Prokaryotes reproduce asexually through binary fission and can divide every 20 minutes in ideal conditions. They gain genetic diversity not through sex, but through horizontal gene transfer: transformation, transduction, and conjugation. These mechanisms, combined with rapid mutation rates and natural selection, are what drive antibiotic resistance.
Difficulty: Introductory | Prerequisites: Prokaryotic Cell Biology study notes (Part 1), basic understanding of DNA replication.
This topic picks up where prokaryotic cell structure leaves off. Once you know that prokaryotes have a single circular chromosome and no nucleus, the next question is: how do they reproduce and evolve? The answer has enormous practical consequences. Prokaryotic genetics explains why bacteria can evolve resistance to antibiotics within years or even months, why E. coli is the workhorse of molecular biology labs, and why horizontal gene transfer makes bacterial evolution fundamentally different from eukaryotic evolution. If you skipped Part 1 (cell structure, Gram staining, domains of life), go back to it first.
Binary fission
The form of asexual reproduction used by prokaryotes. The cell copies its single circular chromosome, elongates, and splits into two genetically identical daughter cells. Think of it as the cell photocopying itself and then dividing down the middle.
Generation time
The time it takes for a population of cells to double. For E. coli under optimal laboratory conditions, this is about 20 minutes. In simple terms, it is how fast the population grows.
Model organism
A species chosen for scientific study because it is easy to maintain, reproduces quickly, has a well-understood genome, and produces results that generalise to other organisms. E. coli and Bacillus subtilis are common prokaryotic model organisms.
Horizontal gene transfer (HGT)
The transfer of genetic material between organisms by a route other than parent-to-offspring (vertical) inheritance. In simple terms, bacteria can swap genes sideways, even between unrelated species. Also called lateral gene transfer.
Transformation
The uptake of free ("naked") DNA from the environment by a bacterial cell. The DNA may come from dead cells that have lysed and released their contents.
Transduction
The transfer of bacterial DNA from one cell to another via a bacteriophage (a virus that infects bacteria). The phage accidentally packages host DNA and injects it into a new cell.
Conjugation
The direct transfer of DNA from one bacterial cell to another through a physical connection called a pilus. Requires cell-to-cell contact.
F-plasmid (fertility plasmid)
A plasmid that carries the genes for building a pilus and for transferring a copy of itself to another cell. A cell with an F-plasmid is called F+ (donor); a cell without one is F- (recipient).
Pilus (plural: pili)
A protein appendage on the surface of a bacterial cell. The sex pilus (or conjugation pilus) connects an F+ cell to an F- cell to allow DNA transfer.
Bacteriophage (phage)
A virus that infects bacteria. Phages can accidentally transfer bacterial DNA during transduction.
Antibiotic resistance
The ability of bacteria to survive exposure to an antibiotic that would normally kill them or stop their growth. Resistance genes can spread through mutation and horizontal gene transfer.
Selective pressure
An environmental factor (such as the presence of an antibiotic) that favours individuals with certain traits and drives evolutionary change. In simple terms, the antibiotic kills susceptible bacteria and leaves the resistant ones to multiply.
Mutation
A change in the DNA sequence. Mutations occur randomly during DNA replication and are the ultimate source of new genetic variation.
Prokaryotes reproduce asexually through binary fission. There is no meiosis, no mitosis with a spindle apparatus, and no sexual reproduction in the eukaryotic sense.
Steps of binary fission:
The circular chromosome attaches to the plasma membrane and begins to replicate. Replication starts at the origin of replication and proceeds bidirectionally around the circle.
As the chromosome copies are made, the cell elongates, pushing the two copies apart.
The plasma membrane pinches inward (invagination) at the midpoint of the cell.
A new cell wall forms (septum formation), dividing the cell into two genetically identical daughter cells.
Genetic implications:
Both daughter cells are clones of the parent. Binary fission does not generate genetic variation on its own.
Genetic variation in prokaryotes comes from two sources: random mutations during DNA replication, and horizontal gene transfer (covered below).
Because there is only one chromosome and no nuclear envelope to break down and reform, the whole process is much faster than eukaryotic cell division.
E. coli can divide every 20 minutes under optimal conditions. That means one cell can produce over 4 billion descendants in just 12 hours.
Why short generation times matter for evolution:
More divisions per unit of time means more opportunities for mutations to occur. Even though the mutation rate per gene per generation is low, the sheer number of generations compounds the effect.
