Question

The Meselson and Stahl experiment starts with E. coli containing 15N/15N labeled DNA grown in 14N media. Which result did Meselson and Stahl observe by sedimentation equilibrium centrifugation to provide strong evidence for the semiconservative model of DNA replication?

Answer 1

Answer:

No 15N/15N labeled DNA was observed in second generation

Explanation:

Meselson and Stahl were doing an experiment to find out the semi-conservative mode of replication of DNA. The first cultured bacteria in 15N medium which was containing a heavy isotope of nitrogen. The DNA synthesized here had more density. After that they transferred the bacteria to the 14N containing medium. When bacteria reproduced and new strands of DNA were synthesized, they observed the following -

• The first generation had hybrid DNA with 15N/14N strands
• The second generation had both hybrid 15N/14N DNA and 14N/14N DNA strands
• There was no 15N/15N DNA observed.

In this way they proved that one strand of DNA is kept conserved during the replication.

Answer 2

Answer:

The first generation has DNA with  15N/14N strands,  

Second generation has  both type of DNA, one contain 15N/14N  strands and other with14N/14N strands.

Explanation:

In DNA replication, the newly formed dsDNA  contains one parental type  strand  and the other newly formed strand, which is formed during replication.

The mode of DNA replication is semi-conservative in which one strand is parental and other is new.

As given, E.coli contain 15N/15N labeled DNA and it was grown in 14N media.

So, here during DNA replication, the first generation has hybrid 15N/14N DNA, one strand( 15N) is parental and other (14N) in newly formed,  and the second generation has both hybrid 15N/14N DNA and 14N/14N DNA. There is no DNA containing 15N/15N strands was observed.