Plasmids

In addition to having a chromosome, many bacteria possess plasmids: small, usually circular DNA molecules. Some plasmids are present in many copies per cell, whereas others are present in only one or two copies. In general, plasmids carry genes that are not essential to bacterial function but may play an important role in the life cycle and growth of their bacterial hosts. There are many different types of plasmids; E. coli alone is estimated to have more than 270 different naturally occurring plasmids. Some plasmids promote mating between bacteria; others produce compounds that kill other bacteria. Plasmids are used extensively in genetic engineering (see Chapter 14), and some of them play a role in the spread of antibiotic resistance among bacteria.

Most plasmids are circular and several thousand base pairs in length, although plasmids consisting of several hundred thousand base pairs have also been found. Each plasmid possesses an origin of replication, a specific DNA sequence where DNA replication is initiated (see Chapter 2). The origin allows a plasmid to replicate independently of the bacterial chromosome (Figure 7.5). Episomes are plasmids that are capable of replicating freely and are able to integrate into the bacterial chromosomes. The F (fertility) factor of E. coli (Figure 7.6) is an episome that controls mating and gene exchange between E. coli cells, a process we will discuss shortly.

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Figure 7.5: A plasmid replicates independently of its bacterial chromosome. Replication begins at the origin of replication (ori) and continues around the circle. In this diagram, replication is taking place in both directions; in some plasmids, replication is in one direction only.
[Photograph: Biology Pics/Science Source.]
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Figure 7.6: The F factor, a circular episome of E. coli, contains a number of genes that regulate its transfer into a bacterial cell, replication, and insertion into the bacterial chromosome. Replication is initiated at ori. Insertion sequences IS3 and IS2 control both insertion into the bacterial chromosome and excision from it.

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CONCEPTS

Bacteria can be studied in the laboratory by growing them on liquid or solid media. A typical bacterial genome consists of a single circular chromosome that contains several million base pairs. Some bacterial genes may be present on plasmids, which are small, circular DNA molecules that replicate independently of the bacterial chromosome.

image CONCEPT CHECK 1

Which of the following statements is true of plasmids?

  1. They are composed of RNA.

  2. They normally exist outside of bacterial cells.

  3. They possess only a single strand of DNA.

  4. They contain an origin of replication.

d