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.
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.
