Bacterial DNA – the role of plasmids
Like other organisms, bacteria use double-stranded DNA as their genetic material. However, bacteria organise their DNA differently to more complex organisms.
Bacterial DNA
Bacteria have a single circular chromosome that is located in the cytoplasm in a structure called the nucleoid. Bacteria also contain smaller circular DNA molecules called plasmids.
Bacterial DNA – a circular chromosome plus plasmids
The DNA of most bacteria is contained in a single circular molecule, called the bacterial chromosome. The chromosome, along with several proteins and RNA molecules, forms an irregularly shaped structure called the nucleoid. This sits in the cytoplasm of the bacterial cell.
In addition to the chromosome, bacteria often contain plasmids – small circular DNA molecules. Bacteria can pick up new plasmids from other bacterial cells (during conjugation) or from the environment. They can also readily lose them – for instance, when a bacterium divides in two, one of the daughter cells might miss out on getting a plasmid.
Every plasmid has its own ‘origin of replication’ – a stretch of DNA that ensures it gets replicated (copied) by the host bacterium. For this reason, plasmids can copy themselves independently of the bacterial chromosome, so there can be many copies of a plasmid – even hundreds – within one bacterial cell.
Plasmids help bacteria to survive stress
Antibiotic resistance
Some plasmids can make their host bacterium resistant to an antibiotic. In this image, two halves of an agar plate containing the antibiotic kanamycin have been spread with the same strain of E. coli. The bacteria spread on the right-hand side of the plate contain a plasmid that confers resistance to kanamycin, so they can grow colonies even when it is present. On the left-hand side of the plate, the bacteria lack the kanamycin resistance plasmid and have been unable to grow.
Plasmids contain just a few genes, but they make a big difference to their host bacterium. The genes are usually not essential for the bacterium’s day-to-day survival – instead, they help the bacterium to overcome occasional stressful situations. For instance, many plasmids contain genes that, when expressed, make the host bacterium resistant to an antibiotic (so it won’t die when treated with that antibiotic). Other plasmids contain genes that help the host to digest unusual substances or to kill other types of bacteria.
Plasmids make themselves indispensable
Keeping a plasmid is hard work for a bacterial cell, because replicating DNA (including plasmid DNA) uses up energy. However, by protecting its bacterial host from stress-related death, a plasmid maximises its chances of being kept around. Under stressful conditions, bacteria with the plasmid will live longer – and have more opportunity to pass on the plasmid to daughter cells or to other bacteria. Bacteria without the plasmid are less likely to survive and reproduce.
Some plasmids take extreme measures to ensure that they are retained within bacteria. For example, some carry a gene that makes a long-lived poison and a second gene that makes a short-lived antidote. These plasmids are effectively holding their host bacterial cell hostage – if they are ever lost from the cell, they won’t be able to provide the antidote and the cell will die.
Plasmid vector
Key features of a typical plasmid vector are an origin of replication (to ensure the vector is copied within bacteria), a gene for antibiotic resistance (to ensure the vector is not lost by bacteria) and a set of recognition sites for restriction enzymes (to make it straightforward to insert foreign DNA into the vector).
Plasmids in biotech – delivering DNA
Plasmids have been key to the development of molecular biotechnology. They act as delivery vehicles, or vectors, to introduce foreign DNA into bacteria. Using plasmids for DNA delivery began in the 1970s when DNA from other organisms was first ‘cut and pasted’ into specific sites within the plasmid DNA. The modified plasmids were then reintroduced into bacteria.
Why plasmids are excellent DNA delivery vectors
Decades after their first use, plasmids are still crucial laboratory tools in biotechnology:
Scientists can force bacteria to keep them. Virtually all plasmids that are used to deliver DNA contain genes for antibiotic resistance. Once bacteria have been treated with a plasmid, scientists grow them in the presence of antibiotic. Only those cells that contain the plasmid will survive, grow and reproduce. The others will be killed by the antibiotic.
They are copied independently. Plasmids can be copied numerous times, regardless of whether the bacterial host is replicating its own DNA, and every time a plasmid vector is replicated, so is the introduced DNA that it contains.
They are circular. DNA that is circular is well suited to incorporate extra DNA sequences. That’s because it can be cut open without falling apart, then snap back together once new DNA has been incorporated.
Read more about how to add foreign DNA to bacteria.
