Bacteria can pick up genes or pieces of DNA from their environment in a process called transformation. In this lesson, we’ll explore how this natural process can be exploited in the lab for genetic engineering of bacteria.
A Clone: To Be or Not To Be
‘But I don’t want to be a clone!’ Wailed Daughter Bacterial Cell.
‘For most of us, that is the way of our world, my young one.’ Sighed Mother Bacterial Cell. ‘For generations I have been passing my genes on to you and your sisters. I am proud that you are all clones of me.’
Daughter Bacterial Cell silently fumed. She wanted to be different.
‘There is one way. . .’ Said Mother Bacterial Cell. ‘Although not all bacteria can do it.’
Daughter Bacterial Cell felt her hopes rise.
‘You could pick up some new genes from the environment.’ Said Mother Bacterial Cell. ‘It would devastate me to see you using someone else’s genes, but I would understand.’
In the bacterial world, cloning is usually the rule. One bacterial cell divides to give rise to two new cells called daughter cells. These cells are exact copies, or clones, of the mother cell. This is also called vertical gene transfer because the genes are being passed down from one cell to the next. But, bacteria do have a few tricks up their sleeves to add a little variety to their gene pool.
Horizontal gene transfer is the transfer or acquisition of genes from other (non-mother) cells, from viruses, or from the environment. Transformation is a specific kind of horizontal gene transfer where bacterial cells take up free DNA found in the environment. Not all bacterial cells can do transformation, those that can are called competent cells. Even competent cells typically don’t do transformation all the time; it is turned on and off based on growth and environmental conditions. Transformation is used in nature and in the lab as a tool for genetic engineering of bacteria.
Transformation in Nature
When bacterial cells die naturally, their cells break apart and the cell material, including pieces of the chromosome, gets spilled out into the environment. If there is another bacterial cell in the vicinity that is competent for transformation, it can take up that free DNA and either incorporate it into their genome or degrade it.
Picking up that new DNA from the environment can give a bacterial cell a new ability or feature that might make it better adapted to its environment. Scientists aren’t really sure how often this happens in nature; it seems to be rare, but there is evidence that it happens.
Transformation in the Lab
Scientists like to use nature as inspiration. In this case, scientists realized they could use transformation to add genes to bacteria for genetic engineering. Say there is a special protein that can be used for industrial or medical purposes, but you need to produce large amounts. Scientists can transform the gene for the protein into a bacterial cell, commonly E. coli, and the bacterial cells can mass produce the protein. But remember, not many bacteria can naturally do transformation. So scientists came up with several tricks to coax them. Here we’ll look at one method.
These are the common steps in artificial transformation:
- Cells and DNA are added to a tube.
- Salt is added to help the DNA get close to the bacterial cell.
- The cells are first chilled then heated rapidly to get the cell membrane to loosen up enough for the DNA to get through.
- After that, the temperature is reduced, the salt is removed, some nutrients are added, and the cells recover.
The first few steps are like anesthesia and surgery to get the DNA inside, and the last step is like the recovery room. Still, even with these lab tricks, only a few cells will loosen up enough to let the DNA get inside. So, in a tube of millions of cells, we need to figure out which cells did the transformation and which cells didn’t.
Finding Transformed Cells
Usually, when scientists do transformation they put the new gene into a plasmid. A plasmid is just a little circle of DNA that can carry genes for genetic engineering and genes useful for finding transformed cells. Plasmids usually have a gene that resists a certain antibiotic. Let’s follow what happens after a typical transformation of E. coli in the lab:
- First, grow some cells to be transformed. These cells have their normal genes that allow them to grow and divide.
- Next, you add your plasmid that is carrying your gene of interest plus a gene for resistance to an antibiotic. You do the chemical and temperature treatments required to transform the plasmid into the cells. Only a few cells become competent and get the plasmid, but which ones? Only a few cells will become competent and take up the plasmid. When antibiotics are added only the cells that underwent transformation survive.
- You only want to keep the cells that got the plasmid. Transferring your cells to a culture plate, you add an antibiotic, which will kill only the cells that didn’t get the plasmid. Most cells will die, and your few transformed cells will remain to grow and divide until they form a visible little mound of cells called a colony.
Transformation is the process of bacterial cells taking up free DNA found in their environment. Transformation occurs in nature, but it is also exploited in research labs to genetically engineer bacteria. There is vertical gene transfer and horizontal gene transfer.
Bacteria that can do transformation are called competent cells. Most bacteria aren’t competent naturally, but using chemical and temperature treatments, scientists can make the cells competent and transform DNA plasmids into them. Even then, most bacteria will not undergo transformation. For this reason, an antibiotic resistance gene is usually included so that when the scientists treat the cells with an antibiotic it will kill the untransformed cells while allowing the transformed cells to survive.