TABLE OF CONTENTS
- 1The Big Picture — Why Bacteria Share DNA
- 2Vertical vs Horizontal Gene Transfer
- 3Genetic Variation — Why It Matters
- 4Transformation — Naked DNA Uptake
- 5Conjugation — Direct Cell-to-Cell Transfer
- 6Hfr Bacteria — High Frequency Recombination
- 7Transduction — Viral-Mediated Gene Transfer
- 8Lytic vs Lysogenic Cycles
- 9Key Comparison Tables
- 10Mnemonics & Memory Aids
Step 1: Read each section — they build on each other. Step 2: After each section, cover the page and explain from memory. Step 3: Use the comparison tables to distinguish the three transfer methods. Step 4: Do all 40 questions. Mark gaps for re-study.
The Big Picture
Topic 3 covered the 5 steps of molecular cloning: isolate gene, insert into vector, transform into host, screen, culture. Topic 4 zooms in on Step 3 — transformation — and explains the biological mechanisms behind it. Molecular biologists did not invent gene transfer in bacteria — they co-opted natural systems that bacteria evolved over billions of years to share genetic material.
Bacteria are described as promiscuous — they readily share DNA with each other through multiple mechanisms. Understanding these natural mechanisms is essential because: (1) they are the basis for laboratory transformation techniques, (2) they explain how antibiotic resistance spreads between bacteria, and (3) they are a major source of genetic variation in prokaryotes.
Molecular cloning works because bacteria naturally take up, share, and integrate foreign DNA. The lab techniques simply make these natural processes more efficient and controlled. Every time you transform E. coli with a plasmid, you are exploiting a system that bacteria evolved for horizontal gene transfer.
Vertical vs Horizontal Gene Transfer
Vertical gene transfer is the standard inheritance we think of — DNA is passed from a parent cell to its daughter cells during cell division. Every daughter gets a copy of the parent's chromosome (and any plasmids). This is how genetic information normally flows down through generations.
Horizontal gene transfer (HGT) is the transfer of genetic material between cells that are NOT parent and offspring — often between completely unrelated bacteria. This is unique to prokaryotes (eukaryotes use sexual reproduction for genetic mixing). HGT occurs by three mechanisms: transformation, transduction, and conjugation. These are the focus of this topic.
Genetic Variation
Genetic variation is the raw material for evolution. Without variation, a population cannot adapt to changing environments — a single disease, antibiotic, or environmental change could wipe out every individual. Horizontal gene transfer is one of the main ways bacteria generate genetic variation (alongside mutation).
Eukaryotes generate variation primarily through sexual reproduction (meiosis + fertilisation = recombination of parental genomes). Prokaryotes do not reproduce sexually — they divide by binary fission, producing clones. Instead, they rely on horizontal acquisition (transformation, conjugation, transduction) to gain new genes from other bacteria.
Inbreeding is the result of reproduction between closely related individuals, reducing genetic variation. In agriculture, monocultures (genetically identical crops) are devastated by single diseases — e.g., the Irish Potato Famine. In bacteria, a population without HGT is vulnerable to a single antibiotic. Variation = survival insurance.
Transformation
4 Uptake of naked DNA from the environment Transformation is the unidirectional transfer of free (naked) DNA from the environment into a bacterial cell, resulting in a phenotypic change in the recipient. The DNA may come from dead lysed bacteria in the environment. The recipient cell must be competent — able to take up DNA through its membrane.
Figure 2: Natural transformation — (1) recipient cell takes up naked donor DNA fragments, (2) donor DNA aligns with
homologous region on chromosome, (3) recombination integrates donor genes, (4) genetically transformed cell results.
Griffith’s Experiment (1928) — Proof of Transformation
Frederick Griffith demonstrated transformation using Streptococcus pneumoniae: he showed that heat-killed virulent (smooth, S) bacteria could transfer their virulence trait to living non-virulent (rough, R) bacteria. The R bacteria were ‘transformed’ into S bacteria by taking up DNA released from the dead S cells. This was the first evidence that DNA carries genetic information. Laboratory Transformation (what you do in the lab) In the lab, transformation is made more efficient by artificially making cells competent using CaCl + heat 2 shock (42°C) or electroporation. You are exploiting the same natural mechanism — just making it happen more reliably with your recombinant plasmid.
Conjugation
5
Direct cell-to-cell DNA transfer via a sex pilus
Conjugation is the unidirectional transfer of DNA from a donor cell (F+) to a recipient cell (F-) through direct physical contact via a structure called the sex pilus (plural: pili). The pilus acts as a bridge between the two cells.
Figure 3: Conjugation — the F+ (donor) cell extends a sex pilus to the F- (recipient) cell. The F factor (plasmid) is replicated and
one copy is transferred through the pilus. Result: both cells are now F+.
The F Factor (Fertility Factor)
The F factor is a special plasmid (also called the sex factor) that carries the genes for pilus formation and DNA transfer. Cells carrying the F factor are called F+ (donors); cells without it are F- (recipients). During conjugation, a copy of the F factor is transferred from F+ to F-, converting the recipient into an F+ cell.
F- conjugation. KEY POINT In standard conjugation (F+ × F-), only the F plasmid is transferred. The recipient becomes F+ but does NOT receive any chromosomal genes from the donor. Chromosomal gene transfer requires Hfr cells (Section 6). Hfr Bacteria 6 High Frequency Recombination — when the F factor integrates into the chromosome Figure 4: Formation of an Hfr cell — the F factor integrates into the bacterial chromosome by recombination. The cell can now transfer chromosomal genes during conjugation. Sometimes, the F factor undergoes a rare recombination event and integrates directly into the bacterial chromosome. When this happens, the cell is called an Hfr cell (High frequency recombination). The F factor is no longer a separate plasmid — it is now part of the chromosome. Why Hfr matters: When an Hfr cell conjugates with an F- cell, it starts transferring DNA from the integrated F factor — but because the F factor is now embedded in the chromosome, chromosomal genes adjacent to the integration site are also dragged along. This means Hfr conjugation can transfer chromosomal genes to the recipient — something standard F+ conjugation cannot do. However, complete transfer of the entire chromosome (including the rest of the F factor) rarely happens because the conjugation bridge usually breaks before transfer is complete. So the recipient typically gets some chromosomal genes but does NOT become F+ (it stays F-).