How are composite transposons formed in bacterial genomes?

Introduction / Context:
Composite transposons are larger mobile elements that capture and mobilize antibiotic resistance or metabolic genes. They form a potent mechanism for rapid genetic innovation and spread of resistance clusters.

Given Data / Assumptions:

• Insertion sequences (IS) are short transposable elements encoding transposase, flanked by inverted repeats.
• Two IS elements can land near one another in direct or inverted orientation.
• The chromosomal DNA between them can be mobilized as a single unit.

Concept / Approach:
When two IS elements flank a region, the pair functions as the ends of a larger transposon. The captured central genes (for example, antibiotic resistance) move with the composite element during subsequent transposition events, facilitating horizontal spread.

Step-by-Step Solution:
Define composite transposon architecture: IS–[cargo genes]–IS.Establish formation mechanism: independent IS insertions bracketing a locus.Conclude option describing two IS elements flanking intervening DNA is correct.

Verification / Alternative check:
Classic examples include Tn9 and Tn10 derivatives where resistance genes lie between IS elements acting as transposition termini.

Why Other Options Are Wrong:

• Single IS deletions do not create composites.
• Plasmid integration is not required for the composite structure.
• “IS into IS” is not the defining mechanism for bracketing chromosomal DNA.

Common Pitfalls:
Confusing simple IS transposition with composite element formation that captures adjacent genes.

Final Answer:
By two IS elements inserting near each other in a chromosome and bracketing intervening genes