A beneficial mutation (such as one conferring antibiotic resistance) can sweep through a population in days or weeks rather than the years or decades it takes in organisms with longer generation times.
Short generation times also mean that natural selection acts faster. If a new antibiotic is introduced, resistant mutants can emerge, survive, and dominate the population within a clinically relevant timeframe.
Putting it in numbers:
If E. coli divides every 20 minutes and has a mutation rate of roughly 1 in 10 billion per base pair per generation, a population of a billion cells (easily reached overnight in a flask) will contain cells with mutations at essentially every position in the genome by the next morning. This is why bacterial populations adapt so quickly.
A model organism is a species used extensively in research because it has characteristics that make it convenient and informative to study.
What makes a good model organism:
Short generation time (results come quickly)
Easy and cheap to maintain in the lab
Small genome that is well-characterised
Genetic tools are available (can manipulate genes easily)
Results generalise to other organisms
Ethical considerations are minimal
Why E. coli is the classic prokaryotic model:
Generation time of 20 minutes
Genome fully sequenced (about 4.6 million base pairs, roughly 4,300 genes)
Enormous library of genetic tools, mutant strains, and published data
Non-pathogenic lab strains (such as K-12) are safe to work with
Decades of accumulated knowledge about its metabolism, gene regulation, and molecular biology
Much of what we know about DNA replication, gene expression, and gene regulation was first worked out in E. coli.
Horizontal gene transfer (HGT) is the main way prokaryotes gain new genetic variation beyond mutation. Unlike vertical transfer (parent to offspring), HGT moves genes sideways between cells, even across species. There are three mechanisms, and exams regularly ask you to compare them.
1. Transformation
A bacterial cell takes up free DNA fragments from the surrounding environment.
The DNA typically comes from nearby cells that have died and lysed (burst open).
The cell must be "competent" (able to take up DNA), which may be a natural state or induced in the lab.
Example: Griffith's experiment (1928) with Streptococcus pneumoniae demonstrated transformation and led to the discovery that DNA is the molecule of heredity.
2. Transduction
A bacteriophage (virus) infects a bacterial cell and accidentally packages some of the host's DNA into new viral particles.
When that phage infects a second bacterium, it injects the first bacterium's DNA into the new host.
Generalised transduction: random fragments of host DNA are packaged. Specialised transduction: only genes near the phage integration site are transferred.
The key point is that the phage acts as a vehicle, carrying DNA between cells without intending to.
3. Conjugation
Requires direct cell-to-cell contact via a pilus.
The donor cell (F+) carries an F-plasmid that encodes the pilus and the machinery for DNA transfer.
The F+ cell extends a pilus to an F- (recipient) cell, pulls the two cells together, and transfers a copy of the F-plasmid.
After transfer, the recipient becomes F+ and can now act as a donor itself.
If the F-plasmid integrates into the host chromosome (creating an Hfr cell, for "high frequency of recombination"), conjugation can transfer chromosomal genes as well, not just the plasmid.
Why HGT matters:
HGT is the reason antibiotic resistance can spread so rapidly through bacterial populations and even jump between species. A resistance gene on a plasmid can be conjugated from a harmless soil bacterium to a pathogenic one in a hospital, making an infection suddenly untreatable.
Antibiotic resistance is one of the most pressing public health issues of the 21st century, and the biology covered in these notes explains exactly how it arises.
How resistance develops, step by step:
A random mutation occurs in a bacterium that happens to reduce the effectiveness of an antibiotic (e.g. a mutation in a target protein, or a mutation that upregulates an efflux pump).
When the antibiotic is present, susceptible bacteria die. The resistant mutant survives and reproduces.
Because generation times are short, the resistant population can become dominant within days.
The resistance gene may sit on a plasmid, which can then be transferred to other bacteria through conjugation, transformation, or transduction.
The result: resistance spreads through the population and even to other species.
Real-world example: MRSA
Methicillin-resistant Staphylococcus aureus (MRSA) acquired the mecA gene, which encodes a modified penicillin-binding protein that methicillin cannot target. This gene is carried on a mobile genetic element (SCCmec) that can transfer between strains. MRSA is now a major cause of hospital-acquired infections worldwide.
Why finishing a course of antibiotics matters:
Stopping antibiotics early leaves behind the bacteria with the highest tolerance. These survivors are the most likely to carry partial resistance, and continued exposure at sub-lethal doses provides the perfect selective pressure for full resistance to evolve.
