Introduction
Nucleotides are the building blocks of DNA, which is also known as deoxyribonucleic acid. There are three of them: one deoxyribose molecule, one with a phosphate group, and one with a nitrogen-containing base Adenine, guanine, cytosine, and thymine are the four nucleotide bases found in DNA. During the formation of a DNA strand, covalent bonds develop between the sugar group of one nucleotide and the phosphate molecule of the next nucleotide, forming a strand of DNA.
DNA replication
DNA replication is the method by which a DNA fragment is copied, and it is also the method by which cells repair their DNA damage. During DNA replication, the DNA molecule makes two new complementary strands that are complementary to the original strands. For each strand of the original DNA, also known as parental DNA, a template for the new strand is created. When adenine is present on the original strand, the nitrogenous base thymine is substituted into the newly formed strand to generate a thymine base. In most bacteria, DNA replication begins at one single origin of replication, wherein proteins that initiate replication bind to a certain sequence of nucleotides and divide the two strands of DNA, forming what is known as a replication bubble in the process. In the following step, replication is carried out in both ways in the two replication forks.
Models of DNA
It is possible to represent DNA replication in three different ways: the conservative model, the semiconservative model, and the dispersive model.
Semi-conservative model:- Within the framework of this paradigm, the two strands of DNA unwind from one another, with each serving as a template for the formation of a new, complementary strand of DNA. As a result, two DNA molecules are formed, one containing the original strand and the other containing a new strand.
Conservative model :- In this scenario, DNA replication results in one molecule that contains both original DNA strands (which is identical to the original DNA molecule) and another molecule that contains two new DNA strands (which is not identical to the original DNA molecule) (with exactly the same sequences as the original molecule).
Dispersive model:- DNA replication leads in the formation of two DNA molecules that are mixes, or “hybrids,” of the DNA from the parent and daughter genomes, respectively. In this concept, each individual strand is made up of a patchwork of both original and newly synthesised genetic material.
Meselson and Stahl Experiment
The Meselson and Stahl Experiment served as an experimental demonstration of semiconservative DNA replication. When Matthew Meselson and Franklin Stahl conducted an experiment on E.coli, which divides every 20 minutes, to explore DNA replication in 1958, they were the first to publish their findings.
Experiment
Using centrifugation in calcium chloride, the isotopes of nitrogen 15N (heavy) and 14N (normal) may be separated based on their densities, which can be determined by measuring the density of the two isotopes of nitrogen (CsCl). Meselson and Stahl cultivated E.coli on a medium containing 15NH4Cl for a number of generations before publishing their findings. As a result, the nucleotide 15N was successfully incorporated into bacterial DNA. Later, they changed the 15NH4Cl media to the more common 14NH4Cl medium. They took the sample at regular intervals of time and tested it for the presence of DNA concentration.
Observation
In sample no. 1 (after 20 minutes), there was bacterial DNA present in a low to moderate density. A sample of DNA with intermediate and light densities was found in the same proportion in Sample No. 2 (after 40 minutes of incubation).
Result
Meselson and Stahl drew on observations and experimental evidence to argue that DNA molecules can reproduce semi-conservatively. A further investigation into the semi-conservative nature of DNA replication, or the copying of cells, DNA, did not stop there. As a follow-up to the Meselson and Stahl experiment, Taylor and team did another experiment on the plant Vicia faba (fava beans), which demonstrated once more that DNA replication is semi-conservative.
Conclusion
According to the results of the experiment carried out by Meselson and Stahl, DNA replicates semi-conservatively, which means that each DNA strand serves as a template for the synthesis of a new, complementary strand in the same molecule.
We now know that semi-conservative Replication of DNA is a universal mechanism shared by all creatures on the planet Earth, despite the fact that Meselson and Stahl conducted their research on the bacterium E. coli at the time.