Biology for AP Courses · Cell Reproduction
Prokaryotic Cell Division
On this page 9 sections
In 30 seconds
Prokaryotes — bacteria and archaea — reproduce by Binary fission Asexual reproduction of prokaryotes: one cell splits into two identical cells. Full entry →, an elegant and fast process in which one cell grows, copies its chromosome, and splits into two genetically identical daughter cells. Because prokaryotes lack a nucleus and membrane-bound organelles, they have no chromosomes in the eukaryotic sense and no mitotic spindle; division is organized around a protein ring and the cell's own growth. The process has three coordinated events: the Origin of replication The specific site on the chromosome where DNA replication begins. Full entry → on the circular chromosome begins DNA replication; the cell elongates while the two chromosome copies separate; and a Septum The dividing wall of membrane and cell wall built at the division site. Full entry → (a dividing wall) forms at the middle, pinching the cell into two. The key structural player is FtsZ A tubulin-related protein that forms a ring at the division site. Full entry →, a tubulin-like protein that assembles into a ring (the Z ring The FtsZ ring at the cell's midpoint that constricts during division. Full entry →) at the future division site — in many ways the bacterial equivalent of the actin contractile ring used in animal cytokinesis, and a cousin of the tubulin that builds eukaryotic spindles. Binary fission is fast and simple, which is exactly why bacterial populations can explode in number when conditions are good.
Why this matters
- Growth curves in medicine and food safety: Because bacteria divide by doubling, a single cell can become millions in hours under favorable conditions — the basis of infection growth, antibiotic timing, and food spoilage. Understanding doubling explains why "a few bacteria" on a counter can become a hazard.
- Antibiotic mechanism context: Several antibiotics (e.g., penicillins) interfere with cell-wall construction, which is most active while bacteria are dividing. This is educational context for why dividing bacteria are especially vulnerable to certain drugs — no treatment details here.
- Biotechnology: Lab work with bacteria (growing cultures, making recombinant proteins) depends on controlling binary fission — this is the workhorse of the biotech industry.
- AP exam: Expect comparison questions: binary fission vs. mitosis — what is similar (DNA duplication, equal distribution) and what is different (no spindle, no nucleus, no chromosomes in the eukaryotic sense).
The college version
Core Concepts
Binary fission, step by step
Binary fission proceeds through a coordinated sequence. First, the single circular chromosome is attached to the plasma membrane, and DNA replication begins at the origin of replication; the two origins move toward opposite ends of the cell as replication proceeds. Second, the cell elongates, increasing in length while the two chromosome copies remain attached to the membrane at their origins. Third, FtsZ proteins assemble into a ring at the midpoint of the cell. Fourth, the ring constricts, and new membrane and cell-wall material builds the septum, which closes to separate the cytoplasm. The result is two daughter cells, each with one complete chromosome. "Fission" literally means splitting — there is no mitosis, no nuclear envelope to break down, and no spindle.
The FtsZ ring: the bacterial division engine
FtsZ is a protein that is structurally related to eukaryotic tubulin (the building block of microtubules). During division, FtsZ polymerizes into a ring on the inner face of the plasma membrane at the cell's midpoint. The Z ring constricts as FtsZ filaments shorten, helping pinch the cell in two; in many bacteria it also acts as a scaffold that recruits the enzymes that synthesize the new cell wall at the division site. The ring's position matters: placing it at the midpoint guarantees the two daughter cells get roughly equal cytoplasm, and proteins that inhibit FtsZ assembly away from the middle help enforce that placement.
Why prokaryotic division is fast and simple
Prokaryotes have a small genome (typically a single circular chromosome — often a few million base pairs, a commonly taught order of magnitude) and no organelles to duplicate, so replication and division are quick. Under ideal laboratory conditions, a commonly taught example is E. coli doubling roughly every 20–30 minutes; the real rate depends heavily on species, nutrients, temperature, and other conditions. Because each division produces two identical cells, populations grow exponentially (1 → 2 → 4 → 8 → …) whenever resources permit.
Plasmids: extra genetic elements
Many bacteria also carry plasmids — small, usually circular DNA molecules that replicate independently of the main chromosome. Plasmids are not required for basic survival but often carry useful genes (e.g., antibiotic resistance). They are distributed to daughter cells during division, though not with the same guarantee as the main chromosome, and they can be passed between bacteria by Conjugation Direct transfer of DNA (often plasmids) between bacteria. Full entry → — a separate process from binary fission that is not reproduction. Plasmids are also the workhorses of genetic engineering, used to carry foreign genes into bacteria.
