16. In semi-conservative mode of DNA replication twoparental strands unwind and are used for synthesis of new strands following the rule of complimentary base

pairing. Synthesis of complimentary strands requirethat DNA synthesis proceeds in opposite direction,while the double helix is progressively unwinding andreplicating in only one direction.one of the DNA strands

is continuously synthesised in the same direction as theadvancing replication fork and is called leading strandstrands

is synthesized the otherwhereasdiscontinuously in segments and is referred to aslagging strands. These short fragments madediscontinuously are labelled as okazaki fragments.These okazaki fragments need to be matured intocontinuous DNA strand by which one of the followingcombination of enzymes?
(1) DNA Pol III and DNA ligase

(2) DNA pol I and DNA ligase

(3) DNA pol II and DNA ligase

(4) DNA gyrase and DNA ligase

 

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Introduction

During semi-conservative DNA replication, the two parental strands unwind and serve as templates for new complementary strands. Because DNA polymerases synthesize DNA only in the 5′ to 3′ direction, replication of the two antiparallel strands occurs differently:

• The leading strand is synthesized continuously in the direction of the replication fork.

• The lagging strand is synthesized discontinuously in short segments called Okazaki fragments.

These Okazaki fragments must be processed and joined to form a continuous DNA strand. This maturation involves the removal of RNA primers, gap filling, and sealing of nicks, which requires specific enzymes.

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Formation of Okazaki Fragments

• The lagging strand template runs 5′ to 3′, so DNA polymerase synthesizes new DNA in short stretches moving away from the replication fork.

• Each Okazaki fragment starts with an RNA primer synthesized by primase.

• DNA polymerase extends these primers to form short DNA fragments.

• This discontinuous synthesis necessitates a maturation process to join fragments into a continuous strand.

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Enzymes Involved in Okazaki Fragment Maturation

1. DNA Polymerase I

   • Possesses 5′→3′ exonuclease activity that removes RNA primers from the Okazaki fragments.

   • Fills in the resulting gaps with DNA nucleotides, synthesizing DNA to replace the removed RNA.

2. DNA Ligase

   • Seals the nicks (breaks in the sugar-phosphate backbone) between adjacent Okazaki fragments by forming phosphodiester bonds.

   • This action completes the synthesis of a continuous lagging strand.

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Other Enzymes and Their Roles

• DNA Polymerase III:

  • The primary enzyme responsible for synthesizing new DNA strands during replication, including the bulk of the lagging strand synthesis.

  • Does not remove RNA primers or seal nicks.

• DNA Polymerase II:

  • Functions mainly in DNA repair, not in Okazaki fragment maturation.

• DNA Gyrase:

  • A topoisomerase that relieves supercoiling ahead of the replication fork, unrelated to Okazaki fragment processing.

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Summary Table

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Correct Answer

(2) DNA Polymerase I and DNA ligase

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Conclusion

The maturation of Okazaki fragments during semi-conservative DNA replication is a crucial step to ensure the lagging strand becomes a continuous DNA molecule. This process requires the coordinated action of DNA Polymerase I, which removes RNA primers and fills in the gaps with DNA, and DNA ligase, which seals the nicks between fragments. Together, these enzymes maintain the integrity and continuity of the newly synthesized DNA strand.