Genetic diversity allows organisms to adapt to changing environmental conditions. In this lesson, we will explore bacterial transduction and how it allows bacteria to transfer genes and increase genetic diversity.
Horizontal Gene Transfer
What if you could share genes with your best friend? Maybe you want her straight hair and blue eyes, and your friend wants your tall height and acute sense of smell.
Yeah, that probably sounds like a bad science-fiction movie. But in the bacterial world, this is definitely possible.In bacteria, vertical gene transfer is the transfer or inheritance of genetic material from parent (or mother) cell to offspring cell. This process effectively generates clones of the parent cell.
In contrast, horizontal gene transfer describes the ability of bacterial cells to transfer genetic material from one cell to another. This process allows for genetic recombination, or the mixing together of two different pieces of DNA to generate a new unique DNA sequence. In bacteria, this process is crucial for generating genetic diversity within populations. In the environment, conditions are constantly changing, and organisms have to adapt to these changes in order to survive. Using genetic recombination to generate genetic diversity makes it possible for organisms like bacteria to adapt to environmental changes.
Mechanisms of Horizontal Gene Transfer
In bacteria, horizontal gene transfer can occur in one of three ways:The first is transformation, which is the ability of some cells to take up freely floating DNA found in the environment. Scientists aren’t exactly sure how many species of bacteria have this ability, but they do know that natural transformation has contributed to the successful adaptation and diversity of several prominent bacterial species.
The second is conjugation, which allows for the transfer of DNA through a structure called a pilus from one cell to another. Bacterial conjugation can be likened to the mating rituals of other species in the sense that it requires direct cell-to-cell contact and involves the exchange of genetic material.The final mechanism of horizontal gene transfer is transduction, which uses bacteria viruses to transfer DNA from one cell to another.
Transduction does not require direct contact like conjugation. And unlike transformation, it needs a third party to help transfer the gene. Let’s explore the process of transduction to get a full understanding of how it works.
We have all heard of a video or image going viral.
Well, where do you think that idea or metaphor came from? Nature, of course! Viruses are notorious for their ability to invade a host, hijack the host cellular machinery, and force it to build millions of copies of the virus. These copies are then released and go on to attack new hosts, spreading through populations.Viruses that infect bacteria are called bacteriophages, which literally translates to ‘bacteria-eater.’ Sometimes, instead of just infecting and hijacking the host, the virus picks up and transfers some of the host cell’s DNA. This process is transduction.
There are two types of transduction. The first is called generalized transduction.
In some cases, the bacteriophage interaction with the bacteria is quite simple – the virus invades, hijacks the host to generate copies of itself, then kills the host cell by popping it like a balloon (called lysis). These new baby bacteriophage then go on to infect and kill other host cells.During this process, the bacteriophage chops up the host chromosome into many small fragments.
When the baby bacteriophages are being assembled, some of these host DNA chunks can accidentally be packaged into the new viral particles. They are then carried to a new host cell, where they are injected and can cause genetic recombination.
The second type of transduction is called specialized transduction.
In this process, instead of just hijacking and killing the cell, the viral DNA goes dormant by incorporating itself into the bacterial DNA chromosome. The host cell survives and continues to grow and divide, passing on the incorporated viral DNA to the clone offspring cells. Eventually, in nightmarish fashion, the bacteriophage reactivates itself and cuts itself free from the host cell genome.
It then hijacks the cell to produce new bacteriophage that lyse the host cell and go on to infect additional cells.The trick is that sometimes, when the virus cuts itself out of the host genome, it takes a chunk of the host DNA with it. Once the bacteriophage infects a new cell, the old host’s DNA that tagged along can be genetically recombined with the new host DNA. Each type of virus inserts into the host genome at the same place each time.
When these viruses cut themselves free from the genome, they typically take with them the genes that are found right beside their insertion spot. This is unlike generalized transduction, where random genes can be picked up by the virus. This is the process of specialized transduction.
The Discovery of Transduction
You may be wondering how transduction was discovered.
In 1952, scientists were working with mutant cells of Salmonella. They noticed that genetic recombination was taking place. They had to rule out the various mechanisms of horizontal gene transfer. First, they ruled out transformation by verifying that the culture was free of contaminating DNA. Since there was no contamination, there was no way for the bacteria to take up freely floating DNA from the environment. Then they ruled out conjugation by keeping the cells isolated from one another by a thin membrane.
If the cells couldn’t touch, there was no way for the bacteria to conjugate. When they still detected genetic recombination, it was time to do some sleuthing. Eventually they discovered the tiny bacteriophage particles in the culture and uncovered the details of transduction.
Let’s review: Transduction is an important mechanism for horizontal gene transfer in bacteria. Transduction is carried out by bacterial viruses called bacteriophages, which infect and kill the host cell. There are two types of transduction. The first is generalized transduction, where the virus randomly chops up the host chromosome and accidentally packages chunks of the genome, transferring them to new host cells.
The second is specialized transduction and results from the virus going dormant by inserting itself in the host chromosome. At some point in the future it reactivates, cutting itself out of the host DNA and sometimes accidentally taking some host genes with it. Transduction seems straight out of science fiction, but believe me, it’s real!
After viewing this video lesson, students should be able to:
- Describe transduction
- Compare and contrast generalized and specialized transduction
- Recall the discovery of transduction