Binary fission vs. mitosis: the comparison
Both processes duplicate the genetic material and distribute one complete copy to each daughter cell, and both use proteins related to tubulin (FtsZ vs. spindle microtubules). The differences are the exam gold: prokaryotes have no nucleus, no membrane-bound organelles, no multiple linear chromosomes, and no spindle; their chromosome is typically a single circle whose replication and segregation are tied to membrane attachment and cell elongation, whereas eukaryotic mitosis uses a spindle to move many chromosomes precisely.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Binary fission | Mitosis | Fission has no spindle, no nucleus, no linear chromosomes; FtsZ ring vs. microtubule spindle. Both still deliver one full DNA copy per daughter. |
| FtsZ | Eukaryotic actin/contractile ring | FtsZ is a tubulin homolog forming the Z ring; the animal cytokinesis ring is actin-based. They play similar roles through different molecules. |
| Plasmids | The main chromosome | The main chromosome is essential and replicated every division; plasmids are accessory, replicate independently, and can be gained or lost. |
| Conjugation | Binary fission | Conjugation exchanges DNA between existing cells; fission produces new cells. Conjugation is not reproduction. |
| "Bacteria double every 20 minutes" | A universal rule | That doubling time is a commonly taught ideal-lab example for E. coli; real rates vary enormously with species and conditions. |
| Prokaryotes have no DNA | The nucleoid is "not really genetic material" | Prokaryotes have a chromosome (often circular) in the nucleoid region — it is just not enclosed in a nucleus. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A bacterium is like a tiny balloon with one long string of instructions inside. To make a copy of itself, it first copies the string, then the balloon stretches longer and longer until it is twice as big, and then it pinches in the middle — like squeezing a balloon — and pops into two identical balloons. Each one gets its own full copy of the instructions.
Worked example
Start with a single E. coli-like cell in a warm flask of nutrient broth, doubling every 20–30 minutes — a commonly taught ideal-condition illustration. After one doubling there are 2 cells; after two, 4; after three, 8. The pattern is 2ⁿ after n doublings. In 8 hours at a 20-minute doubling time, that is 24 doublings: 2²⁴ ≈ 16.7 million cells from one ancestor (assuming resources never run out — in a real culture they do, which is why growth slows and plateaus). Now translate this to the body: the same exponential logic explains why a small number of bacteria in food left at warm temperatures can multiply into a hazardous dose within hours — the arithmetic of binary fission is the reason food-safety rules about refrigeration exist. The contrast with eukaryotic cells is stark: a human cell cycle of ~24 hours (topic 2) would need more than a day just to double once, while a bacterium can double dozens of times in the same window.
Key takeaways
- Binary fission = prokaryotic reproduction: DNA replication → elongation → septum formation → two identical cells.
- The origin of replication starts chromosome copying; the two copies attach to the membrane and separate as the cell grows.
- FtsZ forms the Z ring at the midpoint; FtsZ is related to eukaryotic tubulin.
- No nucleus, no spindle, no linear chromosomes, no mitosis — the key differences from eukaryotes.
- Growth is exponential (doubling): a commonly taught example is E. coli at ~20–30 minutes per doubling under ideal lab conditions — rates vary with species and environment.
- Plasmids replicate independently and can carry genes like antibiotic resistance; conjugation transfers plasmids between bacteria and is separate from reproduction.
- Both binary fission and mitosis conserve the genetic material — one full copy per daughter cell.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
List the steps of binary fission in order.
Show answer
Chromosome replication begins at the origin of replication and the origins move apart; the cell elongates; the FtsZ ring forms at the midpoint; the ring constricts and the septum closes, splitting the cell into two.
What is the role of the FtsZ ring, and which eukaryotic protein is FtsZ related to?
Show answer
FtsZ assembles into the Z ring at the cell's midpoint and constricts to pinch the cell in two (also scaffolding new wall synthesis). FtsZ is related to eukaryotic tubulin.
How does a bacterium ensure that both daughter cells receive one complete copy of the chromosome?
Show answer
The chromosome is attached to the plasma membrane near the origin of replication; as replication proceeds the two origins (and attached copies) are drawn toward opposite ends by cell elongation, so when the septum closes each daughter contains one complete chromosome.
A culture of bacteria doubles every 30 minutes. How many cells exist after 3 hours if it starts from 1 cell? (Show the doubling count.)
Show answer
3 hours = 6 doublings (30 min each): 2⁶ = 64 cells.
Why is conjugation not reproduction?
Show answer
Conjugation transfers DNA (often plasmids) between existing bacteria; it does not create new cells. Reproduction (binary fission) is what increases cell number.
Give two similarities and two differences between binary fission and mitosis.
Show answer
Similar: DNA is duplicated and one complete copy goes to each daughter; both are driven by tubulin-family proteins (FtsZ vs. spindle). Different: prokaryotes lack a nucleus, spindle, and linear chromosomes and divide by ring constriction, while eukaryotes divide by mitosis with a spindle and cytokinesis.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Binary fission
- Asexual reproduction of prokaryotes: one cell splits into two identical cells.
- Origin of replication
- The specific site on the chromosome where DNA replication begins.
- FtsZ
- A tubulin-related protein that forms a ring at the division site.
- Z ring
- The FtsZ ring at the cell's midpoint that constricts during division.
- Septum
- The dividing wall of membrane and cell wall built at the division site.
- Plasmid
- A small, independently replicating circular DNA molecule.
- Conjugation
- Direct transfer of DNA (often plasmids) between bacteria.
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.