Supercharge your learning!

Use adaptive quiz-based learning to study this topic faster and more effectively.

Bacterial gene transfer

Bacteria reproduce asexually, so unlike eukaryotes, genetic variation cannot be introduced during reproduction.

To increase genetic variation, bacteria undergo horizontal gene transfer.

Horizontal gene transfer is the transfer of genetic material between organisms through a means other than reproduction.

Horizontal gene transfer allows for advantageous genes to be passed between bacterial cells without the need for sexual reproduction.

Horizontal gene transfer cannot create genetic variation, it can only spread different versions of genetic material. The underlying genetic variations are ultimately caused by mutations.

Vertical gene transfer refers to the transfer of genetic material from a parent to the offspring.

Vertical gene transfer only occurs through reproduction. Bacteria divide asexually through binary fission, which is a simple three step process:

  1. The bacterial chromosome replicates.
  2. The two chromosome copies attach to the cell membrane on opposite sides of the cell. The cell membrane then grows, increasing the size of the cell.
  3. The cell constricts at the equator and divides, producing two daughter cells, each with a copy of the chromosome.
These E. coli are dividing. Bacteria divide through binary fission.
These E. coli are dividing. Bacteria divide through binary fission.

In horizontal gene transfer, genetic material is transferred between two organisms by methods other than reproduction.

Horizontal gene transfer is the main method of creating genetic variation in bacteria.

There are three methods of gene transfer in bacteria:

  • Transformation: uptake of DNA from the environment.
  • Transduction: transfer of genes by a bacteriophage.
  • Conjugation: exchange of DNA directly between two bacterial cells that are not reproducing.

Horizontal gene transfer is rare in eukaryotes but very common in prokaryotes such as bacteria.

E. coli has acquired 18% of its current genome through horizontal gene transfer.

Transformation is one of three methods of horizontal gene transfer.

During transformation, the bacterium acquires extracellular bacterial DNA (DNA floating freely in the environment).

This DNA is usually from dead bacteria that have been broken down.

Bacteria vary in their ability to take up DNA from their environment. In a laboratory setting, about 1% of bacteria are able to take up DNA.

In the natural environment, this percentage is believed to be slightly higher.

Bacteria that are able to undergo transformation carry specialised proteins on their cell membranes. These proteins allow extracellular DNA to be taken up into the cell.

Transformation is most successful if the DNA comes from a closely related species.

The sections of DNA taken up during transformation can be any part of the bacterial genome.

In the laboratory, transformation is widely used to introduce new genes into bacteria for the purpose of studying gene function.

Most bacteria cannot easily take up DNA from their environment (roughly 1% in a laboratory are able to take up DNA with no aid).

As a result, artificial aids are used to make the bacterial cell membrane permeable to extracellular DNA.

Bacteria are often placed in a calcium chloride solution ($$\ce{CaCl2}$$), and then heat shocked (heated quickly) to make the cell permeable to the DNA strand that needs to be introduced.

Unlike naturally occurring transformation, artificial transformation is passive and does not require specialised membrane proteins.

This means that any type of DNA can be introduced into the bacterial cell, even DNA that has not come from a bacterial genome.

Transduction is one of the three methods of horizontal gene transfer in bacteria.

Transduction is the injection of new segments of bacterial DNA into a bacterium by a bacteriophage (virus).

The viral genome is packed into a protein coat as it leaves a bacterial cell. Sometimes a section of the host's genome is be incorporated.

Therefore, when the bacteriophage infects another cell and the genetic package is injected, the virus may also inject some bacterial genetic material.

The injected bacterial DNA may be incorporated into the host's chromosome by recombination and then expressed in the host cell.

As with transformation, any part of the bacterial genome may be introduced into the host cell, and the two bacterial cells do not need to come into contact with each other for the DNA transfer.