AP Syllabus focus:
‘Some prokaryotic and eukaryotic cells contain plasmids, which are small extra?chromosomal circular DNA molecules carrying additional genetic information.’
Plasmids are important, flexible genetic elements that sit alongside chromosomes. Understanding what they are, where they occur, and what they encode helps explain how cells can rapidly gain new traits without changing their main genome.
What plasmids are
Plasmids are typically small, circular DNA molecules that exist separately from a cell’s chromosomal DNA.
This circular map illustrates common plasmid features, including an origin of replication (ori) that enables independent copying and selectable markers (often antibiotic resistance genes) used to maintain the plasmid in a population. The labeled cloning/expression regions (e.g., promoter, restriction sites, insert) show how plasmids can carry and express additional genetic information beyond the chromosome. Source
They often carry nonessential but advantageous genes, meaning a cell can survive without them but may benefit from having them in certain environments.
Plasmid: a small, usually circular, extra?chromosomal DNA molecule that replicates independently of the chromosome and can carry genes that provide additional traits.
Plasmids are part of a cell’s overall genetic information, but they are not considered the primary hereditary material in the same way as chromosomes.
Plasmids in prokaryotes
How common they are
In prokaryotes (bacteria and archaea), plasmids are widespread. A single cell may contain:
No plasmids (many strains)
One plasmid
Multiple plasmids of different sizes and functions
Key structural and functional features
Prokaryotic plasmids are usually:
Circular double-stranded DNA
Much smaller than the bacterial chromosome
Present at variable copy number (from one or a few copies to dozens per cell)
Many plasmids contain genetic instructions that can increase fitness in particular conditions, such as:
Antibiotic resistance genes (e.g., enzymes that inactivate an antibiotic)
Virulence factors (traits that help pathogens infect hosts)
Metabolic genes (ability to use unusual nutrient sources)
Stress-response traits (survival in toxins or extreme environments)
These genes illustrate the syllabus idea that plasmids carry additional genetic information beyond the chromosome.
This schematic summarizes bacterial conjugation mediated by the F plasmid, tracking how a donor transfers a single DNA strand to a recipient through a conjugative pilus/transfer machinery. It also shows how DNA replication restores double-stranded circular plasmids in both cells, explaining how plasmid-borne traits can spread quickly through a population. Source
Inheritance and stability (core ideas)
When a prokaryotic cell divides, plasmids are generally passed on to daughter cells, but not always equally. Plasmid persistence depends on:
How reliably plasmids are copied before division
How effectively plasmids are distributed to daughter cells
Whether the plasmid provides a selective advantage (cells keeping the plasmid do better in that environment)
Plasmids in eukaryotes
How common they are
Plasmids are less common in eukaryotes than in prokaryotes, but they do exist in some groups. When present, they still match the syllabus description: small extra?chromosomal circular DNA molecules carrying extra genetic information.
Examples and general features (AP-level)
Eukaryotic plasmids are most famously found in some yeasts (for example, naturally occurring circular DNA elements). In eukaryotic cells, plasmids:
Exist separately from nuclear chromosomes
Must be replicated and maintained in a more compartmentalised cell environment
Often rely on specific DNA sequences that help them be copied and retained as cells divide
Because eukaryotic genomes are organised in a nucleus and packaged with proteins, extra?chromosomal DNA faces additional challenges for persistence, helping explain why plasmids are comparatively rare across eukaryotes.
Why plasmids matter for genetic diversity of traits (within cells)
Plasmids expand the range of traits a cell can express without altering its chromosomes. Key implications include:
Cells can show rapid changes in phenotype if they gain or lose a plasmid that encodes a trait.
The same species can have strains with very different capabilities depending on which plasmid genes they carry.
Plasmids can act as “accessory” DNA, making gene content more variable than chromosomal genes alone.
FAQ
No. Plasmids can impose a metabolic cost because the cell must replicate and express extra DNA. In environments without selection for plasmid genes, plasmid-free cells may outcompete plasmid-bearing cells.
Copy number depends on plasmid-specific control of replication initiation. Some plasmids are tightly regulated to low copy number; others replicate more freely and become high copy number.
Some plasmids can occasionally integrate into chromosomal DNA via recombination, becoming part of the chromosome temporarily. Whether this happens depends on shared sequences and recombination mechanisms.
No. Mitochondria and chloroplasts contain their own genomes, but these are considered organellar chromosomes rather than plasmids. Plasmids are separate, additional DNA elements outside chromosomes.
Common approaches include isolating extra?chromosomal DNA and separating it from chromosomal DNA by size and structure, then confirming circular DNA with specific enzymes or sequencing to identify plasmid genes.
Practice Questions
State two features of plasmids. (2 marks)
Plasmids are small DNA molecules. (1)
Plasmids are extra-chromosomal and/or circular and/or carry additional genetic information. (1)
Explain how plasmids can affect the phenotype of a prokaryotic cell and why plasmids are less commonly observed in eukaryotes. (5 marks)
Plasmids carry genes that are not on the main chromosome. (1)
These genes can confer an advantageous trait (e.g., antibiotic resistance, virulence, novel metabolism). (1)
Presence/absence of a plasmid can change the cell’s phenotype. (1)
Eukaryotic plasmids exist but are less common than in prokaryotes. (1)
Eukaryotic cell organisation (e.g., compartmentalisation/maintenance during division) makes stable extra-chromosomal DNA maintenance more difficult. (1)
