Table of Contents
Overview: Why Prokaryotes Matter for Reproduction and Development
Prokaryotes (bacteria and archaea) are single-celled organisms without a true cell nucleus. In the context of this chapter, the focus is how such “simple” cells reproduce, grow, and develop as individuals and as populations. Their strategies are very different from those of multicellular plants and animals, but they follow the same basic biological principles: copying genetic material, dividing, and responding to environmental conditions.
In this chapter, we concentrate on:
- How a single prokaryotic cell reproduces and divides.
- How genetic material is exchanged without sexual reproduction.
- How growth patterns and survival structures (like endospores) arise.
- Why rapid reproduction and genetic exchange make prokaryotes so evolutionarily flexible.
Basic cell structure and general features of prokaryotes are covered elsewhere; here we focus specifically on reproduction, growth, and development.
Asexual Reproduction: Binary Fission
The main mode of prokaryotic reproduction is asexual: one cell divides into two genetically very similar daughter cells. This process is called binary fission.
Basic Steps of Binary Fission
The details vary between bacterial and archaeal groups, but the general pattern is:
- Replication of the chromosome
- Most prokaryotes have a single, circular DNA molecule attached to the inner membrane.
- Replication starts at a defined origin and proceeds around the circle until two complete copies exist.
- Segregation of DNA
- The two DNA molecules move apart to different regions of the cell.
- This is not mitosis: there are no spindle fibers or distinct mitotic phases, but the end result is similar—each future daughter cell receives one chromosome.
- Elongation of the cell
- The cell grows in length, increasing cytoplasm and membrane area.
- Growth is coordinated with chromosome segregation.
- Formation of a division septum
- A ring-like structure of proteins forms at mid-cell (in bacteria, often built from the FtsZ protein).
- New cell wall and membrane material is inserted inward, forming a partition (septum).
- Separation into two daughter cells
- The septum completes, cutting the cytoplasm into two compartments.
- In many species, daughter cells separate; in others, they remain attached in chains or clusters.
Because each division results in two cells, binary fission is a doubling process. Under ideal conditions, some bacteria can divide every 20 minutes or even faster; many others divide slower (hours to days), depending on nutrients and temperature.
Clonal Reproduction and Genetic Identity
Binary fission produces clones—cells that are genetically (almost) identical to the parent, barring mutations. This has several implications:
- Rapid spread of a successful genotype in a stable environment.
- High vulnerability if conditions change, unless genetic variation is introduced by other processes (see genetic exchange below).
- The concept of a “generation” is much shorter for prokaryotes than for most eukaryotes, which greatly accelerates evolution.
Growth Patterns: From Single Cells to Large Populations
Because each cell reproduces by binary fission, prokaryotic populations can grow extremely quickly. Prokaryotic growth often refers to increase in cell number, not the increase in size of individual cells.
Exponential Population Growth
If each cell divides once per generation, and no cells die, population size \(N\) doubles each generation:
$$
N_t = N_0 \cdot 2^n
$$
where:
- \(N_0\) = starting number of cells
- \(n\) = number of generations
- \(N_t\) = number of cells after \(n\) generations
In reality, resources are limited, and growth eventually slows.
Typical Growth Phases in a Closed System
In a closed culture (e.g., a flask with nutrient broth), many bacterial populations show a growth curve with characteristic phases:
- Lag phase
- Cells adjust to new conditions, synthesize enzymes and components needed to use the available nutrients.
- Little or no cell division yet.
- Exponential (log) phase
- Cells divide at their maximum rate for the conditions.
- Cell number increases exponentially; cells are usually most metabolically active and uniform in this phase.
- Stationary phase
- Nutrients become limiting and waste products accumulate.
- Rate of cell division ≈ rate of cell death; total cell number remains roughly constant.
- Many species increase stress resistance or start forming special structures (e.g., endospores in some bacteria) in this phase.
- Death phase (decline phase)
- Conditions become increasingly unfavorable (e.g., severe nutrient depletion, toxic waste products).
- Cell death exceeds cell division; viable cell count decreases.
These phases are especially relevant where dense bacterial growth occurs, such as in food spoilage, infections, or industrial fermentations.
Growth Forms: Planktonic Cells and Biofilms
Prokaryotes do not only live as scattered individual cells; they often form organized communities:
- Planktonic growth
- Cells float freely in a liquid environment and grow as independent units.
- Biofilms
- Cells attach to surfaces (rocks, pipes, teeth, medical devices) and secrete a slimy matrix of polysaccharides and other substances.
- Within a biofilm, cells experience gradients in nutrients and oxygen, form microenvironments, and often show differentiated roles (e.g., surface vs. deeper layers).
- Biofilms can be much more resistant to antibiotics, disinfectants, and environmental stresses than free-living cells.
Biofilm formation is an example of simple developmental organization at the multicellular level, even though each member is a prokaryotic cell.
Genetic Exchange Without “True” Sexual Reproduction
Prokaryotes do not have sexual reproduction in the same sense as animals and plants (no gametes, fertilization, or meiosis). However, they frequently exchange genetic material, which contributes to genetic diversity and rapid adaptation. This is called horizontal gene transfer (HGT), since genes move across lineages instead of strictly from parent to offspring.
