DNA replication -a historical perspective

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By the end of the 19th century, the first chapter of a powerful new science was all but written. The science was genetics and the critical first chapter concerned the rules, governing transmission of hereditary traits from generation to generation. Genetics is the study of inheritance (heredity) of parental characteristics and of variability of the characteristics of an organism. Variability can occur by genetic change and is in fact the basis of evolution.

The first step in understanding heredity was the work of Gregor Johann Mendel, an Australian monk & philosopher who showed in 1865 that crosses (hybrids) of different garden pea varieties had a definite pattern of inheritance of parental characteristics such as color, shape and other properties of the flower and seeds.

Mendel’s research paper remained dormant & unnoticed by the scientific world until 1900. It was in the beginning of 20th century that three botanies, namely Hugo de Vries, working on Oenothera spp, Carl Correns working on Xenia spp, peas and maize and Erich von Tshermak working on various flowering plants, independently drew the conclusion like Mendel. Later these botanists came across the research paper of Mendel & rediscovered it in 1900. Mendel’s discoveries had been confined and extended by connecting the rules of heredity with the property and behaviors of chromosomes, in the dawn of 20th century from the point of view of modern genetics, the important early milestone was the recognition of DNA as the hereditary material, based on transformation, phenomenon of DNA transfer first discovered by Griffith (1928), a British health scientist.

The elucidated structure of DNA was later demonstrated by X ray diffraction patterns of the DNA fibers, by Rosalind Franklin. Thus they marked the birth of modern molecular biology.

From the point of view of modern genetics, the important early milestone was the recognition of DNA as the hereditary material, based on transformation, phenomenon of DNA transfer first discovered by Griffith (1928), a British health scientist. Griffith showed that when mice were injected with a mixture of pneumococci (Streptococcus pneumoniae) containing a few non-capsulated and non-pathogenic cells and a large number of heat – killed capsulated and pathogenic cells, mice died out of pneumonia and live capsulated cells were isolated from their blood. Hence, the non-capsulated cells were transformed into capsulated forms and the phenomenon was called transformation. Griffith showed that the transforming principle could be passed from the transformed cells to their progeny and thus had the characteristics of a gene. However, Avery, McLeod and McCarty established the fact that this transforming principle was DNA in 1944. They showed that DNA of one strain of bacterium. The heat-killed cells of capsulated pneumoccocci in Griffith’s experiments were thus indeed donating their DNA to the non-capsulated cells and passing on the gene for capsule formation. The fact that DNA was the genetic material was confirmed by Hershey and Chase (1952) when they discovered that DNA alone of a bacteriophage enters a bacterial cell during infection and gives rise to a number of progeny phages. The foundation for the modern discipline of molecular genetics was laid in 1953 when James Watson and Francis Crick proposed the double helix model for DNA that explained its replication. The two scientists were awarded Nobel Prize in 1962 for their achievements.

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