Question

you have devised a system that allows you to stain adenine bases with a visible purple dye. you place some yeast cells into a growth media containing the dye solution and allow the dye to become incorporated into both strands of the double helices. you then remove a single yeast cell from the dye during g1 phase and allow it to go through two rounds of cell division in a growth medium that contains no dye, but has all other necessary components needed for replication. all four cells are back in g1 phase at the end of this experiment. if you isolate all the dna from the four cells, what percentage of those dna molecules will contain some purple dye?
0%
25%
50%
75%
100%

Explanation:

Step1: Recall DNA replication principle

DNA replicates semi - conservatively. Each new DNA molecule consists of one old strand and one new strand.

Step2: Analyze first round of replication

When the yeast cell with dye - labeled DNA replicates for the first time in dye - free medium, each of the two daughter DNA molecules has one labeled strand (from the original DNA) and one unlabeled strand.

Step3: Analyze second round of replication

When these two daughter DNA molecules replicate again in dye - free medium, out of the four DNA molecules produced, two will have one labeled strand and two will have no labeled strands.

Step4: Calculate the percentage

The percentage of DNA molecules with some purple dye is $\frac{2}{4}\times100\% = 50\%$.

Answer:

50%