The connection between agriculture and resistance:
Antibiotics used in livestock farming (often at sub-therapeutic doses to promote growth) create enormous selective pressure for resistance in animal-associated bacteria. These resistant strains can transfer resistance genes to human pathogens through HGT.
Students often think binary fission is "simpler mitosis." Binary fission does not involve a mitotic spindle, condensed chromosomes, or the stages of mitosis (prophase, metaphase, etc.). It is a distinct process.
Students often believe horizontal gene transfer is rare or unusual. It is extremely common in prokaryotes and is a major driver of their evolution. The tree of life for bacteria is better described as a web because of how frequently genes move sideways.
Students sometimes think antibiotic resistance means the antibiotic mutates the bacteria. The antibiotic does not cause mutations. Mutations arise randomly during DNA replication. The antibiotic simply selects for pre-existing resistant individuals by killing the susceptible ones.
Students often confuse F+ and F- cells. F+ is the donor (has the F-plasmid, makes the pilus). F- is the recipient.
⚠️ Be able to list and compare the three modes of horizontal gene transfer (transformation, transduction, conjugation). Know what is transferred, how, and whether a vector is involved.
⚠️ Know the role of the F-plasmid and the difference between F+, F-, and Hfr cells.
⚠️ Be able to explain the steps by which antibiotic resistance arises and spreads in a bacterial population. This is a classic exam essay question.
⚠️ Know the steps of binary fission and why prokaryotic reproduction is faster than eukaryotic cell division.
⚠️ Be able to list characteristics that make a species a good model organism.
True or false: Binary fission produces genetically diverse offspring. (False. Both daughter cells are clones of the parent.)
Fill in the blank: The three modes of horizontal gene transfer are ________, ________, and ________. (Transformation, transduction, conjugation.)
True or false: Antibiotics cause mutations that lead to resistance. (False. Mutations arise randomly; antibiotics select for resistant individuals.)
Fill in the blank: A bacterial cell carrying an F-plasmid is designated ________. (F+.)
True or false: Transduction requires direct cell-to-cell contact. (False. Transduction is mediated by a bacteriophage. Conjugation requires direct contact.)
Q: Describe the four steps of binary fission.
A: (1) The circular chromosome attaches to the plasma membrane and begins bidirectional replication from the origin of replication. (2) The cell elongates, pushing the two chromosome copies apart. (3) The plasma membrane pinches inward at the midpoint. (4) A septum forms, dividing the cell into two identical daughter cells.
Q: Compare transformation, transduction, and conjugation. For each, state the source of DNA, the mechanism of transfer, and whether cell-to-cell contact is required.
A: Transformation: free DNA from the environment (usually from lysed cells), taken up directly by a competent cell, no contact required. Transduction: bacterial DNA accidentally packaged inside a bacteriophage, transferred when the phage infects a new cell, no direct contact required. Conjugation: DNA (usually a plasmid) transferred through a pilus from an F+ donor to an F- recipient, requires direct cell-to-cell contact.
Q: Explain how a single antibiotic-resistance mutation can spread to an entire bacterial population and beyond.
A: A random mutation confers resistance in one cell. When the antibiotic is applied, susceptible cells die and the resistant mutant survives and reproduces (natural selection). Because generation times are short, the resistant clone can dominate the population within days. If the resistance gene is on a plasmid, it can transfer to other cells through conjugation and even jump to other species via horizontal gene transfer.
Q: What is an Hfr cell, and why is it significant?
A: An Hfr (high frequency of recombination) cell is a bacterium in which the F-plasmid has integrated into the main chromosome. During conjugation, an Hfr cell can transfer chromosomal genes (not just the plasmid) to an F- recipient, enabling the spread of a wider range of genetic material.
Q: List four characteristics of a good model organism.
A: Short generation time, inexpensive to maintain, well-characterised genome, genetic tools available for manipulation. (Also acceptable: results generalise to other organisms, ethical considerations minimal.)
Horizontal gene transfer connects to evolutionary biology (it complicates the simple branching tree model of evolution for prokaryotes) and to medical microbiology (antibiotic resistance). Binary fission and generation time link back to the chromosome structure covered in Part 1 and forward to population genetics. The concept of model organisms comes up again in genetics (Mendel's peas, Morgan's fruit flies, C. elegans, yeast) whenever you study how a discovery was made and why certain species were chosen.
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