Before we rationalize the answer, it would be useful to be reminded of semi-conservative DNA replication:
Basically in this type of replication, an original DNA yields 2 copies, wherein each copy contains HALF of the original DNA. In short, if you have a double stranded DNA, if that undergoes replication, it will yield TWO double stranded copies, wherein each copy has a strand from the original DNA.
Let us then trace the strand of radioactive nucleotides as it replicates 3 times:
Now that you are reminded of the concept of semi-conservative DNA replication, you will now be able to trace what happens when you replicate a strand that contains radioactive nucleotides. This will be made easier if we visualize this through the diagram below:
To be oriented, the black lines = DNA strands made of NON-radioactive nucleotides, while the red lines = DNA strands made of RADIOACTIVE nucleotides.
Do not forget, that the medium that was used was full of NON-radioactive nucleotides, which means that further replication will yield NEW strands that are made of non-radioactive nucleotides.
After the 1st replication: This will yield 2 copies of DNA. As you would expect, there will be 2 DNA molecules, EACH having HALF of the original DNA. As you can see, 1 copy contains nothing but non-radioactive DNA, while the other copy contains the radiooactive half of the original DNA.
After the 2nd and 3rd replication: Basically, after following the principle of semiconservative replication, you will have 8 total molecules of DNA, each copy of which contains half the DNA that they originated from.
What is this question asking for? You will now need to calculate the proportion of DNA that contains RADIOACTIVE nucleotides! Let’s reread what the question is asking us to do: “correct percentages of original DNA and completely non-radioactive DNA”. If you recall, the “original DNA” is the one that contains both a radioactive and non-radioactive strand. There are 8 total DNA copies after 3 replications. There are 7 completely non-radioactive DNA, while there is only 1 copy that is the same as the original DNA. A fraction of 1/8 would then give you a percentage of 12.5, which means that row 1 or choice B is the only correct answer.
