What Is a Plasmid?
A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Plasmids are most commonly found in bacteria but can also exist in archaea and eukaryotic organisms like yeast. They replicate independently of the main chromosome.
Section 2: Key Features of Plasmids
Plasmids typically carry a small number of genes that are not essential for the cell's normal survival but often provide a selective advantage. These genes can confer traits such as antibiotic resistance, metabolic functions, or virulence. Because they are separate from the chromosome, they can be easily transferred from one bacterium to another.
Section 3: A Practical Example
A common example is an R-plasmid (Resistance plasmid). A bacterium might contain an R-plasmid with a gene that produces an enzyme capable of breaking down an antibiotic like penicillin. While a bacterium without this plasmid would die when exposed to penicillin, the one carrying the plasmid can survive and reproduce, passing the resistance trait to other bacteria.
Section 4: Importance in Science
Plasmids are crucial tools in molecular biology and genetic engineering. Scientists can insert specific genes of interest into a plasmid and then introduce that plasmid into bacteria. The bacteria then act as tiny factories, replicating the plasmid and producing large quantities of the protein encoded by the inserted gene, such as insulin.