Читать книгу Astrobiology - Charles S. Cockell - Страница 117

Organisms carry DNA in forms other than their primary chromosomal DNA. A plasmid is a small DNA molecule that is separate and can replicate independently of the chromosomal DNA in a cell, although it is considered to be part of the total genome of an organism (Figure 5.15). Plasmids are most commonly found as small circular, double-stranded DNA molecules in bacteria, archaea, and eukaryotic organisms. They carry genes that can benefit the survival of the organism in the natural environment. Examples include genes for antibiotic resistance (which makes them of central significance in the transfer of antibiotic resistance between bacteria in hospitals) and resistance to heavy metals (which allows organisms to live in diverse environments with heavy metals, from volcanic hot springs to human-polluted industrial sites). Plasmids can also provide bacteria with the ability to fix nitrogen gas from the atmosphere into useful nitrogen compounds or to degrade certain organic compounds that provide an advantage when nutrients are scarce. Plasmids vary in size from about a thousand to over a million base pairs, and the number of identical plasmids in a single cell can range from one to several thousand depending upon the environment and the species.

Figure 5.15 Plasmids are small circular pieces of DNA. They can be introduced into microorganisms by genetic engineers to produce industrially important products. The plasmid shown here has an origin of replication, which is the sequence that allows it to be copied. It contains a “coding sequence,” which could be a DNA sequence producing an important drug. The “promoter” is a DNA sequence that allows the coding region to be read. The transcription termination sequence tells the genetic machinery when to stop transcribing. It also contains a gene for antibiotic resistance which allows for bacteria successfully carrying the plasmid to be selected by researchers if they want to make sure that only the drug-producing bacteria are growing. The other sites marked on this diagram (e.g. SpeI, kpnI) are locations where specific enzymes (restriction enzymes) cut the DNA and allow the researcher to insert or remove bits of DNA.

Artificial plasmids are widely used in molecular cloning, where they serve as “carriers” of foreign genes that the researcher might wish to incorporate and have read inside a particular organism. For this reason, they are one of the most important tools in genetic engineering (Figure 5.15).

Plasmids are sometimes also called replicons, capable of replicating autonomously within a suitable host. However, plasmids, like viruses, are not considered to be a form of life. You might like to consider whether you agree with this view, and why, in the light of discussions in Chapter 2.