The major mechanisms are:
Transformation
- Prokaryotic cells take up free DNA from the environment (e.g., from dead cells).
- If the DNA is compatible and integrated into the chromosome or maintained in a plasmid, the recipient cell acquires new traits.
- Some bacteria are naturally “competent” and can actively bind and internalize DNA under certain conditions.
Transduction
- DNA is transferred between prokaryotes by viruses that infect bacteria, called bacteriophages.
- During viral replication, fragments of bacterial DNA can be accidentally packaged into virus particles.
- When these particles infect a new cell, they may inject the previous host’s DNA, which can integrate into the new host’s genome.
Conjugation
- DNA is transferred directly from one prokaryotic cell to another via cell-to-cell contact.
- Often mediated by plasmids (small circular DNA molecules) that encode the machinery (e.g., pili) required for transfer.
- One cell acts as a donor, the other as a recipient.
- Conjugation can spread genes for antibiotic resistance, metabolic pathways, or virulence factors through a population or between related species.
These processes can occur alongside binary fission. A cell first acquires new DNA via HGT, then passes it to its descendants by normal cell division. Over time, this creates mosaic genomes and rapid evolutionary change.
Differentiation and Developmental Strategies in Prokaryotes
Although prokaryotic cells are “simple” compared with eukaryotic cells, many show forms of cell differentiation and developmental cycles. These processes allow survival under stress and exploitation of different ecological niches.
Endospore Formation in Some Bacteria
Certain Gram-positive bacteria (notably species in the genera Bacillus and Clostridium) can produce endospores—highly resistant, dormant structures that preserve the cell’s genetic material during unfavorable conditions.
Key Features
- Trigger: Usually nutrient limitation or other environmental stress.
- Process: A single vegetative (actively growing) cell undergoes complex internal rearrangements, forming one endospore inside itself.
- Result: The original cell’s external structures can break down, leaving an endospore that can withstand extreme heat, dryness, chemicals, and radiation.
- Germination: When conditions improve (nutrients, water), the endospore can germinate and return to a vegetative, dividing cell.
Endospore formation is a developmental process involving:
- Asymmetric cell division (two compartments: forespore and mother cell).
- Layers of protective coats and cortex around the forespore.
- Massive changes in gene expression between the compartments.
Only certain bacterial groups can form endospores; archea and most bacteria do not.
Other Survival Structures and Resting Stages
Non-spore-forming prokaryotes may use alternative strategies:
- Cysts: Thick-walled resting cells with reduced metabolic activity.
- Akinetes (in some cyanobacteria): Enlarged cells packed with storage materials and protective layer, allowing survival through winter or drought.
- Viable but non-culturable (VBNC) states: Cells drastically reduce metabolism and become difficult to grow in laboratory conditions, yet remain alive and able to resuscitate under favorable conditions.
These states do not necessarily involve the complex multilayered spore structures seen in endospores, but they serve a similar function: bridging unfavorable periods.
Multicellular-like Development in Some Prokaryotes
Certain prokaryotes show surprisingly complex developmental programs and division of labor, blurring the line between unicellular and multicellular life.
Filamentous Growth and Heterocysts in Cyanobacteria
Some cyanobacteria form filaments of cells connected end to end. Under nitrogen-limiting conditions, some cells within the filament differentiate into heterocysts, specialized for nitrogen fixation.
- Heterocysts:
- Thicker walls to keep out oxygen (which inhibits nitrogenase, the nitrogen-fixing enzyme).
- Do not perform oxygen-producing photosynthesis.
- Provide fixed nitrogen compounds to neighboring cells.
- Neighboring photosynthetic cells:
- Provide carbohydrates to heterocysts.
This creates a simple division of labor and spatial pattern along the filament.
Fruiting Body Formation in Myxobacteria
Myxobacteria are soil-dwelling bacteria that can exhibit coordinated multicellular behavior:
- Individual cells move on surfaces and feed on other microbes.
- Under starvation, thousands of cells aggregate to form a fruiting body.
- Some cells within this structure differentiate into resistant spores.
- When conditions improve, spores germinate, releasing new motile cells.
This process involves:
- Cell–cell communication (chemical signals).
- Coordinated movement and aggregation.
- Differentiation into distinct cell types (e.g., spores vs. supporting cells).
Though they remain individual prokaryotic cells, the collective behavior and division of labor resemble a primitive form of multicellular development.
Reproductive Strategies and Evolutionary Success
The combination of features discussed above makes prokaryotes extraordinarily successful:
- Rapid asexual reproduction by binary fission allows quick exploitation of resources.
- Horizontal gene transfer introduces genetic diversity and new traits without waiting for mutation alone.
- Developmental flexibility (spores, cysts, biofilms, specialized cells) allows survival in extreme and fluctuating environments.
- Simple but effective developmental programs enable some to form organized communities or structures (biofilms, fruiting bodies, filaments with specialized cells).
In the broader context of reproduction, growth, and development, prokaryotes illustrate how even very small, structurally simple organisms can display complex life strategies that rival, and in some ways surpass, those of multicellular eukaryotes.