Question

$Escherichiacoli$ with completely radioactive DNA was allowed to replicate in a non-radioactive medium for two generations. Percentage of bacteria with radioactive DNA is

• A. 25%
• B. 50%
• C. 100%
• D. 12.5%

Answer 1

The correct option is B

50%

DNA replication of $Escherichiacoli$
DNA replication of $Escherichiacoli$ in the non-radioactive medium follows the below mentioned model:
$Escherichiacoli$ with completely radioactive DNA (RDNA)was allowed to replicate in a non-radioactive medium for two generations (40 minutes).
In the first generation a single band was formed during centrifugation. It consists of only RDNA- non-radioactive DNA double helix (hybrid DNA).

DNA replication of $Escherichiacoli$
●In the second generation (that is after 40 minutes) two bands of equal width were formed. One with RDNA - non radioactive DNA double helix (hybrid DNA) and the other one with only non- radioactive DNA double helix (light DNA). This shows that 50% of DNA is radioactive.
●Thus, the experiment proves the semiconservative replication of the DNA.
Final answer
(b) 50%