BOTANY Higher Secondary Second Year - Textbooks Online

Replication
fork
Super
coil
DNA
polymerase
RNA
primer
Fig. 3.9 Semiconservative method of replication of DNA
their experiments on Escherichia coli using radioactive isotopes. The
replication of chromosome in E. coli is completed in 40 minutes.
During replication of DNA, the two complementary strand of DNA
uncoil and separate from one end in a zipper like fashion. The enzyme
helicase unwinds the two strands and as a result replication fork is formed.
As the DNA unwinds, the part of the DNA that is found above the
replication fork becomes supercoils. These are called positive supercoils.
An enzyme called topoisomerase releases these supercoils. Based on
separated DNA strands, new strands grow by the addition of nucleotides.
DNA polymerase I, II and III are involved in this elongation. However,
these enzymes are not capable of initiating DNA synthesis.
For the synthesis of new DNA, two things are required. One is RNA
primer and the enzyme primase. The DNA polymerase moves along the
newly formed RNA primer nucleotides, which leads to the elongation of
DNA. In the other strand, DNA is synthesized in small fragments called
Okazaki fragments. These fragments are linked by the enzyme called
135
Topoisomerase
Helicase
Helix destabilising protein
Primase
RNA primer
Okazaki fragments
Ligase
DNA polymerase