A competent cell is a cell, typically a bacterium, that has been prepared to accept foreign genetic material from its surrounding environment. The fundamental ability of a cell to take up this extracellular DNA is known as competence. When a cell successfully incorporates this new genetic material, the specific process is termed transformation. This biological mechanism is a cornerstone of molecular biology, allowing scientists to introduce new genes or genetic instructions into a host organism.
Defining the Competent State
For a cell to be considered competent, its cell envelope must allow the passage of foreign DNA. Deoxyribonucleic acid (DNA) molecules are inherently hydrophilic and carry a strong negative electrical charge due to their phosphate backbone. The bacterial cell membrane, however, is a hydrophobic and negatively charged lipid bilayer, which naturally repels the external DNA.
This double barrier prevents the easy movement of DNA across the cell boundary into the cytoplasm. Therefore, competence is the physiological state where the cell’s membrane permeability is temporarily altered. The cell must either produce specialized protein channels or be physically manipulated to neutralize the repelling charges and create pores.
Natural Competence
Certain bacterial species have evolved the capacity to become competent naturally. Species like Bacillus subtilis and Streptococcus pneumoniae activate this state in response to specific environmental cues, such as nutrient starvation, high cell population density signaled through quorum sensing molecules, or the presence of DNA-damaging agents.
The biological function of natural competence is DNA repair and the acquisition of new genetic traits. By taking up environmental DNA, the cell gains access to new genetic information that can be incorporated into its own chromosome through homologous recombination, thereby driving genetic variation.
This active uptake process relies on a complex set of specialized proteins, collectively referred to as the Com machinery. A key component is the DNA uptake pilus, a protein structure that binds to and pulls the double-stranded DNA toward the cell surface. As the DNA passes through the cell envelope, one strand is often degraded, and the remaining single strand is transported into the cytoplasm by membrane channel proteins like ComEC.
Inducing Competence in the Laboratory
Most bacteria are not naturally competent and must be treated in a laboratory setting to induce the ability to take up DNA. This artificial competence is achieved primarily through two distinct methods: chemical transformation followed by heat shock, and electroporation.
The chemical method involves treating the cells with a cold solution containing divalent cations, typically calcium chloride (\(CaCl_2\)). The positively charged calcium ions are thought to shield the negative charges of both the DNA and the bacterial cell wall and membrane. This neutralization lessens the electrostatic repulsion between the two components. Following this treatment, the cells are subjected to a brief, rapid increase in temperature, often to \(42^\circ C\). This thermal pulse is believed to create temporary pores or gaps in the cell membrane, allowing the DNA to quickly pass through before the membrane structure is restored.
Alternatively, electroporation uses a brief, high-voltage electrical pulse to accomplish the same goal. The electric field temporarily destabilizes the cell membrane’s lipid bilayer structure. This momentary disruption creates transient pores through which the DNA molecules can enter the cell. Electroporation generally achieves much higher transformation efficiencies than the chemical method, making it the preferred choice for applications requiring a large number of transformed cells.
Essential Role in Genetic Engineering
Competent cells are a fundamental tool for molecular biology and genetic engineering applications. The ability to perform transformation is the first step in the process of gene cloning. It allows scientists to introduce a vector, such as a plasmid, carrying a gene of interest into a host cell.
This enables the replication of the foreign DNA within the host, which is essential for creating large quantities of the genetic material for study. Competent cells are also used to facilitate the expression of recombinant proteins. By transforming the cell with a plasmid containing the coding sequence for a specific protein, the cell’s own machinery can be used to manufacture the desired therapeutic or industrial protein.