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  • What type of replication occurs during conjugation?
  • What occurs to produce an Hfr E. coli cell?
  • What types of traits are encoded on plasmids?

Transposition

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.

Diagram of a transposon. 1: A typical transposon encodes the enzyme transposase, surrounded by inverted repeat sequences. A segment of chromosome shows that the transposon is interspersed between genes. The transposon is made of a gene for transposase and small bands labeled inverted repeat sequence on either side of the gene. 2: Transposase facilitates recombination between inverted repeats. Transposon is cut from its original location and inserted into a new location. This is shown by an oval labeled transposase causing the DNA segment for fold upon itself so the inverted repeats are nearly touching. 3: Transposon targets specific sequences in DNA that will be duplicated, forming direct repeats on either side of the inserted transposon sequence. This is shows as the transposon now sitting in the middle of a gene labeled disrupted gene.
Transposons are segments of DNA that have the ability to move from one location to another because they code for the enzyme transposase. In this example, a nonreplicative transposon has disrupted gene B. The consequence of that the transcription of gene B may now have been interrupted.
  • What are two ways a transposon can affect the phenotype of a cell it moves to?

[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

Part 3

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.

  • Why does the removal of infected tissue stem the infection?
  • What are some likely complications of this method of treatment?

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Key concepts and summary

  • Horizontal gene transfer is an important way for asexually reproducing organisms like prokaryotes to acquire new traits.
  • There are three mechanisms of horizontal gene transfer typically used by bacteria: transformation , transduction , and conjugation .
  • Transformation allows for competent cells to take up naked DNA, released from other cells on their death, into their cytoplasm, where it may recombine with the host genome.
  • In generalized transduction , any piece of chromosomal DNA may be transferred by accidental packaging of the degraded host chromosome into a phage head. In specialized transduction , only chromosomal DNA adjacent to the integration site of a lysogenic phage may be transferred as a result of imprecise excision of the prophage.
  • Conjugation is mediated by the F plasmid, which encodes a conjugation pilus that brings an F plasmid-containing F + cell into contact with an F - cell .
  • The rare integration of the F plasmid into the bacterial chromosome, generating an Hfr cell , allows for transfer of chromosomal DNA from the donor to the recipient. Additionally, imprecise excision of the F plasmid from the chromosome may generate an F’ plasmid that may be transferred to a recipient by conjugation.
  • Conjugation transfer of R plasmids is an important mechanism for the spread of antibiotic resistance in bacterial communities.
  • Transposons are molecules of DNA with inverted repeats at their ends that also encode the enzyme transposase, allowing for their movement from one location in DNA to another. Although found in both prokaryotes and eukaryotes, transposons are clinically relevant in bacterial pathogens for the movement of virulence factors, including antibiotic resistance genes.

Fill in the blank

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

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________ 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

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True/false

Asexually reproducing organisms lack mechanisms for generating genetic diversity within a population.

False

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Short answer

Briefly describe two ways in which chromosomal DNA from a donor cell may be transferred to a recipient cell during the process of conjugation.

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Describe what happens when a nonsense mutation is introduced into the gene encoding transposase within a transposon.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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