What is the small circular DNA molecule in bacteria that can be transferred between cells?

• A. Plasmid
• B. Chromosome
• C. Genome
• D. Nucleoid

Answer: (A)

The main concept being tested is how bacteria exchange genetic material through extrachromosomal DNA elements called plasmids. Plasmids are small circular DNA molecules that exist outside the bacterial chromosome and replicate independently. Their key feature is the ability to be transferred between cells, enabling the spread of traits like antibiotic resistance through horizontal gene transfer via mechanisms such as conjugation, transformation, or transduction. This makes plasmids the best description for the molecule described in the question.

In contrast, the chromosome is the larger, essential piece of genetic material that contains most of the organism’s genes. The genome refers to the entire set of genetic material of the organism, including all chromosomes (and any plasmids). The nucleoid is the region within the cell where the chromosome is located, not a separate DNA molecule.