A plasmid is a small, circular piece of DNA found in many bacteria and some eukaryotic cells. Unlike chromosomal DNA, which is linear and essential for basic life functions, plasmids can carry additional genes that may provide advantages, such as antibiotic resistance. They replicate independently of chromosomal DNA, allowing for easy transfer between cells. Plasmids are widely used in genetic engineering and biotechnology. Scientists can insert specific genes into plasmids to create recombinant DNA, which can then be introduced into host cells for research or therapeutic purposes. This technique is crucial for producing proteins, studying gene function, and developing new treatments.