A collection of blog posts connected to my teaching on biomedical sciences and biochemistry degrees. All views and opinions expressed are my own, and not connected to my past, present or future employers.
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Transduction is driven by bacteriophages, which are viruses that specifically target and infect bacterial cells. In the video, I explain the mechanics of transduction and look at how bacteriophages can inadvertently transfer DNA from one bacterium to another.
Bacteriophages operate through two distinct life cycles: the lytic and lysogenic cycles. During the lytic cycle, the bacteriophage attaches to a bacterial cell, injects its DNA, and takes over the bacterium to produce new viruses. This ultimately leads to the bursting (lysis) of the bacterial cell, releasing the newly formed phages to continue the infection cycle. In the lysogenic cycle, the bacteriophage DNA is integrated into the bacterial chromosome, where it can lie dormant until conditions favour a return to the lytic cycle.
Some bacteriophages, such as the P22 phage, can mistakenly package bacterial DNA instead of their own during the assembly of new viruses. When these phages go on to infect other bacteria, they transfer the captured bacterial DNA, effectively driving genetic exchange between bacteria. This mechanism contributes to bacterial evolution in the wild and serves as a valuable tool in the lab for gene mapping and studying bacterial genetics.
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Gene mapping in bacteria is a process that has helped scientists understand the order and location of genes on a bacterial chromosome.
In this post, I will look at a method of gene mapping using bacterial conjugation, specifically focusing on a technique known as interrupted mating. Although this technique is historical and has been largely replaced by genome sequencing, it remains an important method that sheds light on genetic exchange in bacteria.
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The Role of Hfr Cells in Gene Mapping
The process of using conjugation to map genes in bacteria was developed using a specific type of bacteria discovered in the 1950s, known as Hfr (High Frequency of Recombination) cells. These cells are particularly efficient at transferring genes during conjugation, a process where DNA is exchanged between two bacterial cells.
In Hfr cells, the F (fertility) factor, which is normally an independent plasmid, gets incorporated into the bacterial chromosome. This integration means that during conjugation, the F factor and parts of the bacterial chromosome can be transferred to a recipient cell.
Two outcomes can occur during this process:
F' Factors: If the F factor is excised from the chromosome, it may carry with it some bacterial genes, forming what is known as F' factors.
Chromosome Transfer: Alternatively, the entire bacterial chromosome, including the F factor, can be transferred to the recipient cell.
The Concept of Interrupted Mating
Interrupted mating is a technique that was used for gene mapping. To understand how it works, let’s consider an example involving two bacterial strains:
Donor Bacteria (Hfr positive): This strain has the genes to produce amino acids leucine and threonine but is sensitive to the antibiotic ampicillin.
Recipient Bacteria (Hfr negative): This strain lacks the genes for leucine and threonine production but is resistant to ampicillin.
In this experiment, the donor and recipient bacteria are mixed and allowed to conjugate. The goal is to map the location of the genes that produce leucine and threonine on the donor’s chromosome. Here’s how the process unfolds:
Conjugation Begins: The bacteria are mixed, and DNA transfer begins from the Hfr donor to the recipient.
Interrupted Mating: At specific time intervals, the mating process is interrupted, effectively stopping the transfer of DNA.
Plating on Selective Media: After interrupting the mating, the bacteria are plated on media that either lacks leucine or threonine and contains ampicillin. The donor bacteria, which are sensitive to ampicillin, will die. The recipient bacteria will only grow if they have received and integrated the necessary genes from the donor to produce the missing amino acids.
Mapping Gene Order: By examining which plates show bacterial growth at different time points, scientists can determine the order in which the genes were transferred. For instance, if bacteria start growing on the leucine-lacking plate before the threonine-lacking plate, it indicates that the leucine gene is closer to the origin of transfer than the threonine gene.
Applications and Limitations of the Technique
This method of mapping was useful because it allows scientists to estimate the relative positions of genes on a bacterial chromosome. For example, using this technique, researchers were able to map the E. coli K12 genetic map into a timeline of 100 minutes, representing the time required for the full chromosome to transfer at 37 °C.
However, the technique has limitations:
It is mostly applicable to E. coli and closely related bacteria.
Mapping the entire chromosome is rare and works best for genes that are close to each other.
The F plasmids involved are large, of low copy number, and may not be ideal for genetic manipulation.
Despite these limitations, the interrupted mating technique provides valuable insights into genetic exchange mechanisms in bacteria. While modern approaches like genome sequencing have largely replaced it, understanding this historical method helps us appreciate the exchange of DNA between bacteria.
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Additional Reading
The video was produced with help from the following resources:
In this video, I look at the three methods bacteria use to share genetic information: Conjugation, Transduction, and Transformation.
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Bacteria typically increase their numbers through a process akin to cloning; that is, they make exact copies of themselves. While this method is efficient, it presents a significant evolutionary limitation: genetic diversity can only arise through random mutations. In the absence of a mechanism for exchanging genes, bacteria would be stuck in a genetic standstill, unable to benefit from the rapid spread of advantageous traits. Bacteria solve this problem by horizontal gene transfer (HGT), which is also called lateral gene transfer (LGT).
To help the spread of advantageous genes in a bacteria population, the bacteria use three methods to exchange DNA:
Conjugation: This process involves direct physical contact between two bacteria through a structure called a sex pilus. A donor bacterium with an F plasmid (F+) forms a bridge to a recipient bacterium (F-), transferring a copy of its DNA. This method not only promotes genetic diversity but also allows for the rapid spread of advantageous traits such as antibiotic resistance.
Transduction: In this method, bacteriophages (viruses that infect bacteria) play a crucial role. When a virus infects a bacterium, it sometimes incorporates fragments of the host's DNA into its own genetic material. As the virus infects new bacterial cells, it transfers these DNA fragments, facilitating genetic exchange.
Transformation: This process occurs when bacteria take up free-floating DNA from their environment, often released by dead bacterial cells. The acquired DNA is then incorporated into the recipient's genome, providing new genetic traits that can be beneficial for survival and adaptation.
In the video, I examine the three methods.
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Additional Reading
The video was produced with help from the following resources:
