Why DNA Replication is Semiconservative?

THe Watson-Crick double helical model for DNA suggested that the basis for copying gcnctic material is base complementarity. The base sequence of one strand of a DNA moleculc determines the sequence on the other strand as the two strands are complementary to each other.

The complementarity of two strands provides a means for the accurate replication of the DNA molecule. During replication, the two strands separate into two single strands and then appropriate complementary nucleotides are assembled on each exposed strand to form two DNA molecules.

These two new DNA molecules have one strand each from the original molecule replicated and one strand each newly synthesized. Thus, as in every DNA, one parental strand is conserved and one new strand is synthesized, this mode of replication is known as semiconsevative replication.

The semiconservative mode of replication was experimentally proved by Mathcw Mcselson and Franklin Stahl of California Institute of Technology in 1958 with E.coli. They grew bacterial strain in a medium containing radioactive isotope of Xitrogen (X ^{15 Ammonium chlorides}) as the only source of Xitrogen. After several generations, all the nitrogenous bases of bacterial DNA got labeled with X¹⁵ Xitrogen.

These radio-labeled DNA have greater density than DNA with normal nitrogenous bases. Then the culture of bacteria was washed to make them free from the medium and was transferred to medium containing normal X¹⁴ ammonium chloride. After one generation the density of DNA extracted from the cultured bacteria was intermediate. This is because during one generation time each DNA double helix had separated and an old strand (with X¹⁵) had synthesized a new complementary strand (with X¹⁵).

The intermediate density is usually referred to as hybrid density. After two generations of growth in the normal medium with X¹⁴, half of the DNA was of hybrid density and half was light or normal. The density of DNA was compared by density gradient centrifugation in concentrated solution of Cesium chloride. As the growth generations continued in light or normal medium more and more DNA present would be light. This confirmed the semiconservative mode of replication.

Scmiconscrvative mode of DNA replication in bacteria was also demonstrated by J.Cairns using autoradiography.J.H.Taylor and his co-workers established scmiconscrvative mode of replication in Vicia faba (eukaryote). Semiconservative mode of replication of chromosome can be visualized through an examination of chromosomes that arc allowed two rounds of replication in medium containing bromodcoxy-uridinc and staining replicated chromosomes with fluorescent dye and Gicmsa.

The newly synthesized strand of each DNA stain differently from old strand. Such chromosomes where the two strands of DNA arc stained differently arc called harlequin chromosomes. Presence of harlequin chromosomes confirm semiconservative mode of replication.