Mechanisms that generate variation in prokaryote populations. Transduction, transformation, conjugation, transposable elements.
- In transformation, a bacterium takes up a piece of DNA floating in its environment.
- In transduction, DNA is accidentally moved from one bacterium to another by a virus.
- In conjugation, DNA is transferred between bacteria through a tube between cells.
- Transposable elements are chunks of DNA that "jump" from one place to another. They can move bacterial genes that give bacteria antibiotic resistance or make them disease-causing.
When you hear the word "clone," what do you think of? Maybe Dolly the sheep, or experiments carried out in molecular biology labs. But it's also true that the bacteria around you—on your skin, in your gut, growing on your kitchen sink—are "cloning" themselves all the time!
Bacteria reproduce by splitting in two via binary fission. Binary fission makes clones, or genetically identical copies, of the parent bacterium. Since the "child" bacteria are genetically identical to the parent, binary fission doesn't provide an opportunity for genetic recombination or genetic diversity (aside from the occasional random mutation). This contrasts with sexual reproduction.
Still, genetic variation is key to the survival of a species, allowing groups to adapt to changes in their environment by natural selection. That's true for bacteria as well as plants and animals. So it's not too surprising that prokaryotes can share genes by three other mechanisms: conjugation, transformation, and transduction.
In transformation, a bacterium takes in DNA from its environment, often DNA that's been shed by other bacteria. In a laboratory, the DNA may be introduced by scientists (see biotechnology article). If the DNA is in the form of a circular DNA called a plasmid, it can be copied in the receiving cell and passed on to its descendants.
Left: plasmid taken up by transformation.
Right: linear DNA fragment taken up by transformation and swapped into the bacterial chromosome by homologous recombination.
Why would this be important? Imagine that a harmless bacterium takes up DNA for a toxin gene from a pathogenic (disease-causing) species of bacterium. If the receiving cell incorporates the new DNA into its own chromosome (which can happen by a process called homologous recombination), it too may become pathogenic.
In transduction, viruses that infect bacteria move short pieces of chromosomal DNA from one bacterium to another "by accident."
Yep, even bacteria can get a virus! The viruses that infect bacteria are called bacteriophages. Bacteriophages, like other viruses, are the pirates of the biological world—they commandeer a cell's resources and use them to make more bacteriophages.
However, this process can be a little sloppy. Sometimes, chunks of host cell DNA get caught inside the new bacteriophage as they are made. When one of these "defective" bacteriophages infects a cell, it transfers the DNA. Some bacteriophages chop the DNA of their host cell into pieces, making this transfer process more likely.
Virus infects cell by injecting its DNA. Bacterial DNA is fragmented and viral DNA is replicated. New viral particles are made and exit the cell. One contains host DNA instead of viral DNA. When this virus infects a new host, it injects the bacterial DNA, which can recombine with the chromosome of the new hows.
Archaea, the other group of prokaryotes besides bacteria, are not infected by bacteriophages but have their own viruses that move genetic material from one individual to another.
In conjugation, DNA is transferred from one bacterium to another. After the donor cell pulls itself close to the recipient using a structure called a pilus, DNA is transferred between cells. In most cases, this DNA is in the form of a plasmid.
- An F+ donor cell contains its chromosomal DNA and an F plasmid. It has a rodlike pilus. A recipient F- cell has only a chromosome and no F plasmid.
- The donor cell uses its pilus to attach to the recipient cell, and the two cells are pulled together.
- A channel forms between the cytoplasms of the two cells, and a single strand of the F plasmid is fed through.
- Both of the cells now have an F plasmid and are F+. The former recipient cell is now a new donor and can form a pilus.
Donor cells typically act as donors because they have a chunk of DNA called the fertility factor (or F factor). This chunk of DNA codes for the proteins that make up the sex pilus. It also contains a special site where DNA transfer during conjugation begins.
