Which transposable element consists of inverted repeat sequences at both ends and carries a gene encoding the transposase enzyme (but no additional cargo genes)?

Introduction / Context:
Transposable elements are mobile DNA segments. Two common prokaryotic forms are insertion sequences (IS elements) and composite transposons. IS elements are the most minimal units and encode only the functions required for their movement. This question differentiates IS elements from other mobile genetic elements.

Given Data / Assumptions:

• Element has inverted repeats (IRs) at both ends.
• Element encodes transposase.
• No mention of extra genes such as antibiotic resistance.

Concept / Approach:
IS elements include terminal inverted repeats and a transposase gene. Composite transposons consist of two IS elements flanking additional genes (e.g., resistance genes). Viruses and plasmids are broader entities and do not fit the precise structural description of IRs + transposase only.

Step-by-Step Solution:

Verification / Alternative check:
Classic examples such as IS1, IS3 families in bacteria have IRs and a transposase, confirming the definition.

Why Other Options Are Wrong:

• Composite transposon: Requires flanking IS elements around additional genes; not minimal.
• Virus: Encodes viral replication and capsid proteins; not just IRs + transposase.
• Plasmid: Autonomous circular DNA; lacks the IR/transposase architecture by definition.

Common Pitfalls:

• Assuming all transposons carry resistance genes; IS elements do not.
• Confusing IRs with palindromic sequences in other contexts.

Final Answer:
Insertion element