Explain Watson-Crick model for semiconservative DNA replication. Also explain the experimental proof of same which was given by Matthew Meselson and Franklin Stahl.
Answers
★ Solution :-
In order to understand this question, we must know what these models and theories were. Let's discuss them one - by - one. But firstly we need to understand that what is DNA.
DNA or Deoxyribo - Nucleic Acid is a doubly helical molecule which is doubly stranded. It is protein molecule which consists of chromosomes.
Watson - Crick Model
This model is for semi - conservative DNA replication. Watson and Crick proposed a basic mechanism for DNA replication. In the diagram given by them, they stated the sugar - phosphate backbone like structure by lines and the base pairs to the line randomly. This model was based on Hydrogen bonds as the basis of proteins which are present in DNA strands. The structure given by Watson - Crick model was ::
- Two sugar - phosphate backbones on the outside of the molecule which were held by hydrogen bonds with the nitrogenous bases inside the molecule. This means when the two pairs are pulled apart, they still will be attracted by the bond present in between them.
We know that the structure of a DNA molecule is double helix. Now it forms like zip. We start opening this zip one by one so it seems like one part is opened and another they seems winded. Now if we go on this logic then we see that on unwinding the double helix, the two strands in the molecule of DNA will expose single bases on each strand.
But we know that in pairing of two DNA the strands should expose two complementary bases. And then these complementary bases will start forming a new double helix which is identical to the one from which helix are unzipped . This means the newly added nucleotide comes from a pool of nucleotides that are present in the cell itself.
If we search the result of this replication, then the newly formed molecule will consist of one parental nucleotide and one new synthesised nucleotide. This supposition was found true in both Prokaryotes and Eukaryotes. But it is a hypothetical mode of replication. Thus these are called as Semi - Conservative mode of replication.
Meselson - Stahl Experiment
Matthew Meselson and Franklin Stahl conducted an experiment using E. Coli bacteria to check the semi - conservative replication of DNA. They firstly started growing E. Coli bacteria in a medium where there was sufficient nutrients and isotope of Nitrogen. We know that in DNA molecule, Nitrogen is an efficient component and helps in it's synthesis. So the bacteria took the nitrogen present in surrounding to synthesise it's biological and DNA molecules.
After the cells of the bacteria were divided, the two scientists took the samples and used the method of Density Gradient Centrifugation.
The resultant helical structures were seperated. Then it was found that these seperated molecules formed different generations of new synthesised molecules. This means that each strand in the found DNA molecule served as a template to form a new synthesised molecule which on combining forms a new DNA molecule.
>> More to Know :-
⇒ Semi - Conservative Replication : In this type of replication, the newly formed duplet contains a parental chain and a newly formed synthesised chain.
⇒ Conservative Replication : In this type of replication, the newly formed duplet contains both chains as newly formed synthesised chain.
Answer:
★ Solution :-
In order to understand this question, we must know what these models and theories were. Let's discuss them one - by - one. But firstly we need to understand that what is DNA.
DNA or Deoxyribo - Nucleic Acid is a doubly helical molecule which is doubly stranded. It is protein molecule which consists of chromosomes.
Watson - Crick Model
This model is for semi - conservative DNA replication. Watson and Crick proposed a basic mechanism for DNA replication. In the diagram given by them, they stated the sugar - phosphate backbone like structure by lines and the base pairs to the line randomly. This model was based on Hydrogen bonds as the basis of proteins which are present in DNA strands. The structure given by Watson - Crick model was ::
Two sugar - phosphate backbones on the outside of the molecule which were held by hydrogen bonds with the nitrogenous bases inside the molecule. This means when the two pairs are pulled apart, they still will be attracted by the bond present in between them.
We know that the structure of a DNA molecule is double helix. Now it forms like zip. We start opening this zip one by one so it seems like one part is opened and another they seems winded. Now if we go on this logic then we see that on unwinding the double helix, the two strands in the molecule of DNA will expose single bases on each strand.
But we know that in pairing of two DNA the strands should expose two complementary bases. And then these complementary bases will start forming a new double helix which is identical to the one from which helix are unzipped . This means the newly added nucleotide comes from a pool of nucleotides that are present in the cell itself.
If we search the result of this replication, then the newly formed molecule will consist of one parental nucleotide and one new synthesised nucleotide. This supposition was found true in both Prokaryotes and Eukaryotes. But it is a hypothetical mode of replication. Thus these are called as Semi - Conservative mode of replication.
Meselson - Stahl Experiment
Matthew Meselson and Franklin Stahl conducted an experiment using E. Coli bacteria to check the semi - conservative replication of DNA. They firstly started growing E. Coli bacteria in a medium where there was sufficient nutrients and isotope of Nitrogen. We know that in DNA molecule, Nitrogen is an efficient component and helps in it's synthesis. So the bacteria took the nitrogen present in surrounding to synthesise it's biological and DNA molecules.
After the cells of the bacteria were divided, the two scientists took the samples and used the method of Density Gradient Centrifugation.
The resultant helical structures were seperated. Then it was found that these seperated molecules formed different generations of new synthesised molecules. This means that each strand in the found DNA molecule served as a template to form a new synthesised molecule which on combining forms a new DNA molecule.
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>> More to Know :-
⇒ Semi - Conservative Replication : In this type of replication, the newly formed duplet contains a parental chain and a newly formed synthesised chain.
⇒ Conservative Replication : In this type of replication, the newly formed duplet contains both chains as newly formed synthesised chain.