Transformation of host cells using these vectors.

The idea

Transformation is the uptake of a recombinant vector by a host cell. For bacteria, cells and plasmids are mixed in ice-cold calcium chloride solution, which makes the cell wall and cell-surface membrane more permeable, and then given a brief heat shock at about 42 degrees Celsius; a small proportion of the cells take up a plasmid. Electroporation, a short high-voltage pulse that opens temporary pores in the membrane, is an alternative. For plants a plasmid carried by the bacterium Agrobacterium tumefaciens can insert DNA into the plant genome; for animal cells a virus vector, liposomes that fuse with the cell-surface membrane, or micro-injection into a fertilised egg are used.

What earns the marks

• Mixing host cells and recombinant plasmids in a calcium ion solution and applying heat shock.
• Explaining that this makes the cell wall or cell-surface membrane more permeable, so the plasmid can enter.
• Stating that only a small proportion of the host cells are transformed.
• Naming an alternative vector or method, such as a virus, a liposome, electroporation or the Agrobacterium plasmid.
• The need to identify transformed cells afterwards, using a marker gene.

Common mistakes and how to put them right

• Saying the plasmid enters by active transport, when it crosses a membrane made temporarily permeable.

• Confusing transformation with transcription or translation, which are stages of protein synthesis.

• Assuming every host cell is transformed, so no selection is needed.

• Calling the plasmid a host cell, or calling the bacterium the vector.

• Describing heat shock without the calcium ion step, so nothing explains the increased permeability.

Exam tips

• Give both halves of the bacterial method - calcium ions and heat shock - and say what they do to the membrane.
• Always add the sentence that only a small proportion of cells are transformed; it sets up the marker gene question that usually follows.
• Match the method to the organism: plasmids and heat shock for bacteria, viruses or liposomes for animal cells.