Bacterial genetics — The chromosome of most bacteria typically exists as which overall physical form?

Introduction / Context:
Prokaryotic chromosomes differ from eukaryotic chromosomes in both structure and packaging. Understanding the common topology of bacterial chromosomes helps explain replication origins, supercoiling, and segregation mechanisms, as well as exceptions like linear chromosomes in a few species.

Given Data / Assumptions:

• We are discussing the main chromosomal DNA, not plasmids.
• “Closed circle” refers to covalently closed circular DNA (cccDNA).
• Generalizations apply to “almost all” bacteria, acknowledging rare exceptions.

Concept / Approach:
Most bacteria have a single, covalently closed circular chromosome that is highly supercoiled and associated with nucleoid-associated proteins. While the DNA double helix is a helix at the molecular level, “helix” alone does not describe the chromosomal topology. Open circular DNA would have a nick rather than being covalently closed, and a linear form is uncommon (though present in some genera like Streptomyces and Borrelia). Therefore, the typical configuration is a closed circle.

Step-by-Step Solution:

Verification / Alternative check:
Electron micrographs of isolated bacterial chromosomes show circular, supercoiled DNA consistent with covalently closed topology.

Why Other Options Are Wrong:

• Open circle: implies a nicked, non-supercoiled form—not the normal chromosomal state.
• Helix: describes secondary structure, not chromosome shape.
• Linear chromosome: rare exception, not typical.

Common Pitfalls:
Confusing plasmid topology with the chromosome; both are often circular, but the question targets the main chromosome.

Final Answer:
a closed circle