36. Two experiments were performed. In the first one, Okazaki fragments were prepared from a replicating cell of E. coli grown in the presence of ³²P. In the other,the two strands of E. coli chromosome were separated

into a H strand and L strand, immobilized onto anitrocellulose membrane and hybridized with the Okazaki fragments prepared in the first experiment.

Which one of the following options correctly describes the observation?

(1) Okazaki fragments will hybridize to only H strand

(2) Okazaki fragments will hybridize to only L strand

(3) Okazaki fragments will hybridize with both H and L strands

(4) Because the H and L strands have been prepared from different cultures of E. coli, the Okazaki fragments will hybridize to neither

Background: Okazaki Fragments and DNA Replication in E. coli

DNA replication in Escherichia coli is semi-discontinuous, with the leading strand synthesized continuously and the lagging strand synthesized discontinuously in short segments called Okazaki fragments. These fragments are later joined to form a complete DNA strand.

• The leading strand corresponds to the H strand (heavy strand) or the strand synthesized continuously.

• The lagging strand corresponds to the L strand (light strand) or the strand synthesized discontinuously via Okazaki fragments.

The Experiment Setup

• Okazaki fragments were isolated from replicating E. coli cells labeled with radioactive ^32P.

• The two parental strands of E. coli DNA were separated into H and L strands and immobilized on a membrane.

• The labeled Okazaki fragments were hybridized to these immobilized strands.

Expected Observation

Okazaki fragments are short DNA pieces synthesized on the lagging strand template. They are complementary to the lagging strand template, which corresponds to the L strand.

Therefore:

• Okazaki fragments will hybridize specifically to the L strand, since they are complementary to it.

• They will not hybridize to the H strand, which corresponds to the leading strand template.

Why Not Both Strands?

• Leading strand synthesis is continuous, producing long, high molecular weight DNA fragments, not short Okazaki fragments.

• Okazaki fragments are unique to the lagging strand, where discontinuous synthesis occurs.

• Hence, Okazaki fragments hybridize only to the lagging strand template (L strand).

Correct Answer

(2) Okazaki fragments will hybridize to only L strand

Summary Table

Conclusion

In E. coli, Okazaki fragments isolated from replicating cells hybridize specifically to the lagging strand (L strand) of the chromosome. This reflects the semi-discontinuous nature of DNA replication, where only the lagging strand is synthesized in short fragments.