Explain the process of DNA replication with the help of a replicating fork. (5 marks)
(i) It is a complex process and involves the participation of several enzymes and other protein
factors. Replication begins at a definite region known as origin of replication or ori. In bacte-
rium or virus, DNA has only one origin of replication. Eukaryote DNA is a giant molecule. There
are a number of origins because of its large size and association with protein.
(ii) Unwinding of DNA. Two strands of DNA separate. There are enzymes which unwind and
separate the two strands of DNA. It involves breaking of weak hydrogen bonds. Enzyme is
(iii) However the two strands do not open in one stretch due to very high energy requirement, but
the point of seperation procees slowly from one end to the other. It gives the appearance of 'y'
shaped called the replication fork.
DNA strands save as template.
(iv) Enzyme DNA dependent DNA polymerase plays an important role in adding the nucleotides on
(v) Replication is energetically a very expensive process. Nucleotides are not directly added.
Deoxyribonucleoside triphosphates are used. They serve dual purposes. It acts as substrate. It
also provides energy for polymerisation reaction. The two terminal phosphates in a
Deoxyribonucleoside triphosphate are high energy phosphates, same as in the case of ATP.
(vi) DNA polymerase can polymerise the nucleotides only in the 5' 3' direction. Since the two
strands of the DNA are antiparallel in orientation, replication of the two templates proceed in
two opposite directions. On one strand (with polarity 3' 5') the synthesis of DNA is
continuous, while on the other template strand (with polarity 5'3'), the synthesis of DNA is
discontinuous, i.e., short stretch of DNA are synthesized. The continuous strand is known as
leading strand. Short segments of DNA are known as okazaki fragments.
(vii) The short segments of DNA are later joined by the enzyme DNA ligase.
(viii) After the completion of DNA replication each DNA molecule has one parental and one new
synthesized strand. So it is known as semi-conservative mode of replication.