Nucleotides can be radiolabeled before they are incorporated into newly synthesized dna and, therefore, can be assayed to track their incorporation. in a set of experiments, a student-faculty research team used labeled t nucleotides and introduced these into a culture of dividing human cells at specific times. which of the following questions might be answered by using the method described?
a. when do spindle fibers attach to chromosomes?
b. how many picograms of dna are made per cell cycle?
c. what is the length of the s phase of the cell cycle?
d. how many cells are produced by the culture per hour?

Juan and carol were studying invertebrates in biology. they knew that segmented or earth worms preferred a dark, moist habitat. during this lab, they would be investigating the responses of organisms called planaria or dugesia tigrina. these were simple flatworms that still had a one-way digestive system and a very simple nervous system. juan and carol placed the planaria in a petri dish containing cool, distilled water that was partially covered with black paper. they shined a light on the dish. next, they removed the paper and placed a small amount of chicken liver at one end of the dish. they added a few large salt crystals to the water. finally, they added drops of hot water to the cool water in the petri dish. their results can be seen in the data table. according to their experiment, all but one conclusion is valid.