Explain bacterial genetic exchange (conjugation, transformation, transduction)
WHAT is happening (the big picture)
The three differ by how the DNA travels:
| Mechanism | DNA carrier | Cell contact needed? | DNA source |
|---|---|---|---|
| Conjugation | plasmid via pilus/bridge | Yes (direct) | living donor cell |
| Transformation | free naked DNA from environment | No | dead/lysed cell |
| Transduction | bacteriophage (virus) | No | previous host bacterium |
1. Conjugation — the plasmid handshake
WHY it works — derive it step by step:
- The F⁺ (donor) cell carries the F plasmid, which has the genes (tra genes) to build a sex pilus.
- HOW: the pilus of F⁺ contacts F⁻ (recipient), then retracts, pulling the two cells together → forms a conjugation bridge (mating channel).
- One strand of the F plasmid is nicked at the oriT site and transferred as a single strand into the recipient.
- Both cells synthesise the complementary strand (rolling-circle replication) → now both cells are F⁺.
Hfr cells (why chromosomal genes can transfer): If the F plasmid integrates into the bacterial chromosome, the cell becomes Hfr (High frequency recombination). Now transfer starts dragging the chromosome along. Transfer is usually interrupted before the whole chromosome moves, so genes near oriT transfer first — this is how scientists map gene order (interrupted mating experiment: time = distance).
2. Transformation — picking up naked DNA
HOW / WHY:
- A bacterium must be competent — having surface proteins that bind and import DNA. Competence is often triggered by stress/high cell density.
- Naked DNA fragment binds → one strand is degraded, the other enters.
- The incoming strand pairs with a homologous region of the chromosome and is inserted by homologous recombination.
3. Transduction — the virus courier
Two types — WHY two? because it depends on how the phage packages DNA:
(a) Generalised transduction (lytic cycle error):
- Phage infects, chops up host DNA, and replicates.
- HOW the transfer happens: during packaging the phage accidentally stuffs a piece of bacterial DNA into a phage head instead of phage DNA.
- This defective phage injects bacterial DNA into a new cell → recombination. Any gene can be moved (hence "generalised").
(b) Specialised transduction (lysogenic cycle):
- A temperate phage integrates into the chromosome as a prophage at a specific site.
- On excision it sometimes takes adjacent host genes with it.
- Only genes near the integration site transfer (hence "specialised").
Steel-man your mistakes
Active recall
Recall Quick self-test (cover the answers!)
- Which mechanism needs direct cell-to-cell contact? → Conjugation
- Which needs a virus? → Transduction
- Which uses free DNA from the environment? → Transformation
- What converts F⁻ into F⁺? → transfer of the F plasmid by conjugation.
- What is an Hfr cell? → cell with F plasmid integrated into the chromosome.
- Which experiment proved transformation? → Griffith's (1928).
Recall Feynman: explain to a 12-year-old
Bacteria can't have babies the fancy way, so they swap gene "trading cards" while alive.
- Conjugation: two bacteria hold hands with a tiny tube and one passes a card across.
- Transformation: a bacterium finds a card lying on the ground (dropped by a dead one) and picks it up.
- Transduction: a virus is like a mail truck that accidentally delivers a card from the last house to the next one. All three let bacteria get new powers (like resisting medicine) super fast.
Flashcards
What is horizontal gene transfer?
Name the three mechanisms of bacterial genetic exchange.
Which mechanism requires direct cell-to-cell contact via a pilus?
What structure builds the conjugation bridge?
How many DNA strands transfer during conjugation, and why?
What is an F⁺ cell vs F⁻ cell?
What is an Hfr cell?
Why are Hfr cells useful in genetics?
Define transformation.
What is a competent cell?
Which experiment demonstrated transformation?
What is the "transforming principle" (Avery et al.)?
Define transduction.
Generalised transduction occurs during which phage cycle?
Specialised transduction occurs during which phage cycle?
Which transduction type can transfer any gene?
Which transduction type only transfers genes near the prophage site?
Why is conjugation clinically important?
Connections
- Plasmids and F factor
- Bacteriophage lytic and lysogenic cycles
- Homologous recombination
- Antibiotic resistance mechanisms
- Griffith and Avery experiments
- Binary fission (vertical transfer)
- DNA as genetic material
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, bacteria binary fission se divide hote hain — matlab har daughter cell ek exact copy (clone) hoti hai. To phir gene mixing kaise ho? Isi liye bacteria ke paas teen "jugaad" hote hain naya DNA lene ke liye, bina sex ke. Inko hum horizontal gene transfer kehte hain, aur yeh evolution aur antibiotic resistance ke phailne ke liye bahut important hai.
Conjugation ka matlab hai direct handshake: donor cell (F+) ek pilus (chhota tube) banata hai, recipient (F-) ko pakadta hai, aur ek single strand plasmid DNA bridge se paar bhej deta hai. Cells fuse nahi hote — sirf ek copy jaati hai, isliye donor bhi F+ rehta hai aur recipient bhi F+ ban jaata hai. Agar F plasmid chromosome me ghus jaye to cell Hfr ban jaata hai aur chromosomal genes bhi transfer hone lagte hain.
Transformation me koi contact nahi, koi virus nahi. Aas-paas ke marey hue bacteria ka naked DNA environment me pada hota hai, aur ek competent cell use utha kar apne genome me daal leta hai. Yahi Griffith ke famous 1928 experiment me hua tha. Transduction me ek bacteriophage (virus) courier ka kaam karta hai — packaging ke time galti se bacterial DNA phage head me chala jaata hai aur agle cell me deliver ho jaata hai.
Yaad rakhne ka trick: Conjugation = Contact, Trans-form = Free DNA Floor se, Trans-duct = virus wala duct (pipe/carrier). Bas yeh teen distinguish karlo, exam me question pakka aata hai!