Some of these genes encode traits for antibiotic resistance or resistance to heavy metal, while others may produce virulence factors that enable a bacterium to colonize a host and overcome its defences or have specific metabolic functions that allow the bacterium to utilize a particular nutrient, including the ability to degrade recalcitrant or toxic organic compounds. Many of the genes carried by a plasmid are beneficial for the host cells, for example: enabling the host cell to survive in an environment that would otherwise be lethal or restrictive for growth. Plasmids almost always carry at least one gene. A few types of plasmids can also insert into the host chromosome, and these integrative plasmids are sometimes referred to as episomes in prokaryotes. Smaller plasmids make use of the host replicative enzymes to make copies of themselves, while larger plasmids may carry genes specific for the replication of those plasmids. A typical bacterial replicon may consist of a number of elements, such as the gene for plasmid-specific replication initiation protein (Rep), repeating units called iterons, DnaA boxes, and an adjacent AT-rich region. The self-replicating unit, in this case, the plasmid, is called a replicon. In order for plasmids to replicate independently within a cell, they must possess a stretch of DNA that can act as an origin of replication. There are two types of plasmid integration into a host bacteria: Non-integrating plasmids replicate as with the top instance, whereas episomes, the lower example, can integrate into the host chromosome. Later in 1968, it was decided that the term plasmid should be adopted as the term for extrachromosomal genetic element, and to distinguish it from viruses, the definition was narrowed to genetic elements that exist exclusively or predominantly outside of the chromosome and can replicate autonomously. The term plasmid was introduced in 1952 by the American molecular biologist Joshua Lederberg to refer to "any extrachromosomal hereditary determinant." The term's early usage included any bacterial genetic material that exists extrachromosomally for at least part of its replication cycle, but because that description includes bacterial viruses, the notion of plasmid was refined over time to comprise genetic elements that reproduce autonomously. 11 Software for bioinformatics and design.Plasmids vary in size from 1 to over 400 k bp, and the number of identical plasmids in a single cell can range anywhere from one to thousands under some circumstances. Unlike viruses, which encase their genetic material in a protective protein coat called a capsid, plasmids are "naked" DNA and do not encode genes necessary to encase the genetic material for transfer to a new host however, some classes of plasmids encode the conjugative "sex" pilus necessary for their own transfer. This host-to-host transfer of genetic material is one mechanism of horizontal gene transfer, and plasmids are considered part of the mobilome. Plasmids are transmitted from one bacterium to another (even of another species) mostly through conjugation. However, plasmids, like viruses, are not generally classified as life. Plasmids are considered replicons, units of DNA capable of replicating autonomously within a suitable host. Synthetic plasmids are available for procurement over the internet. In the laboratory, plasmids may be introduced into a cell via transformation. Artificial plasmids are widely used as vectors in molecular cloning, serving to drive the replication of recombinant DNA sequences within host organisms. While chromosomes are large and contain all the essential genetic information for living under normal conditions, plasmids are usually very small and contain only additional genes that may be useful in certain situations or conditions. In nature, plasmids often carry genes that benefit the survival of the organism and confer selective advantage such as antibiotic resistance. They are most commonly found as small circular, double-stranded DNA molecules in bacteria however, plasmids are sometimes present in archaea and eukaryotic organisms. Illustration of a bacterium showing chromosomal DNA and plasmids (Not to scale)Ī plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently.
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