39. In the figure above, replication of DNA beginning from the origin of replication of the chromosome of a newly identified bacterium having a double stranded circular DNA genome is shown. Characterization of DNA

polymerase responsible for genome replication showedthat DNA synthesis occurred in to direction and it depends on the presence of a primer (as is the case in

Escherichia coli). Polarities ot DNA (5′ or 3′) are as shown. Replication begjns at a point marked ‘O’ on the left of the bubble. and both the parent strands were replicated concurrently. The longer arrow inside the

bubble shows the leading strand, whereas the shorter arrows (marked a, b, c) show the Okazaki fragments. The model depicts a:

(i) bidirectional mode of replication wherein synthesis of the Okazaki fragment marked ‘c’ occurs prior to those marked ‘a’ and ‘b’

(2) bidirectional mode of replication wherein synthesis of the Okazaki fragment marked ‘a’ occurs prior to those marked ‘b’ and ‘c’

(3) unidirectional mode of replication wherein synthesis of the Okazaki fragment marked ‘c’ occurs prior to those marked ‘a’ and ‘b’.

(4) unidirectional mode of replication wherein synthesis of the Okazaki fragment marked ‘a’ occurs prior to those marked ‘b’ and ‘c’.

 Introduction

DNA replication in bacteria typically involves a circular chromosome and proceeds via a bidirectional mode of replication, where two replication forks move in opposite directions from a single origin of replication. Each fork synthesizes a leading strand continuously and a lagging strand discontinuously through Okazaki fragments.

A newly identified bacterium with a double-stranded circular genome replicates similarly to Escherichia coli, requiring primers for DNA synthesis and proceeding bidirectionally. Understanding the order in which Okazaki fragments are synthesized on the lagging strand is essential to interpreting replication dynamics.

Key Features of DNA Replication in Circular Bacteria

• Bidirectional replication: Two replication forks move away from the origin (marked ‘O’) in opposite directions.

• Leading strand synthesis: Continuous DNA synthesis in the 5′ to 3′ direction along the template strand.

• Lagging strand synthesis: Discontinuous synthesis via Okazaki fragments, each initiated by a primer.

• Okazaki fragment order: The fragment farthest from the replication fork is synthesized first, followed by fragments closer to the fork as it progresses.

Analysis of the Given Statements

• The longer arrow inside the replication bubble represents the leading strand synthesized continuously.

• The shorter arrows labeled a, b, and c represent Okazaki fragments synthesized discontinuously on the lagging strand.

Synthesis Order of Okazaki Fragments

• The Okazaki fragment farthest from the replication fork (usually ‘a’) is synthesized first because the lagging strand is synthesized in short segments moving away from the fork.

• Subsequent fragments (‘b’ then ‘c’) are synthesized closer to the replication fork as it progresses.

• This order reflects the discontinuous nature of lagging strand synthesis.

Correct Interpretation of the Options

1. Option (1): Bidirectional replication where synthesis of fragment ‘c’ occurs prior to ‘a’ and ‘b’

   • Incorrect, because ‘c’ is closest to the fork and synthesized last.

2. Option (2): Bidirectional replication where synthesis of fragment ‘a’ occurs prior to ‘b’ and ‘c’

   • Correct, matches the expected order of Okazaki fragment synthesis.

3. Option (3): Unidirectional replication with fragment ‘c’ synthesized first

   • Incorrect, replication is bidirectional, and ‘c’ is synthesized last.

4. Option (4): Unidirectional replication with fragment ‘a’ synthesized first

   • Incorrect, replication is bidirectional as per the description.

Final Answer

(2) Bidirectional mode of replication wherein synthesis of the Okazaki fragment marked ‘a’ occurs prior to those marked ‘b’ and ‘c’

Summary Table

Conclusion

The newly identified bacterium replicates its circular genome via bidirectional replication, similar to E. coli. In this process, Okazaki fragment synthesis on the lagging strand proceeds such that the fragment farthest from the replication fork (marked ‘a’) is synthesized first, followed by fragments closer to the fork (‘b’ and then ‘c’). This reflects the discontinuous synthesis characteristic of lagging strand replication.