Bacterial genetics – What is the approximate size of the Escherichia coli genome expressed in kilobases (kb)?

Introduction:
Genome size provides context for gene number, regulatory complexity, and replication timing. E. coli K-12 is a benchmark organism in microbiology and molecular genetics, and its genome size is widely cited in textbooks and databases.

Given Data / Assumptions:

• Genome size is commonly reported in base pairs or kilobases/megabases.
• E. coli K-12 laboratory strains have well-characterized genomes.
• Approximation within a few percent is acceptable in survey questions.

Concept / Approach:
The E. coli K-12 genome is approximately 4.6 megabases (Mb), which equals about 4600 kilobases (kb). Among the choices, 4500 kb is the closest reasonable approximation. Natural isolates may vary slightly, but ~4.5–4.7 Mb is a robust rule of thumb for teaching purposes.

Step-by-Step Solution:

Verification / Alternative check:
Reference genome assemblies report sizes near 4.6 Mb for K-12 MG1655, supporting the 4500–4600 kb approximation used in educational contexts.

Why Other Options Are Wrong:

• 3000/1500 kb: far too small for E. coli.
• 5500/6500 kb: larger than commonly reported laboratory strains.

Common Pitfalls:
Confusing kilobases (kb) with kilobytes (kB) or megabases (Mb), and forgetting that 1 Mb = 1000 kb in genomics shorthand.

Final Answer:
4500 kilobase