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Genetic elements called transposons (transposable elements), or “ jumping genes ,” are molecules of DNA that include special inverted repeat sequences at their ends and a gene encoding the enzyme transposase ( [link] ). Transposons allow the entire sequence to independently excise from one location in a DNA molecule and integrate into the DNA elsewhere through a process called transposition . Transposons were originally discovered in maize (corn) by American geneticist Barbara McClintock (1902–1992) in the 1940s. Transposons have since been found in all types of organisms, both prokaryotes and eukaryotes. Thus, unlike the three previous mechanisms discussed, transposition is not prokaryote-specific. Most transposons are nonreplicative, meaning they move in a “cut-and-paste” fashion. Some may be replicative, however, retaining their location in the DNA while making a copy to be inserted elsewhere (“copy and paste”). Because transposons can move within a DNA molecule, from one DNA molecule to another, or even from one cell to another, they have the ability to introduce genetic diversity. Movement within the same DNA molecule can alter phenotype by inactivating or activating a gene.
Transposons may carry with them additional genes, moving these genes from one location to another with them. For example, bacterial transposons can relocate antibiotic resistance genes, moving them from chromosomes to plasmids. This mechanism has been shown to be responsible for the colocalization of multiple antibiotic resistance genes on a single R plasmid in Shigella strains causing bacterial dysentery. Such an R plasmid can then be easily transferred among a bacterial population through the process of conjugation.
[link] summarizes the processes discussed in this section.
| Summary of Mechanisms of Genetic Diversity in Prokaryotes | |
|---|---|
| Term | Definition |
| Conjugation | Transfer of DNA through direct contact using a conjugation pilus |
| Transduction | Mechanism of horizontal gene transfer in bacteria in which genes are transferred through viral infection |
| Transformation | Mechanism of horizontal gene transfer in which naked environmental DNA is taken up by a bacterial cell |
| Transposition | Process whereby DNA independently excises from one location in a DNA molecule and integrates elsewhere |
Despite continued antibiotic treatment, Mark’s infection continued to progress rapidly. The infected region continued to expand, and he had to be put on a ventilator to help him breathe. Mark’s physician ordered surgical removal of the infected tissue. Following an initial surgery, Mark’s wound was monitored daily to ensure that the infection did not return, but it continued to spread.
After two additional rounds of surgery, the infection finally seemed to be contained. A few days later, Mark was removed from the ventilator and was able to breathe on his own. However, he had lost a great deal of skin and soft tissue on his lower leg.
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A small DNA molecule that has the ability to independently excise from one location in a larger DNA molecule and integrate into the DNA elsewhere is called a ________.
transposon or transposable element
________ is a group of mechanisms that allow for the introduction of genetic material from one organism to another organism within the same generation.
Horizontal gene transfer
Asexually reproducing organisms lack mechanisms for generating genetic diversity within a population.
False
Briefly describe two ways in which chromosomal DNA from a donor cell may be transferred to a recipient cell during the process of conjugation.
Describe what happens when a nonsense mutation is introduced into the gene encoding transposase within a transposon.
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