binary fission
Summary
Binary fission is the asexual reproduction of prokaryotes: the cell replicates its circular chromosome, elongates, and splits into two genetically identical daughter cells. Because it makes clones, variation in bacteria comes mainly from mutation and from transformation, conjugation, and transduction, not from fission itself.
Detail
Meaning: Binary fission is how bacteria and archaea divide. The circular chromosome is copied starting at a single origin of replication, and the two copies move to opposite ends of the elongating cell. The protein FtsZ (a tubulin homologue) forms a ring at the midpoint, the membrane and cell wall grow inward to form a septum, and the cell pinches into two daughter cells. There is no spindle, no condensed chromosomes and no distinct mitotic phases.
On the MCAT: Passages give a growth curve (often log of cell number versus time) and ask you to read the lag, exponential (log), stationary and death phases. During exponential growth the population doubles each generation time. Populations also grow only as long as nutrients last, so growth levels off when resources run out. Questions may also ask why a bacterial population can evolve quickly: short generation times plus mutation, and gene transfer by transformation, conjugation and transduction.
Equation: N = N₀ × 2ⁿ, where N is the final number of cells (unitless), N₀ is the starting number of cells (unitless), and n is the number of generations (unitless), equal to elapsed time divided by generation time (both in s).
Common confusion: Binary fission is not mitosis. Mitosis belongs to eukaryotes and uses a spindle. Fission also is not sexual reproduction, even though conjugation moves DNA between cells: conjugation transfers DNA but does not by itself produce offspring, so it is not reproduction.
Sources
- Campbell Biology
- Brock Biology of Microorganisms
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.