Question

Matthew Meselson and Franklin Stahl grew E.coli(doubling time is $20\min$ in medium containing $14N{H_4}Cl$for many generations. Then the E.coli was transferred to a medium containing$14N{H_4}Cl$. After 40minutes, the cells are harvested and DNA extracted and subjected to cesium chloride density gradient centrifugation. The proportion of light and hybrid DNA densities will be
A) $50\%$ light and $50\%$hybrid DNA
B) $100\%$light DNA
C) $100\%$hybrid DNA
D) $25\%$light and $75\%$hybrid DNA

Answer 1

DNA replication is the process in which two identical copies of DNA from one original DNA molecule are formed.DNA replication is present in all living organisms as the most necessary part for biological inheritance. The cell possesses the property of division for replication of DNA molecules.

Complete Answer:

Hybrid DNA is the DNA molecule which formed from the two different parents DNA.
DNA replication model is three types: semi-conservative, conservative, and dispersive.
Semi-conservative replication. In this model, the two strands of DNA separate from each other, and each single strand acts as a template for formation of a new, complementary DNA strand. This leads to making the two DNA molecules. In these two molecules one strand is from original DNA and another is a new strand.
Conservative replication. In this model, the result of DNA replication is the one molecule which made up of both original DNA strands (similar to the original DNA molecule) and another molecule that made up of two new strands (having the same sequences as the original molecule).
Dispersive replication. In the dispersive model, after replication of DNA two DNA molecules formed are mixtures, or “hybrids,” of parental and daughter DNA. InDispersive replication, each individual strand is a combination of original and new DNA.
Meselson and Stahl carried their experiments on DNA replication using E. coli bacteria as a model system which shows the DNA replicated by a semi conservative model which is a universal method of replication.
So, the culture of E. coli which has$15N$that adds in $14N{H_4}Cl$, after 20 minutes which is first generation only hybrid DNA will be present in the medium. 40 minutes later the second generation will come. There will be $50\%$light and $50\%$hybrid DNA in the medium.
Then by density gradient centrifugation, two separate bands are observed, one of light DNA and one of hybrid DNA.

So, the correct answer $50\%$ light and $50\%$ hybrid DNA.

Note: 1)Density gradient centrifugation is the method to separate the particles.
2) The cell possesses the property of division for replication of DNA molecules.
3) In Dispersive replication, each individual strand is a combination of original and new DNA.