Plasmids are self-replicating, extra chromosomal segments of double-stranded circular and naked DNA that are present in a bacterial cell. They are not a part of the bacterial chromosome or nucleoid, but exist as separate entities within the cell. Plasmids are typically smaller in size compared to the bacterial chromosome and can range from a few kilobases to hundreds of kilobases in length.

Key Points:
- Self-replicating: Plasmids have the ability to replicate independently of the bacterial chromosome. They contain all the necessary elements required for their replication, including an origin of replication and replication machinery.
- Extra chromosomal: Plasmids are separate from the bacterial chromosome and exist as independent entities within the cell. They can be present in multiple copies per cell.
- Circular DNA: Plasmids have a circular structure, unlike the linear structure of the bacterial chromosome. This circular configuration allows for efficient replication and stability.
- Naked DNA: Plasmids do not have a protein coat or envelope like bacteriophages. They are present as naked DNA molecules within the bacterial cell.
- Present in a bacterial cell: Plasmids are commonly found in various bacterial species. They can be transferred between bacterial cells through horizontal gene transfer mechanisms such as conjugation, transformation, and transduction.

Functions of Plasmids:
- Antibiotic resistance: Plasmids often carry genes that confer resistance to antibiotics. This allows bacteria to survive in the presence of antibiotics, making them more difficult to treat.
- Virulence factors: Some plasmids carry genes that encode for toxins or other virulence factors, which enhance the pathogenicity of the bacteria.
- Metabolic capabilities: Plasmids can contain genes that enable bacteria to metabolize specific compounds, such as the ability to degrade certain organic substances or utilize unusual carbon sources.
- Conjugation: Plasmids often carry genes required for the transfer of genetic material between bacterial cells through conjugation. This allows for the spread of plasmids and their associated traits within bacterial populations.

In conclusion, plasmids are self-replicating, extra chromosomal segments of circular and naked DNA that are present in a bacterial cell. They play important roles in bacterial adaptation, survival, and evolution by carrying genes that confer various advantages to the host bacteria.