What is a plasmid?

A plasmid is a small, circular piece of DNA that exists independently of chromosomal DNA within a cell. Plasmids are commonly found in bacteria and can carry genes that provide advantageous traits, such as antibiotic resistance. Their circular structure allows them to replicate independently of the bacterial chromosome, which enables them to be passed from one bacterial cell to another through processes such as conjugation. This characteristic is pivotal in genetic engineering and biotechnology, where plasmids are often utilized as vectors to transfer genes of interest into other organisms.

The presence of plasmids in bacterial cells also plays a significant role in horizontal gene transfer, contributing to genetic diversity and adaptation. In summary, the defining feature of plasmids is their small, circular composition and their ability to replicate independently, making option C the correct description of what a plasmid is.