NEET Botany Molecular Basis of Inheritance Questions Solved

BOARD

(a) Explain the process of DNA replication with the help of a schematic diagram.

(b) In which phase of the cell cycle does replication occur in Eukaryotes? What would happen if cell-division is not followed after DNA replication?

(a) (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 bacterium 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 helicase.

(iii) However the two strands do not open in one stretch due to very high energy requirement, but the point of separation proceeds slowly from one and to the other. It gives the appearance 'y' shaped structure called the replication fork. DNA strands save as template.

(iv) Enzyme DNA dependent DNA polymerase plays an important role in adding the nucleotides on DNA template.

(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'$\to$3' direction. Since the two strands of the DNA are antiparallel in orientation, replication of the two templates proceed in two opposite directions. On one strand (with polarity 3'$\to$5') the synthesis of DNA is continuous, while on the other template strand (with polarity 5'$\to$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.

(b) In eukaryotes, the replication of DNA takes place at S-phase of the cell cycle.

If DNA replication is not followed by cell division, it would result into polyploidy (a chromosomal anomaly).

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