Bacteria are commonly used as host cells for making copies of DNA in the lab because they are easy to grow in large numbers. Their cellular machinery naturally carries out DNA replication and protein synthesis.
are incredibly versatile organisms that have the unique ability to take in foreign and replicate (or copy) it. This gives them an evolutionary advantage and helps them survive changes in their environment. For example, bacteria can acquire DNA that makes them resistant to antibiotics.
The bacterialis contained on a single, circular . This genetic material floats freely in the , unlike organisms where the genetic material is enclosed within a nuclear .
Bacteria may sometimes contain smaller circles of DNA, called plasmids, which have a much smaller number of genes. Plasmids can be swapped between bacteria in a process called.
Using plasmids in the lab
Plasmids can be used as vectors to carry foreign DNA into a cell. Once inside the cell, theis copied by the cell’s own machinery.
In the lab, plasmids are specifically designed so that the DNA they contain will be copied by bacteria.
Laboratory-designed plasmids contain a small number of genes that help. These include:
- An origin of . This is the specific sequence of nucleotides where DNA replication begins.
- A multiple cloning site. This site contains recognition sites for specific restriction enzymes. These restriction enzymes can be used to ‘cut’ the plasmid so foreign DNA can be ‘pasted’ in by .
- A order to survive in a particular , for example, when a specific is present. . This gene codes for a the bacteria need in
Inserting genes into plasmids
The piece of DNA or gene of interest is cut from its original DNA source using a and then pasted into the plasmid by ligation.
The plasmid containing the foreign DNA is now ready to be inserted into bacteria. This process is called transformation.
Before transformation, bacteria are treated with a chemical called calcium chloride, which causes water to enter into the cells and makes them swell. These swollen bacteria are then known as competent bacteria.
Next, plasmid DNA (containing the foreign DNA) is mixed with the competent bacteria and the solution is heated. The plasmid DNA enter the bacteria through small pores created in the cell membranes. Once in the host cell, the plasmid DNA is copied many times by the bacteria’s own DNA replicating machinery.
How do you know if it worked?
After transformation, bacteria are grown on a nutrient rich food called. Only bacteria containing a plasmid with antibiotic resistance will grow in the presence of antibiotic.
For example, if the bacteria are grown on agar containing the antibiotic, only the bacteria that have been transformed with a plasmid containing the resistance gene for ampicillin will survive.
Transformed bacteria can then be grown in large amounts. The DNA of interest, or the protein coded for by the DNA, can then be isolated and purified.
When is transformation used?
Bacterial transformation is used:
- To make multiple copies of DNA, called DNA cloning.
- To make large amounts of specific human proteins, for example, human , which can be used to treat people with .
- To genetically modify a bacterium or other cell.