Questions: Bacterial Chromosome Structure and Gene Organization

Question 1

You treat bacteria with novobiocin, a drug that specifically inhibits DNA gyrase. What would you expect to happen to the bacterial chromosome?

• A. The chromosome becomes more negatively supercoiled and more compact, since gyrase inhibition prevents relaxation
• B. The chromosome becomes less compact as negative supercoiling is lost, and replication is impaired
• C. The chromosome fragments because gyrase is required for maintaining DNA strand integrity
• D. The chromosome converts from circular to linear form because gyrase maintains the covalently closed state

Answer: (B)

DNA gyrase introduces negative supercoils that compact the chromosome and facilitate strand separation for replication and transcription. Inhibiting gyrase shifts the balance toward topoisomerase I's relaxing activity, reducing negative supercoiling and making the chromosome less compact. Reduced supercoiling also impairs replication and transcription (which require the torsional strain of negative supercoiling to facilitate strand separation). Option 0 confuses gyrase's direction of action: gyrase introduces negative supercoils; inhibiting it removes them.

Question 2

A newly discovered bacterium has most of its functionally related genes dispersed as individual genes rather than organized into operons. What is the most likely regulatory consequence?

• A. More precise gene expression, since individual promoters allow each gene to be fine-tuned independently
• B. Less coordinated expression of functionally related genes, requiring separate regulatory signals for each
• C. Faster transcription overall, since polycistronic mRNAs take longer to transcribe than individual mRNAs
• D. Reduced genome size, since operons require additional regulatory DNA that individual genes avoid

Answer: (B)

The operon architecture allows a single regulatory event (one promoter turning on or off) to coordinate simultaneous expression of all genes in a metabolic pathway. Without operons, each gene in a pathway needs its own independent regulatory signal, making coordinated responses slower and less certain. The lac operon's advantage is that lactose induces production of all three enzymes at once; without the operon structure, the cell would have to independently regulate each gene, risking temporal mismatches in enzyme availability.

Question 3

Bacterial chromosomes are organized into topologically independent supercoiling domains so that a break in one domain relaxes supercoiling throughout the entire chromosome.

• T. True
• F. False

Answer: False

The whole biological purpose of topological domain organization is the opposite: each domain maintains its own supercoiling state independently. A nick (single-strand break) in one domain relaxes only that loop; neighboring domains remain compacted and functional. This compartmentalization protects the overall chromosome structure from local damage. If all domains were topologically continuous, a single nick would collapse the supercoiling of the entire chromosome.

Question 4

Bacterial chromosomes are more gene-dense than eukaryotic chromosomes, with the majority of their sequence encoding proteins or structural RNAs.

• T. True
• F. False

Answer: True

Approximately 85–95% of bacterial chromosome DNA codes for proteins or structural RNAs, with minimal non-coding intergenic sequence. By contrast, only about 1.5% of the human genome encodes proteins. This extreme gene density in bacteria reflects strong selective pressure to maintain small, rapidly-replicated genomes. A typical E. coli cell can copy its 4.6 million base pair chromosome in about 40 minutes, and genome economy is a key factor enabling this speed.

Question 5

Why do bacteria use DNA supercoiling to compact their chromosomes rather than the histone-based nucleosome system used by eukaryotes? What functional advantage does supercoiling provide beyond compaction?

Model answer: Bacteria lack histones and instead use DNA gyrase (which introduces negative supercoils), topoisomerase I (which relaxes them), and nucleoid-associated proteins (NAPs like HU, IHF, H-NS) to compact and organize their chromosomes. Beyond compaction, negative supercoiling provides a critical functional advantage: the torsional strain of underwound DNA makes strand separation energetically easier, directly facilitating the strand opening required for both DNA replication and transcription initiation. Organization into topologically independent domains adds further functional value by insulating local damage and allowing region-specific regulation of supercoiling density, which affects gene accessibility.

This explains why antibiotics targeting DNA gyrase (fluoroquinolones, aminocoumarins) are effective: they disrupt both chromosome compaction and the essential enzymatic activities that depend on proper supercoiling levels. The nucleosome system eukaryotes use provides compaction and chromatin-based gene regulation, but bacteria achieve analogous functions through a fundamentally different (and simpler) mechanism suited to their smaller genomes and faster replication rates.