Lesson 6 of 6 / Viral and bacterial genetics
Bacterial reproduction and gene transfer
How can bacteria gain variation without meiosis?
I can explain binary fission, transformation, transduction and F-plasmid conjugation.
I can explain binary fission and bacterial transformation, transduction and F-plasmid conjugation.Syllabus
Syllabus 9477, Core 2(g). Explain binary fission and bacterial transformation, transduction and F-plasmid conjugation.
About 8 min
Make a guess. You are not marked.
Bacteria do not reproduce sexually. Can a bacterium still gain genes from another bacterium?
Show the answer
By conjugation, transformation or transduction.
Horizontal gene transfer moves DNA between cells by contact, uptake from the environment or a phage.
The key idea
Binary fission transmits an existing genome; horizontal transfer introduces DNA by uptake, phages or direct cell contact.
Explore the mechanism
What carries DNA to the next cell?
The chromosome is copied and distributed as one bacterium divides into two. This transmits a genome and increases cell number.
Purple loops = bacterial chromosomes; brown lines/circles = transferred DNA/plasmid. These are route models, not a claim that all incoming DNA is stably inherited.
Explanation
In binary fission, a typical bacterial chromosome replicates, the copies separate as the cell elongates, and division of membrane and wall produces daughter cells. This is asexual reproduction, without a nucleus, mitotic spindle or meiosis. Mutations can make descendants differ from the original genome.
Transformation is uptake of free DNA from the environment by a competent bacterium. The DNA may recombine with chromosomal DNA or persist if it is a suitable plasmid. DNA uptake does not guarantee stable inheritance; it must be maintained or incorporated.
Transduction transfers bacterial DNA through a phage. For example, a phage particle can accidentally package bacterial DNA and deliver it to another bacterium, where recombination may occur. The phage is the transfer vehicle, not a sexual gamete.
Conjugation requires cell contact. In the F-plasmid model, an F-positive donor establishes a connection with an F-negative recipient, transfers a copy of one plasmid strand, and complementary strands are synthesised. Both can finish with an F plasmid. The donor does not normally lose its only copy, and this transfer is not binary fission. Hfr mechanisms are outside the specified scope.
Step by step
- 1
Identify the vehicle
Free DNA, a phage or direct contact?
- 2
Track inheritance
DNA must replicate or integrate to persist.
- 3
Separate transfer from reproduction
Transfer changes genotype without necessarily increasing cell number.
Worked example
Work through the evidence
DNA transfer continues across a filter that blocks cells but allows phages; destroying extracellular free DNA does not stop it. Which route is supported?
One way to explain it
Transduction is supported: a phage can cross the filter and protect its DNA cargo from extracellular DNA-degrading enzyme.
Why this answer works
- Contact-dependent conjugation is blocked by separation.
- Unprotected free-DNA transformation would be sensitive to DNA degradation.
Watch out for this
A student says: "Conjugation is bacterial meiosis producing four haploid cells." What is wrong with this?
Show the answer
It transfers DNA between cells. Bacteria reproduce by binary fission in this model.
Check your understanding
After complete F-plasmid transfer and DNA synthesis, what is the usual result?
Show the answer
Both donor and recipient carry an F plasmid.
Replication restores double-stranded copies in both cells.
Quick recall
How can bacteria gain variation without meiosis?