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● 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 ​N as nitrogen
14​
source. Samples were taken from this media and their DNA was

extracted.

● E .coli​ivides every 20 minutes. Therefore, the DNA extracted after 20
d
minutes had a hybrid density.
● DNA extracted after 40 minutes had equal amount of hybrid and light
intensities.

● This implies that the newly synthesised DNA obtained one of its
strands from the parent. Thus, replication is semi­conservative.

Mechanism of DNA Replication

● Replication occurs in S phase of cell cycle.

● Enzyme involved ­ DNA polymerase (DNA dependent DNA polymerase)
● Replication requires energy.

Source of energy − Deoxyribonucleoside triphosphates (DNTPs)
● DNTPs have dual purpose − Act as substrates and provide energy also

● Replication initiates at specific regions in DNA called origin of
replication.
● DNA polymerase polymerises a large number of nucleotides in a very

short time.
● During the course of replication, two parent strands do not completely