Q.DNA cannot pass through cell membrane of bacteria, why? How is a bacterial cell made competent to take up foreign DNA?

The bacterial cell membrane is a selectively permeable barrier built from a phospholipid bilayer. This structure has hydrophobic (water-fearing) fatty acid tails facing inward and hydrophilic (water-loving) phosphate heads facing outward toward the aqueous environments inside and outside the cell. The interior of this bilayer is essentially a non-polar, oily zone that repels charged or polar molecules.

DNA is a large, highly charged macromolecule. Its sugar-phosphate backbone bristles with negatively charged phosphate groups, making the entire molecule intensely hydrophilic. Water molecules cluster around these charges, and the DNA strand is effectively "dressed" in a hydration shell. This is precisely why DNA dissolves readily in aqueous solutions inside cells but cannot simply drift through a lipid membrane.

When a hydrophilic molecule like DNA approaches the lipid bilayer, it encounters an energetic barrier. Stripping away the hydration shell and forcing the charged backbone into the hydrophobic core of the membrane would require enormous energy—energy that is not available under normal physiological conditions. Small, uncharged molecules like oxygen or carbon dioxide can slip through, but DNA, with its size and charge, cannot. …

When we introduce foreign DNA—a recombinant gene constructed in the lab—into a host cell, that cell doesn't remain "ordinary." It has now acquired new genetic information that wasn't part of its original genome. The terminology for such a cell depends on the method of gene transfer and the type of host, but the underlying idea is the same: the cell has been genetically altered.

Why the terminology matters

In recombinant DNA technology, we build a gene of interest (say, human insulin) into a vector (like a plasmid), then deliver that construct into a bacterial, yeast, or mammalian cell. The cell that successfully takes up and expresses this foreign DNA becomes our factory for producing the desired protein. Naming it precisely helps distinguish successful recipients from the millions of cells that didn't take up the DNA.

The specific terms

1. Transformant

   When a bacterial cell (like E. coli) or a yeast cell takes up recombinant DNA—usually a plasmid—the process is called transformation. The recipient cell is therefore a transformant. This is the most common term in molecular cloning and is what you'll encounter in NEET/JEE contexts when discussing plasmid-based gene transfer.

2. Transfectant

   When animal cells (mammalian cell lines, for instance) receive foreign DNA, the process is termed transfection. The resulting cell is a transfectant. The distinction is historical: "transformation" in animal cells originally referred to oncogenic conversion (turning normal cells cancerous), so a different word was adopted for gene transfer.

3. Recombinant cell / Transformed cell …