Bacteria reproduce mainly by binary fission, a process in which one cell copies its DNA and divides into two genetically identical daughter cells. This matters because a single bacterium can produce a large population in a short time when conditions are favorable. Fast reproduction helps bacteria colonize new environments, form biofilms, spoil food, and cause infections.
Understanding this process also helps explain why sanitation, antibiotics, and vaccines are important tools in public health.
Binary fission begins when the circular bacterial chromosome is copied, then the two DNA copies move toward opposite ends of the cell. The cell elongates, builds a dividing wall called a septum, and separates into two cells. Bacteria can also gain new genes through horizontal gene transfer, including conjugation, transformation, and transduction.
These gene-sharing processes do not create new cells directly, but they can spread traits such as antibiotic resistance through a bacterial population.
Key Facts
- Binary fission produces two daughter cells from one parent cell.
- Bacterial chromosomes are usually circular DNA molecules located in the nucleoid region.
- Basic growth model: N = N0 × 2^n, where n is the number of generations.
- Generation time is the time required for a bacterial population to double.
- Conjugation transfers DNA between bacteria through direct contact, often using a pilus.
- Horizontal gene transfer can spread antibiotic resistance without parent-to-offspring reproduction.
Vocabulary
- Binary fission
- Binary fission is asexual reproduction in which one bacterial cell divides into two genetically identical daughter cells.
- Nucleoid
- The nucleoid is the region of a bacterial cell where its main chromosome is located.
- Plasmid
- A plasmid is a small circular DNA molecule that can carry extra genes, such as antibiotic resistance genes.
- Conjugation
- Conjugation is horizontal gene transfer in which one bacterium passes DNA to another through direct cell-to-cell contact.
- Transduction
- Transduction is horizontal gene transfer in which a virus transfers bacterial DNA from one cell to another.
Common Mistakes to Avoid
- Calling binary fission mitosis is wrong because bacteria are prokaryotes and do not use a nucleus, spindle fibers, or the full mitotic process.
- Assuming daughter cells are always completely identical is wrong because mutations can occur during DNA copying, creating genetic variation.
- Confusing horizontal gene transfer with reproduction is wrong because conjugation, transformation, and transduction move genes between cells but do not directly make new cells.
- Adding bacteria linearly instead of exponentially is wrong because each division can double the population, so growth often follows N = N0 × 2^n under ideal conditions.
Practice Questions
- 1 A culture starts with 50 bacteria. If the bacteria complete 6 rounds of binary fission, how many bacteria are present, assuming none die?
- 2 A bacterium has a generation time of 20 minutes. Starting with 200 cells, how many cells are present after 2 hours under ideal conditions?
- 3 Explain how horizontal gene transfer can make a bacterial population more difficult to treat with antibiotics, even though it is not the same as reproduction.