35. If bacterial genome and plasmid are allowed to replicate in same manner using same replication machinery then

(1) Plasmid DNA will complete replication first

(2) Bacterial DNA will complete replication first

(3) Both will replicate at same time

(4) It depends on GC content of both DNA

Introduction

In bacteria, both the chromosomal genome and plasmids replicate using the host’s replication machinery. However, when allowed to replicate under the same conditions and using the same enzymes, plasmids generally complete replication faster than the bacterial chromosome. This difference arises primarily due to variations in size, replication origin structure, and regulatory mechanisms.

Key Reasons Why Plasmid DNA Replicates Faster

1. Smaller Size of Plasmids

   • Plasmids are typically much smaller than bacterial chromosomes, often ranging from a few thousand to several hundred thousand base pairs, whereas bacterial genomes are millions of base pairs long.

   • Due to their smaller size, plasmids require less time for complete replication.

2. Replication Mechanisms

   • Plasmids often replicate via specialized mechanisms such as theta replication, strand displacement, or rolling-circle replication.

   • Rolling-circle replication, for example, can rapidly produce multiple copies of plasmid DNA in a continuous manner.

   • These mechanisms can be more efficient and faster than the bidirectional replication typical of bacterial chromosomes.

3. Copy Number Control and Regulation

   • Plasmids have evolved tight regulatory systems to maintain their copy number within the host cell.

   • This regulation ensures that plasmid replication is initiated frequently enough to maintain multiple copies but avoids unnecessary replication that could burden the host.

   • The replication initiation of plasmids is often controlled by plasmid-encoded initiator proteins that recognize plasmid-specific origins, allowing rapid and autonomous replication.

4. Replication Origin Structure

   • Plasmid origins of replication are often simpler and more accessible compared to the complex bacterial chromosomal origins.

   • This facilitates quicker initiation of replication rounds.

Why Bacterial Chromosomal DNA Replicates More Slowly

• The bacterial chromosome is large and complex, requiring coordinated replication from a single origin (oriC) with bidirectional forks moving around the circular chromosome.

• Replication must be tightly coordinated with cell division and other cellular processes.

• The chromosome contains essential genes and regulatory sequences that impose constraints on replication timing and control.

Summary Table: Comparison of Replication Speed

Correct Answer

(1) Plasmid DNA will complete replication first

Conclusion

When bacterial genome and plasmid DNA replicate using the same replication machinery, plasmids generally complete replication faster due to their smaller size, specialized replication mechanisms, and efficient regulatory control. This allows plasmids to maintain multiple copies within the host cell without significantly impeding bacterial growth.