The use of marker genes to detect genetically modified (GM) cells or…

The idea

Only a small fraction of host cells take up a recombinant plasmid, so marker genes are used to identify them. A marker gene is inserted into the vector. However, plasmids can reseal without the gene of interest. Traditional methods use two antibiotic resistance genes. Cells are grown on agar with the first antibiotic; only transformed cells survive. Replica plating onto the second antibiotic identifies cells with the recombinant plasmid, as the desired gene disrupts the second marker. Modern markers avoid spreading antibiotic resistance: green fluorescent protein (GFP) makes recombinant cells glow under UV light, and enzyme markers like β-galactosidase turn a colourless substrate blue when the gene is intact.

What earns the marks

• The purpose of the marker gene: to show that the plasmid has been taken up, or to identify transformed cells.
• Explaining that the marker gene is inserted into the vector, allowing detection of cells that have taken up the plasmid.
• Naming a marker and its detection method, such as fluorescence under ultraviolet light or a colour change with an enzyme marker.
• Explaining how insertional inactivation (e.g., disrupting a second marker gene) distinguishes recombinant plasmids from empty plasmids.

Common mistakes and how to put them right

• Saying the marker gene 'marks the gene', without stating that it identifies the cells that have taken up the plasmid.

• Confusing cells with empty plasmids and recombinant plasmids.

  Put it right: A single marker only shows plasmid uptake, while insertional inactivation of a second marker confirms the desired gene is present.

• Naming the enzyme marker as lactase instead of β-galactosidase, or forgetting that fluorescent markers require ultraviolet light for detection.

Exam tips

• When asked how transformed cells are identified, state both the marker used and the specific method of detection (e.g., UV light for GFP).
• If asked for a modern alternative to antibiotic resistance, suggest fluorescent or enzyme markers and state that they prevent the spread of antibiotic resistance to pathogenic bacteria.