DNA is all around you.
So are bacteria. Did you know that those bacteria can pick up and use that DNA? In this lesson we will examine the process of transformation and how bacteria are able to make use of environmental DNA.
Bacterial Sexual Reproduction
Do bacteria have sex? This may seem like a strange question. In reality, the answer is somewhat complicated. Just as a refresher, we are all familiar with the process of sexual reproduction in mammals. A boy animal and a girl animal both contribute their genetic material that is then mixed and matched and results in a baby animal. This baby is not a clone of either parent.
That is, its DNA is not identical to the mother or to the father. Instead, the baby’s DNA is a mixture of the two. This mixing of the parental DNA is referred to as recombination and results in a unique DNA code and a unique animal contributing to genetic diversity within the species.Now for the bacteria. You probably guessed that no, bacteria do not have sex – and you would be right, at least in the classical sense.
For the most part, bacteria are clones of themselves. A single bacterial cell will copy its DNA, then the cell will split in half, giving each half of the cell a copy of the DNA. These are effectively clones. This process continues indefinitely, giving rise to large populations of cells that are all clones of one another.
Now this is where it gets more interesting. Going back to bacterial cell division, we can see that each new bacterial cell gets its DNA from the original. We could call this vertical gene transfer, or the inheritance of genes from the parent (original) cell to the offspring (new) cell. But there is another kind of gene transfer that takes place in bacterial populations referred to as horizontal gene transfer. In the very simplest sense, this involves the transfer of genes from one cell to another without cell division taking place.
It can happen between two cells of the same species, or it can happen between two cells of different species. This allows for bacterial populations to take advantage of recombination to generate genetic diversity. This would be like swapping genes with your siblings or even a stranger walking down the street.
Horizontal Gene Transfer
There are three recognized mechanisms for horizontal gene transfer in bacteria. Conjugation involves the transfer of DNA from one cell to another through a structure called a pilus. Transduction involves the transfer of genes between bacterial cells using a virus called a bacteriophage. Transformation is the ability of some cells to take up freely floating DNA found in the environment.
Let’s talk more about the process of transformation. Bacterial cells that are able to take up free-floating DNA from the environment are called competent cells. Bacteria are not always competent. Sometimes when growth conditions are optimal (in other words, when the cells are happy and well-fed), they are not competent for DNA uptake.
However, in response to some signal, whether it be stress, starvation or other factors, a cell can turn on genes that allow it to become competent.In natural environments, as bacterial cells grow and eventually die, their cells are lysed (or popped open), spilling their DNA into the environment. The DNA chromosome typically is broken up into chunks that then float freely. At this point a competent cell could take up one or more of those chunks.
Once inside the cell, the DNA either has to be incorporated into the host cell DNA chromosome or it ends up being degraded. Usually only a few genes are able to be incorporated. If a gene that is taken up is similar to a gene already found in the chromosome, the two DNA sequences will line up beside one another and then swap, replacing the old gene with the new one. This is that process of recombination that we discussed before, where genetic material is mixed and matched, resulting in new sequences.
What could be an advantage of being competent for transformation? Well, bacterial cells are typically living in chaotic environmental conditions. For example, in a human host, they could be dealing with nutrient starvation, the immune response or other stressful conditions. If a bacterial cell can take up a new gene that confers some advantage within that environment or the host, then that cell can express that gene and outcompete its neighbors, becoming dominant in the population. In disease, this can result in the evolution of organisms that are able to cause more serious disease or are resistant to antibiotics.Now, before we start thinking that all bacteria are doing transformation, really it has only been observed in natural conditions in about 40 bacterial species. Within populations of these competent cells, only about 1% actually carry out the full process, and they are only able to take up DNA that is very similar to their own.
Transformation as a Tool for Biotechnology
Transformation is obviously an important mechanism for generating genetic diversity in natural populations. But transformation is also one of the key tools used in modern biotechnology and has allowed major genetic engineering advances. Scientists can design and build chunks of DNA containing genes they are interested in, then transfer these genes into competent bacterial cells that they can then coax into expressing the genes. Imagine there is a protein that an industry wants to manufacture in large quantities. If they can put the gene for this protein into bacterial cells, they can then use these cells as a factory to express the gene, build the protein, and generate massive quantities.
To sum up, the inheritance of genetic material in bacteria is complicated, with both vertical inheritance from parent cell to offspring cell and horizontal gene transfer from one cell to another. One of the mechanisms for horizontal gene transfer is transformation. In this process, bacterial cells become competent at taking up free DNA from their environment. They then can incorporate this DNA into their genome, generating genetic diversity within the population. The process of transformation is not only important in natural populations but has also become a tool used in biotechnology to create bacterial strains that express desired genes.
After completing this lesson, you will see that bacterial transformation can be natural or induced for study and that it involves free DNA.
You could also be able to cite three recognized mechanisms for horizontal gene transfer and provide details about the transformation process.