If the F factor is transferred during conjugation, the receiving cell turns into an F donor that can make its own pilus and transfer DNA to other cells. Here's one analogy: this process is sort of like how a vampire can turn other people into vampires by biting them.
Transposable elements are also important in bacterial genetics. These chunks of DNA "jump" from one place to another within a genome, cutting and pasting themselves or inserting copies of themselves in new spots. Transposable elements are found in many organisms (including you and me!), not just in bacteria.
In bacteria, transposable elements sometimes carry antibiotic resistance and pathogenicity genes (genes that make bacteria disease-causing). If one of these transposable elements "jumps" from the chromosome into a plasmid, the genes it carries can be easily passed to other bacteria by transformation or conjugation. That means the genes can spread quickly through the population.
One way that transposons can move around the genome is by copying themselves and inserting the copy into a new location. In this diagram, a transposon in the bacterial chromosome is copied and inserts into a plasmid.
In bacteria, reproduction can be very fast, with a generation taking little more than a few minutes for some species. This short generation time, together with random mutations and the mechanisms of genetic recombination we saw in this article, allow bacteria (and other prokaryotes) to evolve very quickly.
Is that a good thing? It depends on your perspective. Rapid evolution means that bacteria can adapt to environmental changes, such as the introduction of an antibiotic, very quickly. That's good for them—but bad for us, when we are the ones with the infection!
Check your understanding
Want to join the conversation?
- How does a transposable element do the cutting and pasting? Does it use an enzyme from the cell or does it have its own enzyme?(11 votes)
- Many§ transposable elements will encode a transposase — an enzyme to catalyze its movement to another location. The exact mechanisms vary among different types of transposons, but cutting and pasting is done by some of them.
You can read more about this in these wikipedia articles:
§Note that transposons can also have parasites — these are known as non-autonomous transposons. They tend to be smaller versions of the autonomous transposons and "steal" the enzyme produced by autonomous transposons to move themselves.(4 votes)
- Can you tell the enzyme how is catalyzes transformation(2 votes)
- Good question, but there is not one specific enzyme responsible for transformation. Transformation requires the expression of about 40 genes, each of which could express a unique protein. It's a complex process and people are still working out the details of how it actually works. Some of the proteins involved are probably enzymes but we don't know exactly what they all do.(3 votes)
- Why are the process of transformation, conjugation, and transduction “easy” to do with prokaryotic cells like bacteria, but much less commonly encountered in eukaryotic cells?(2 votes)
- Because prokaryotes' DNA exits as a single loop and is relatively shorter than for eukaryotes due to the fact that multicellular organism have a greater need for a variety of proteins.(3 votes)
- What is the relationship between a sex pilus and a mating bridge?(2 votes)
- A mating bridge is a connection between two bacterial cells that provides a passageway for DNA in bacterial conjugation.
A mating bridge is different from a sex pilus, which is a structure made by an F+ strain bacterium in bacterial conjugation
Sex pilus acts as an attachment site that promotes the binding of bacteria to each other. In this way, an F+ strain makes physical contact with an F− strain. Once contact is made, the pili shorten and thereby draw the donor and recipient cells closer together. A conjugation bridge is then formed between the two cells, which provides a passageway for DNA transfer.(3 votes)
- what might have happened to no longer existing prokaryotic genes in our genome?(2 votes)
- When a donor cell transfer its DNA to a recipient thus created a new donor cell, my question is where the recipient and donor cell come from?(1 vote)
- The donor and recipient bacteria can be any two living bacteria. They can live anywhere in your body that is permissible to bacteria, for example on your skin, in your mouth or on your hands. The site in your body with the most bacteria is the gut, so that's where most of these exchanges probably take place.(1 vote)
- so we can take the dna as if its like all the other substances taken in by cells through their cell membrane?
and also under conjugation when the virus invades the bacterial cell it takes the dna of the host as if it is its own ?(1 vote)
- What is homologous recombination? ("the receiving cell incorporates the new DNA into its own chromosome (which can happen by a process called homologous recombination)")(1 vote)