Plasmids are extra-chromosomal covalently-closed circular molecules of double stranded DNA (Deoxyribonucleic acid). Since the DNA is double-stranded, plasmids are capable of autonomous molecule replication. Plasmids are the essential tools of genetic engineering, where scientists isolate specific sections of DNA that encode for specific traits. Plasmids are also used as vectors in molecular biolgy studies. Plasmids are widely distributed, being available for a multiplicity of host cells. Since they have double-stranded DNA, they are not dependent altogether on their host cell. Plasmids are dispensable to their host cell. The phenotype plasmids encode for may be useful in particular conditions of bacterial growth. For example, there may be a type of plasmid that encodes for existence under harsh conditions, such as low nutrients, or extreme temperatures.