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Question 4 of 5

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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.

The increase in radioactivity of the embryos with time probably results from

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synthesis of new proteins by the developing embryos

Correct Answer: 
No

synthesis of radioactive thymine by the developing embryos

Correct Answer: 
No

oxidation of radioactive thymine

Correct Answer: 
No

incorporation of radioactive thymine in new cell membranes

Correct Answer: 
No

incorporation of radioactive thymine in new DNA during replication

Correct Answer: 
Yes

The graph shows an increase in the radioactive nucleotide, thymine, after fertilization. As the new sea urchin embryos grow, cell division occurs. The radioactive thymine is incorporated into new DNA in the growing embryos.

Question Difficulty: 
medium