Plasmids are small circular DNA molecules found in many bacteria, separate from the main bacterial chromosome. They often carry useful genes, such as antibiotic resistance genes, that help bacteria survive in certain environments. Scientists use plasmids as tools to move genes into bacteria because they are easy to copy and can be engineered in the lab.
Bacterial transformation matters because it allows cells to take up new DNA and gain new traits.
Key Facts
- A plasmid is circular DNA that replicates independently of the bacterial chromosome.
- Bacterial transformation is the uptake of foreign DNA by a bacterial cell.
- Selectable markers, such as antibiotic resistance genes, help identify bacteria that received a plasmid.
- Only transformed bacteria grow on agar containing the matching antibiotic.
- A recombinant plasmid contains DNA from two or more sources, often written as vector DNA + inserted gene.
- Protein yield can be estimated with total protein = protein per cell x number of cells.
Vocabulary
- Plasmid
- A plasmid is a small circular DNA molecule that can replicate separately from the main bacterial chromosome.
- Transformation
- Transformation is the process in which a bacterial cell takes up DNA from outside the cell.
- Selectable marker
- A selectable marker is a gene that lets scientists identify cells that contain a desired plasmid.
- Recombinant DNA
- Recombinant DNA is DNA made by combining genetic material from different sources.
- Gene expression
- Gene expression is the process by which information in a gene is used to make RNA or protein.
Common Mistakes to Avoid
- Assuming every bacterium takes up the plasmid is wrong because transformation efficiency is usually low, so only some cells become transformed.
- Forgetting the selectable marker is wrong because scientists need a way to separate plasmid-containing bacteria from bacteria that did not take up DNA.
- Thinking antibiotic resistance means the bacteria make the desired protein is wrong because resistance only shows plasmid uptake, not necessarily strong expression of the inserted gene.
- Confusing plasmids with bacterial chromosomes is wrong because plasmids are smaller, usually circular DNA molecules that replicate independently from the main chromosome.
Practice Questions
- 1 A transformation experiment plates 200 microliters of cells from a 1.0 mL recovery culture and 80 colonies grow. Estimate the total number of transformed cells in the full recovery culture.
- 2 A plasmid has 4000 base pairs and an insulin gene insert has 900 base pairs. What is the total size of the recombinant plasmid in base pairs?
- 3 A student plates transformed bacteria on agar with ampicillin, but no colonies grow. Give two possible biological or experimental reasons for this result.