Plasmids are a type of covalent, closed, circular double stranded DNA molecule that naturally exists in bacterial and fungal cells and can replicate independently of chromosomal DNA. They are also known as ccDNA and typically range in size from 1 to 600 kb.
A vector is a type of DNA molecule that can self replicate, in which a segment of DNA is cleaved without affecting its replication, and can be used to replace or insert foreign (target) DNA to bring the target DNA into the host cell. Common vectors include plasmids, bacteriophages, viruses, etc.
Plasmid vectors are artificially constructed plasmids based on natural plasmids to adapt to laboratory operations. Compared with natural plasmids, plasmid vectors typically carry one or more selective marker genes (such as antibiotic resistance genes) and an artificially synthesized multi clone site sequence containing multiple restriction enzyme recognition sites, while removing most non essential sequences to minimize molecular weight for genetic engineering operations.
Plasmid construction is a commonly used experimental technique in molecular biology research. The principle relies on the action of restriction endonucleases, DNA ligases, and other modifying enzymes, which respectively cut and modify the target gene and vector DNA appropriately, connect the two together, and then introduce them into the host cell to achieve the correct expression of the target gene in the host cell.