucleic acid was measured in counts per minute. The results obtained are shown in the figure below.

The figure presents a graph in the first quadrant. The horizontal axis is labeled, Time After Fertilization in minutes, and is numbered, from left to right, 0 to 300, in increments of 50. The vertical axis is labeled Radioactive Counts in thousands per minute, and is numbered, from bottom to top, 0 to 400, in increments of 100. A line is shown starting at 0 on the horizontal axis and 0 on the vertical axis. The line moves diagonally up and to the right with multiple plateaus that give it a stair like appearance. The approximate values in the graph are as follows: Time After Fertilization in minutes, 0; Radioactive counts in thousands per minute, 0.; Time After Fertilization in minutes, 50; Radioactive counts in thousands per minute, 20.; Time After Fertilization in minutes, 100; Radioactive counts in thousands per minute, 50.; Time After Fertilization in minutes, 150; Radioactive counts in thousands per minute, 115.; Time After Fertilization in minutes, 200; Radioactive counts in thousands per minute, 200.; Time After Fertilization in minutes, 250; Radioactive counts in thousands per minute, 350.; Time After Fertilization in minutes, 300; Radioactive counts in thousands per minute, 475.

An appropriate control to show that this experiment measures DNA synthesis and not RNA synthesis would be to perform the same procedures but

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not fertilize the eggs

Correct Answer: 
No

sample the embryos at longer time intervals

Correct Answer: 
No

add radioactive uracil instead of radioactive thymine

Correct Answer: 
Yes

fertilize the eggs in sea water that does not contain radioactive thymine

Correct Answer: 
No

count the number of cells in the embryos at the beginning and at the end of the experiment

Correct Answer: 
No

RNA contains uracil instead of thymine. Therefore, in order to test whether RNA synthesis occurred — and to prove that this experiment measures DNA synthesis and not RNA synthesis — the same procedures would need to be repeated with radioactive uracil substituted for radioactive thymine.

Question Difficulty: 
medium