PLASMIDS

It is an extra chromosomal DNA molecule separate from the chromosomal DNA which is able to replicate independently of the chromosomal DNA. Most commonly found as small circular, double-stranded DNA molecules in bacteria, plasmids are sometimes present in archaea and eukaryotic organisms. In nature, plasmids carry genes that may benefit survival of the organism (e.g. antibiotic resistance), and can frequently be transmitted from one bacterium to another (even of another species) via horizontal gene transfer. Artificial plasmids are widely used as vectors in molecular cloning, serving to drive the replication of recombinant DNA sequences within host organisms.

• They vary from 1kb to 1000kb

• The term plasmid was first introduced by the American molecular biologist Joshua Lederberg  in 1952.
• The plasmid does not contribute to the genome of the bacterial cell it is present in but often translates proteins of significance. For example
• It contains the antibiotic resistance gene that helps the bacteria survive from that specific antibiotic.
• It can provide the bacteria with an ability to fix elemental nitrogen or to degrade calcitrant organic compounds which provide an advantage under conditions of nutrient deprivation.

Fig. 34 . This image shows a line drawing of a bacterium with its chromosomal DNA and several plasmids within it.

Plasmids used in genetic engineering are called vectors. Plasmids serve as important tools in genetics and biotechnology labs, where they are commonly used to multiply (make many copies of) or express particular genes. Many plasmids are commercially available for such uses. The gene to be replicated is inserted into copies of a plasmid containing genes that make cells resistant to particular antibiotics and a multiple cloning site (MCS, or polylinker), which is a short region containing several commonly used restriction sites allowing the easy insertion of DNA fragments at this location. Next, the plasmids are inserted into bacteria by a process called transformation . Then, the bacteria are exposed to the particular antibiotics. Only bacteria that take up copies of the plasmid survive, since the plasmid makes them resistant. In particular, the protecting genes are expressed (used to make a protein) and the expressed protein breaks down the antibiotics. In this way, the antibiotics act as a filter to select only the modified bacteria. Now these bacteria can be grown in large amounts, harvested, and lysed (often using the alkaline lysis method) to isolate the plasmid of interest. Another major use of plasmids is to make large amounts of proteins. In this case, researchers grow bacteria containing a plasmid harboring the gene of interest. Just as the bacterium produces proteins to confer its antibiotic resistance, it can also be induced to produce large amounts of proteins from the inserted gene. This is a cheap and easy way of mass-producing a gene or the protein it then codes for, for example, insulin or even antibiotics. A plasmid can contain inserts of up to 30-40 kbp . To clone longer lengths of DNA, lambda phage with lysogeny genes deleted, cosmids , bacterial artificial chromosomes, or yeast artificial chromosomes are used.

Replication of plasmid

Non Integrative Replication

In this type of replication the plasmid DNA once introduced into the cell grows as per the copy number and the multiplication of the cells