Tolc med 2018 q 27 biology

answer: A
can someone explain? thanks


the DNA is marked if it contains the ³²P
we assume that the bacteriophage replicates in an environment without radioactive P

replication is semi conservative so each time we keep 1 original DNA strand, and 1 new strand synthesised from the non radioactive/non marked elements found inside of the cell
say ³²X is a marked DNA strand and X is an unmarked DNA strand

initially we have the following DNA:
³²X ³²X

after replication an mitosis:
³²X X
³²X X

after replication and mitosis
³²X X
³²X X

this happens again and again until we produce 100 phages.
However since replication is semi conservative, only 2 phages will have the ³²P original strand