Q.(a) Why should a bacterium be made 'competent' ?

(a) Why should a bacterium be made 'competent'?

Concept: Bacterial Transformation

1. Bacterial cells naturally possess a cell wall and cell membrane that prevent the uptake of large molecules like DNA.
2. 'Competence' refers to the induced ability of a bacterial cell to take up foreign DNA from its surroundings.
3. This artificial competence is crucial in genetic engineering to introduce recombinant DNA (often in the form of plasmids) into bacteria. …

Concept and Intuition

In biotechnology, a fundamental challenge is introducing foreign genetic material (like a gene of interest) into a host cell. Cells, whether bacterial, plant, or animal, have natural barriers (cell walls and membranes) designed to protect their internal environment and prevent the entry of foreign substances, including DNA. To overcome these barriers and achieve genetic transformation, specific techniques are employed to either make the host cell receptive to DNA uptake or to physically force the DNA into the cell.

Part (a) addresses the need to prepare bacterial cells to accept DNA, a process called 'competence'. Without this preparation, the bacterial cell's natural defenses would simply reject or degrade any external DNA.

Part (b) focuses on two physical methods, microinjection and the gene gun, which are used primarily for eukaryotic cells (plant and animal cells) where bacterial competence methods are not applicable due to their different cellular structures and mechanisms. These methods bypass natural barriers by directly delivering DNA into the cell's interior.

Step-by-step explanation

(a) Why should a bacterium be made 'competent'?

1. Natural Barriers: Bacteria, like all cells, possess a cell wall and a cell membrane. These structures act as protective barriers, regulating what enters and exits the cell. They naturally prevent the entry of large molecules like DNA, which carries a negative charge due to its phosphate backbone and is therefore repelled by the negatively charged cell membrane.
2. Transformation: For genetic engineering purposes, we often need to introduce foreign DNA (e.g., a recombinant plasmid containing a gene of interest) into a bacterium. The process by which a bacterial cell takes up foreign DNA from its surroundings is called transformation.
3. Defining Competence: A bacterium is considered 'competent' when it is in a physiological state where its cell wall and membrane become permeable, allowing it to take up extracellular DNA. Most bacteria are not naturally competent, meaning they cannot readily take up foreign DNA.
4. Inducing Competence: To make bacteria competent in a laboratory setting, they are typically treated with specific chemicals and physical conditions.
   • Chemical Treatment: Often, bacteria are treated with a divalent cation like calcium chloride (CaCl₂). The Ca²⁺ ions are thought to neutralize the negative charges on both the DNA and the bacterial cell membrane, reducing electrostatic repulsion.
   • Heat Shock: Following chemical treatment, the cells are subjected to a brief period of high temperature (heat shock), usually around 42^circC, followed by rapid cooling on ice. This sudden temperature change is believed to create temporary pores in the cell membrane, allowing the DNA to enter.
   • Electroporation: Another method is electroporation, where bacterial cells are exposed to a brief, high-voltage electric pulse. This pulse creates transient pores in the cell membrane, through which DNA can pass.
5. Role in Biotechnology: Making bacteria competent is crucial for molecular cloning. It enables the introduction of recombinant plasmids (vectors carrying desired genes) into bacterial hosts. Once inside, these plasmids can replicate and express the foreign gene, allowing for gene cloning, protein production, or further genetic manipulation. Without competence, the foreign DNA would remain outside the cell, and transformation would not occur.

(b) Explain the role of 'microinjection' and 'gene gun' in biotechnology.

These are direct gene transfer methods, primarily used for eukaryotic cells (animal and plant cells), which are generally more difficult to transform than bacteria.

1. Microinjection:
   • Mechanism: Microinjection involves using a very fine glass micropipette (needle) to directly inject DNA (or RNA, proteins, etc.) into the nucleus or cytoplasm of a single living cell under microscopic guidance. The cell is typically held in place by a blunt holding pipette.
   • Role in Biotechnology:
     • Animal Cell Transformation: It is widely used for introducing foreign DNA into animal cells, especially large cells like oocytes (egg cells) and early embryos. This is a key technique for creating transgenic animals, where the foreign DNA integrates into the host genome and is passed on to offspring. …