Replication of DNA

After the Watson-Crick model of DNA, it was the biggest question for scientists about how DNA replicates. For this purpose, three models were presented for the replication of DNA. These models are:

Conservative model
Dispersive model
Semi-conservative model

Conservative model.

According to this model, the two parent strands of DNA separate from each other.
They started to replicate and form two new daughter strands of DNA according to their respective nitrogenous bases.
Both parent strands act as the template strands for new (or daughter) DNA.
After the formation of two daughter strands of DNA from its parent strands, both old (parent) and new (daughter) strands separate from each other.
Both daughter strands unite and form a totally new and unique strand of DNA.
Then, both parent strands are again joined together and in this way, the DNA remains conserved.

Dispersive model.

As the name shows, the DNA breaks into pieces. In simple words, nitrogenous bases separate from each other.
Then every nitrogenous base of DNA undergoes replication.
Then the fractured (broken or dispersed) parent strand of DNA started to form new copies of its nitrogenous bases.
These newly formed copies of nitrogenous bases of DNA recombine with their dispersed copies of nitrogenous bases from the strand of DNA and form two sets of DNA.
The newly formed strands of DNA are the hybrid of both old (parent) and new (daughter) templates of DNA.
In this model, it seems that parent DNA dispersed and formed the hybrid of old and young templates of DNA.

Semiconservative model.

This model was suggested by Watson and Crick.
According to this model, the strands of parent DNA separate from each other.
Then each (complete) strand undergoes replication and forms two new single strands of DNA (daughter strands) according to their respective sequence of nitrogenous bases.
Then one strand from the parent's DNA unites with its (single) daughter strand and forms a complete double-stranded DNA.
The second copy of DNA formed in the same way.
In this way, the half of the DNA remains conserved and so is called the semi-conservative model of replication of DNA.

Now the question is which model is correct?

This mystery was solved by two scientists, Matthew Meselson and Franklin Stahl. They had performed their experiment in 1958, which we called as Meselson-Stahl experiment.

In this experiment, they used a heavy isotope of nitrogen and inserted this isotope in the Escherichia coli (or E. coli) bacteria.

Meselson-Stahl experiment

Matthew Meselson and Franklin Stahl used two heavy isotopes of nitrogen to determine the replication method of DNA. One is the nitrogen-14 isotope and the other is the nitrogen-15 isotope. They can determine the replication method of DNA from their experiment by expecting three kinds of result possibilities:

If the DNA replicates with a conservative mode of replication then the newly formed DNA must have one pure-weight isotope of nitrogen and another has a heavy-weight nitrogen isotope.
If DNA replicates with the dispersive mode of replication, then the newly formed strands of DNA must be a mixture of both light and heavy-weight nitrogen isotopes.
If DNA follows the semi-conservative mode of replication, then half of the strand of DNA must have a nitrogen isotope of heavy-weight, and the half strand of DNA must have nitrogen of lightweight.

They observed many generations of this bacteria and in the end, concluded that a semiconservative way of replication occurred in E. coli bacteria because of two different isotopes in the strand of DNA.

Almost all organisms replicate their DNA by the semiconservative method.