You probably didn’t know that bacteria can engage in sexual reproduction. It is not what you think. In this lesson we will explore the process of bacterial conjugation and its impact on genetic variability in bacteria.
Bacterial Sexual Reproduction
Have you ever had the bacterial version of the birds and the bees talk? No? Well, it is definitely time. I am sure this won’t come as a big shock to you, but bacteria don’t have sex – at least in the conventional sense.
Bacterial cells reproduce by making clones of themselves. The mother cell copies its DNA chromosome, then splits her cell in half, keeping one chromosome and giving one to the new daughter cell. By convention, these cells are called mother and daughter cells, but really they are actually clones. They have the exact same genetic material. In a bacterial population, this process continues, one cell dividing into two again and again and again, resulting in huge populations of cells that are all clones of one another. It is considered vertical gene transfer when DNA is passed from mother to offspring cell, and this is what happens in nature the vast majority of the time.But bacteria are tricky little creatures, and they have some ingenious ways of generating genetic diversity.
For instance, bacteria are able to swap genes with their neighbors. This would be kind of like you deciding you want your buddy’s black hair, so he does you a favor and gives you his gene for black hair and VOILA! Now you have black hair. This is called horizontal gene transfer and refers to the ability of some bacterial cells to acquire new genes from neighboring cells in their environment.
Now instead of being a clone daughter cell, the cell has new genetic diversity, a mix between the vertically transferred mother cell DNA and the horizontally transferred neighbor cell DNA.Remember that we said that bacteria don’t have sex? Well, in an evolutionary sense, sex is crucial for allowing mixing and matching of genes, resulting in genetic diversity within a species. We can now see that while bacteria don’t have conventional sex, they do have horizontal gene transfer mechanisms for generating genetic diversity.
Horizontal Gene Transfer
There are three ways that bacteria are able to carry out horizontal gene transfer:
- Transduction uses bacterial viruses, called bacteriophages, to transfer DNA from one infected cell to another.
- Transformation is the ability of some cells to take up freely floating DNA found in the environment.
- Conjugation allows for the transfer of DNA through a structure called a pilus from one cell to another.
For conjugation to take place, two live bacterial cells must come into direct contact with one another. Contact between the cells is accomplished using a conjugation pilus.
The term pilus (plural: pili) refers to a protein-based, hair-like structure that extends from the bacterial cell. Some pili are used to attach to surfaces, but the special conjugation pilus is used specifically to attach to other cells and facilitate DNA transfer. The cell that will transfer DNA is called the donor cell and builds the conjugation pilus attached to its cell. The conjugation pilus is a hollow, pipe-like structure that connects the cytoplasm of the donor cell to the cytoplasm of the recipient cell.
Once the cytoplasm of the donor and recipient cell are physically linked, it is time for the transfer of the DNA. If the donor cell contains a plasmid, a circular piece of extrachromosomal DNA, the plasmid can be transferred to the recipient cell. This is done by copying the plasmid and sending the strand of copied DNA to the recipient through the conjugation pilus. The final result is a copy of the plasmid in both the donor and the recipient cell. Perhaps most interesting is the fact that the plasmid, in addition to other genes, carries genes that allow the recipient cell to become a conjugation donor itself! Now the recipient cell can also spread the plasmid to new cells it encounters in its environment.
Some bacterial cells take conjugation a step further and instead of transferring extrachromosomal plasmids, they transfer portions of the entire chromosome! Just like during the transfer of plasmid DNA, a single strand of the chromosome is transferred through the conjugation pilus and the original copy is kept in the donor cell. Plasmids are much smaller than the chromosome and typically are transferred in their entirety.
However, the chromosome is so large that it can take up to 100 minutes to transfer the entire thing. Rarely can the donor and recipient stay docked for long enough to allow this to take place. Instead, only a portion of the chromosome is transferred before the conjugation pilus is detached. It is very important to note that the genes that make chromosome transfer possible are the very last genes to be sent to the recipient. So, if the conjugation pilus becomes detached partway through the transfer, the recipient cell does not become a new donor cell.
Conjugation in Nature
Some forms of horizontal gene transfer can only occur between organisms that are closely related.
Conjugation transfer of plasmids is the exception to this rule. Plasmids can be transferred not only within species – for example from one E. coli cell to another – but also between species. So an E. coli cell could transfer a plasmid to a Staphylococcus species.Not all bacteria are capable of making a conjugation pilus, but they can still carry out the process of conjugation.
Instead of a sex pilus, some cells have a sticky protein complex on the surface of the cell that allows them to adhere to a donor cell and then open a pore linking the cytoplasm of the two cells. In both possible mechanisms for conjugation, cell-to-cell contact is required.Many bacteria are becoming resistant to antibiotics. Researchers have discovered that in the majority of cases, the genes that make the cells resistant to antibiotics are carried on plasmids. The transfer of plasmids by conjugation is very efficient, allowing plasmids carrying antibiotic resistance genes to spread rapidly through a bacterial population. This makes controlling the spread of antibiotic resistance genes very difficult.
Let’s recap: Conjugation is a mechanism for the horizontal transfer of genes between bacterial cells. This is the only mechanism of horizontal gene transfer that requires physical contact between two living cells. The DNA donor cell docks to the recipient cell using the hollow conjugation pilus.
This links the cytoplasm of the two cells, allowing DNA to be transferred. The transferred DNA is copied in the donor cell, and the new copy is sent through the pilus to the recipient. Conjugation transfer of plasmids is the main mechanism for the spread of antibiotic resistance genes through bacterial populations.
After viewing this video, you should be able to define bacterial conjugation as the process used by cells to reproduce.