`star` Scheme proposed for Replication
`star` The experimental Proof


● While proposing the double helical structure for DNA, `color{Violet}"Watson and Crick"` had immediately proposed a `color{Violet}"scheme"` for `color{Violet}"replication of DNA"`.

● To quote their original statement that is as follows:

`star` It has not escaped our notice that the `color{Violet}"specific pairing"` we have postulated immediately suggests a possible `color{Violet}"copying mechanism"` for the `color{Violet}"genetic material"` (`color{brown}"Watson and Crick"`, `color{brown}"1953"`).

● The scheme suggested that the two strands would `color{Violet}"separate"` and `color{Violet}"act as a template"` for the synthesis of `color{Violet}"new complementary strands"`.

● After the completion of replication, each DNA molecule would have `color{Violet}"one parental"` and `color{Violet}"one newly synthesised"` strand.

● This scheme was termed as `color{brown}"semiconservative DNA replication"`.


● It is now proven that DNA replicates `color{Violet}"semiconservatively"`. It was shown first in `color{Violet}"Escherichia coli"` and subsequently in `color{Violet}"higher organisms"`, such as plants and human cells.

● `color{brown}"Matthew Meselson"` and `color{brown}"Franklin Stahl"` performed the following experiment in 1958:

● They grew `color{Violet}"E. coli"` in a medium containing `text()^(15) NH_4Cl` (`text()^(15)N` is the `color{Violet}"heavy isotope"` of nitrogen) as the only `color{Violet}"nitrogen source"` for many generations.

● The result was that `text()^(15)N` was `color{Violet}"incorporated"` into newly synthesised DNA (as well as other nitrogen containing compounds).

● This `color{Violet}"heavy DNA"` molecule could be distinguished from the `color{Violet}"normal DNA"` by `color{Violet}"centrifugation"` in a `color{Violet}"cesium chloride"` (`color{Violet}"CsCl"`) density gradient

● NOTE: It is to be noted that `text()^(15)N` is `color{Violet}"not a radioactive isotope"`, and it can be separated from `text()^(14)N` only based on `color{Violet}"densities"`).

● Then they `color{Violet}"transferred the cells"` into a medium with normal `text()^(14) NH_4Cl` and `color{Violet}"took samples"` at various definite time intervals as the `color{Violet}"cells multiplied"`, and extracted the `color{Violet}"DNA"` that remained as double-stranded helices.

● The various samples were `color{Violet}"separated independently"` on CsCl gradients to measure the `color{Violet}"densities of DNA"`.

● Thus, the DNA that was extracted from the culture after `color{Violet}"one generation"` after the transfer from `text()^(15)N` to `text()^(14)N` medium [that is after `color{Violet}"20 minutes"`; E. coli divides in 20 minutes] had a `color{Violet}"hybrid or intermediate"` density.

● DNA extracted from the culture after `color{Violet}"another generation"` [that is after `color{Violet}"40 minutes"`, II generation] was composed of `color{Violet}"equal amounts"` of this hybrid DNA and of ‘light’ DNA

● Very similar experiments involving use of `color{Violet}"radioactive thymidine"` to detect distribution of newly synthesised DNA in the chromosomes was performed on `color{Violet}"𝘝𝘪𝘤𝘪𝘢 𝘧𝘢𝘣𝘢"` (faba beans) by `color{Brown}"Taylor"` and colleagues in 1958.

● The experiments proved that the `color{Violet}"DNA in chromosomes"` also replicate `color{Violet}"semiconservatively"`.