AP Syllabus focus:
‘Bacterial transformation introduces foreign DNA into cells, enabling production of genetically modified organisms that express new traits or proteins.’
Transformation is a foundational biotechnology tool that moves DNA into cells to change genotype and phenotype.
Schematic overview of artificial bacterial transformation: a plasmid carrying foreign DNA enters a bacterium after membrane permeability is increased (e.g., chemical/heat shock or electroporation). Once inside, the introduced DNA can be maintained and expressed, producing an observable phenotype such as antibiotic resistance. This diagram links DNA uptake to the appearance of new traits, which is the core logic behind creating bacterial GMOs. Source
In AP Biology, focus on how foreign DNA enters cells and how this enables genetically modified organisms.
Core ideas: transformation and GMOs
Key terms
Foreign DNA can be introduced into a cell to alter the traits it can produce.
Transformation: The uptake and expression of external DNA by a cell, resulting in a heritable or observable change in that cell’s characteristics.
A successful transformation often depends on the DNA being maintained and expressed in the recipient cell.
Genetically modified organism (GMO): An organism whose genome has been deliberately altered using introduced DNA to produce new traits or proteins.
What transformation enables
New traits (e.g., tolerance to an environmental condition)
New proteins (e.g., a medically useful protein or an enzyme)
Selectable phenotypes that allow scientists to identify transformed cells
How transformation works in bacteria (conceptual workflow)
DNA source and delivery
Bacteria commonly take up DNA as:
Map of the cloning plasmid pUC19, illustrating how plasmids function as DNA vectors in bacterial transformation. The labeled features emphasize that plasmids carry an origin of replication (supporting stable maintenance) and selectable-marker regions (supporting survival under selection). This visual anchors the idea that a plasmid can deliver and maintain foreign genes in a bacterial host. Source
Plasmids (small circular DNA molecules used as vectors to carry inserted genes)
Less commonly, linear DNA that must recombine into the chromosome to persist
To increase uptake, cells may be made competent (able to take up DNA) using:
Chemical treatment (often calcium salts) followed by heat shock
Electroporation (brief electric pulse to create transient membrane pores)
Selecting transformed cells
Because only a fraction of cells take up DNA, transformed cells are identified using a selectable marker carried on the introduced DNA:
Most commonly, a marker that provides resistance to a specific antibiotic or tolerance to another selective condition
Cells are grown under selection so that non-transformed cells fail to grow while transformed cells survive
Diagram showing antibiotic-based selection after transformation: cells that received a plasmid with an antibiotic-resistance gene survive and divide on antibiotic-containing media, while cells lacking the plasmid are eliminated. This illustrates how a selectable marker enriches for the rare transformed cells in a large population. The outcome is a population dominated by transformants that can maintain and express the introduced DNA. Source
Expression of a new trait or protein
For a GMO phenotype, the introduced gene must be expressed:
A gene needs appropriate regulatory sequences (such as a promoter) that function in the host
The host’s cellular machinery transcribes and translates the gene, producing the new protein that causes the new trait
GMOs: what “new traits or proteins” means in practice
Stable vs. transient outcomes
Stable transformation: introduced DNA is maintained (as a plasmid or integrated into the chromosome) and passed to daughter cells during division
Transient expression: introduced DNA is expressed temporarily but not maintained across many generations
Common goals of creating GMOs
Produce a specific protein in large amounts (for research or industrial use)
Engineer a metabolic pathway change (altering which molecules a cell can synthesize)
Add a trait that changes survival or performance under defined conditions
What AP Biology expects you to be able to explain
Transformation introduces foreign DNA into cells.
Transformed cells can become GMOs that express new traits or proteins.
Transformation efficiency is low, so selection is used to identify transformed cells.
A GMO phenotype requires the introduced DNA to be present and expressed.
FAQ
Efficiency depends on cell health, DNA quality, and method.
Chemical competence: careful chilling, correct salt concentrations, brief heat shock timing
Electroporation: optimised voltage and low-salt DNA preparations to prevent arcing
Small plasmids typically enter cells more readily than very large constructs.
It’s convenient because survival is an unambiguous readout under antibiotic selection.
Concerns include potential horizontal transfer of resistance genes and regulatory restrictions. Alternatives include metabolic markers or non-antibiotic selection systems.
Transient expression occurs when introduced DNA is expressed temporarily but is not maintained during cell divisions.
Stable expression requires plasmid maintenance (origin of replication) or genomic integration, allowing inheritance of the introduced trait across generations.
Selection shows the marker is present, but confirmation may involve:
Colony PCR targeting the insert
Restriction digest analysis after plasmid extraction
Sequencing across the insertion site
Reporter readouts if a reporter gene is included
Typical measures include working in appropriate containment, sterilising waste, and using disabled strains.
Controls may include auxotrophic strains, plasmids designed to limit spread, and strict labelling and storage procedures.
Practice Questions
Define bacterial transformation and state one outcome of transformation in biotechnology. (2 marks)
Correct definition: uptake of foreign/external DNA by a bacterial cell (1)
One valid outcome: production of a new trait or expression/production of a novel protein (1)
Describe how a plasmid can be used to create genetically modified bacteria and how transformed cells are identified. (5 marks)
Plasmid used as a vector carrying foreign DNA/gene of interest (1)
Bacteria made competent and plasmid introduced (e.g., heat shock or electroporation) (1)
Only a fraction of cells take up plasmid (1)
Plasmid contains a selectable marker (e.g., antibiotic resistance) (1)
Growth on selective medium identifies transformants (only transformed cells survive/grow) (1)
