Question
Biology Question on the molecular basis of inheritance
Ten E. coli cells with 15N- dsDNA are incubated in a medium containing 14N nucleotides. After 60 minutes, how many E. coli cells will have DNA totally free from 15N?
A
20 cells
B
40 cells
C
60 cells
D
80 cells
Answer
60 cells
Explanation
Solution
The bacterium Escherichia coli can divide every 20 minutes. Therefore, in 60 minutes, 3 rounds of divisions will take place.
- First round of division: 20 daughter cells are formed from the 10 parent E. coli cells. All 20 cells have hybrid 15N-14N DNA.
- Second round of division: Each DNA forms two daughter cells, one of which is a hybrid and the other is completely 14N14N. Therefore, 20 cells form 40 daughter DNA molecules, out of which 20 are hybrids and 20 are free from 15N.
- Third round of division: 20 14N-14N cells from the previous generation give rise to 40 14N-14N cells 20 hybrid cells from the previous generation give rise to 20 hybrids + 20 14N-14N cells.
Therefore, the total number of 14N-14N cells at the end of 3 rounds of division is 40+20 = 60.
Hence, the correct option is (C) : 60 cells.