What does it mean that replication is semi conservative ? How was the feature of replication demonstrated in bacterial cell

Please find below the solution to the asked query

DNA replication is the phenomenon in which a duplicate copy of DNA is synthesised. In replication, two strands of the DNA helix separate and each strand acts as a template for synthesising new complementary strands. After completion of replication, the two copies so produced will have one parental and one newly synthesised strand. This scheme of replication is called semi-conservative replication.

The feature of semi conservative replication was demonstrated Messelson and Stahl using bacterium E.coli.

The salient features of their experiment include: 

• E.coli was grown in a medium containing heavy isotope ¹⁵N as the nitrogen source.
• ¹⁵N was incorporated into newly synthesised DNA as well and the DNA became heavy DNA.
• Heavy DNA molecule can be differentiated from normal DNA by density gradient centrifugation using cesium chloride as the gradient.
•   Then, cells were again transferred into a medium with ¹⁴N as nitrogen source. Samples were taken from this media and their DNA was extracted.

• E .coli divides every 20 minutes. Therefore, the DNA extracted after 20 minutes had a hybrid density.
• DNA extracted after 40 minutes had equal amount of hybrid and light densities.
•  This implies that the newly synthesised DNA obtained one of its strands from the parent. Thus, replication is semi-conservative.

Hope this information will clear your doubts about the semi-conservative replication.
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