Bacterial Transformation: Videos & Practice Problems

Bacterial transformation is a process where bacteria take up DNA from their environment. This can happen naturally when bacteria die and release their DNA, or it can be induced experimentally. In natural transformation, DNA from lysed bacteria is released into the environment and can be taken up by competent bacteria, which are in a physiological state that allows DNA uptake. Experimentally, scientists can introduce DNA into a bacterial culture, and through methods like high salt or high temperature, make the bacteria competent to take up the DNA. The DNA can exist as plasmids or fragments that integrate into the bacterial chromosome, forming structures like heteroduplexes.

Competent cells are bacteria that have the ability to take up extracellular DNA from their environment. This competence can be a natural physiological state or induced experimentally. Naturally competent bacteria have specific mechanisms to uptake DNA, while in the lab, competence is often induced by treating bacteria with high salt concentrations or heat shock, which makes their cell membranes more permeable to DNA. Competent cells are crucial for transformation experiments, as they allow the uptake and incorporation of foreign DNA, which can be used for gene cloning, protein expression, and genetic mapping.

Bacterial transformation is used in gene mapping by determining the proximity of genes on a DNA molecule. Scientists fragment the DNA and incubate it with competent bacteria. During transformation, closely located genes are more likely to be taken up together, a process known as double transformation. By analyzing which genes are co-transformed, researchers can infer their relative positions on the chromosome. The closer two genes are, the higher the likelihood they will be taken up together, allowing for the mapping of gene loci based on transformation frequencies.

In bacterial transformation, plasmid DNA and chromosomal DNA are taken up differently. Plasmid DNA, which is circular and extrachromosomal, can enter the bacterial cell and replicate independently. This is commonly used in laboratory settings for cloning and protein expression. Chromosomal DNA, on the other hand, is usually linear and fragmented. Once inside the bacterial cell, it is often digested into single strands and integrated into the bacterial chromosome through homologous recombination, forming structures called heteroduplexes. This integration allows for the stable incorporation of new genetic material into the bacterial genome.

The main experimental methods to induce competence in bacteria include chemical and physical treatments. Chemical methods often involve treating bacteria with calcium chloride (CaCl₂), which increases the permeability of the cell membrane to DNA. Physical methods include heat shock, where bacteria are exposed to a sudden increase in temperature, and electroporation, where an electric field is applied to create temporary pores in the cell membrane. These methods make the bacterial cell membrane more permeable, allowing the uptake of extracellular DNA during transformation.