Which of the following model organisms has only one single chromosome (is haploid)? Xenopus (frog) Escherichia coli (E. coli bacterium) Drosophila melanogaster (fruitfly) Mus musculus (mouse) C. elegans (roundworm)

*Escherichia coli (E. coli bacterium)*<br /><br />Most bacteria, including *E. coli*, have a single circular chromosome and are haploid. The other organisms listed are all eukaryotes and have multiple linear chromosomes and are typically diploid (or have diploid stages in their life cycle).<br />