Do eukaryotes have plasmids?

• A. Sometimes
• B. Never
• C. Always
• D. Rarely

Answer: (A)

Plasmids are small circular DNA molecules that replicate independently of chromosomal DNA. They are common in bacteria and archaea, where they often carry useful genes. In eukaryotes, having a plasmid is not the general rule, since most eukaryotes rely on linear chromosomes inside the nucleus. Yet there are notable exceptions: some fungi, like baker’s yeast, carry a circular, independently replicating plasmid (the 2-micron plasmid), and certain organelles in plants and other eukaryotes (mitochondria and chloroplasts) contain circular DNA that can function in a similar way to plasmids. In addition, extrachromosomal circular DNA elements can be found in some human cancer cells, though these aren’t part of normal eukaryotic genomes.

Because these exceptions exist, eukaryotes can have plasmids, but they are not universal among eukaryotes. That’s why the best answer is “Sometimes.”