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Genetics of
Prokaryotes

Master plasmids, transformation, conjugation, and transduction: the key mechanisms of horizontal gene transfer in bacteria for the MCAT.

MCAT Biology

The Prokaryotic Genome

Prokaryotes lack a membrane-bound nucleus. Their genetic material resides in a nucleoid region: a single, circular, double-stranded DNA chromosome. Unlike eukaryotes, prokaryotes have no histones (though they have histone-like proteins) and replicate via binary fission, not mitosis.

Chromosome
Single circular dsDNA, haploid, in the nucleoid region
Plasmids
Small, circular, extrachromosomal DNA; carry accessory genes
Replication
Binary fission: rapid, asexual, produces clones

Plasmids: Extrachromosomal DNA

Bacterial Chromosome (circular dsDNA) F Plasmid Fertility factor R Plasmid Resistance genes Col Colicin genes

Why Horizontal Gene Transfer Matters

Prokaryotes reproduce asexually by binary fission, producing genetically identical clones. Without mechanisms for genetic variation, bacteria would evolve slowly. Horizontal gene transfer (HGT) solves this problem by allowing bacteria to acquire new genes from other organisms, without vertical inheritance from parent to offspring.

1
Transformation: uptake of free DNA from the environment
2
Conjugation: direct DNA transfer via cell-to-cell contact (pilus)
3
Transduction: DNA transfer mediated by bacteriophages (viruses)

Transformation: Uptake of Free DNA

Lysed Cell (dead/disrupted) Free DNA Competent Cell Takes up DNA Transformant New gene integrated

Conjugation: The Bacterial "Mating"

\(F^+\) Donor

 F Sex Pilus

\(F^-\) Recipient

 No F plasmid

Now \(F^+\)

Hfr and F' Conjugation

When the F plasmid integrates into the bacterial chromosome, the cell becomes Hfr (High frequency recombination). During conjugation, Hfr cells transfer chromosomal DNA along with part of the F plasmid.

Hfr Cell
F plasmid integrated into chromosome. Transfers chromosomal genes during conjugation. Recipient rarely becomes \(F^+\) (incomplete F transfer).
F' Cell
F plasmid excises imprecisely, carrying bacterial genes. Transfers these genes to recipients, creating partial diploids (merodiploids).

Transduction: Phage-Mediated Transfer

1. Infection 2. Packaging Error 3. New Infection Donor Cell Lysis Phage packages host DNA by mistake Host DNA Recipient Gets donor genes Transductant Generalized: Any host gene packaged Specialized: Only genes near prophage site

Key Distinction: Three Mechanisms

Feature Transformation Conjugation Transduction
DNA Source Free DNA in environment Living donor cell Bacteriophage
Cell Contact Not required Required (pilus) Not required
Vector None Sex pilus Phage particle
DNA Transferred Small fragments Plasmid or chromosomal Random or specific genes
Key Requirement Competence factors F plasmid in donor Prior phage infection

Generalized vs Specialized Transduction

Generalized Transduction
Occurs during the lytic cycle. Host DNA is accidentally packaged into phage heads instead of phage DNA. Any gene can be transferred. Random.
Specialized Transduction
Occurs during lysogenic cycle excision. Prophage excises imprecisely, taking adjacent host genes. Only specific genes near the integration site are transferred.

Prokaryotic Gene Regulation: Operons

Promoter Operator Gene A Gene B Gene C Repressor RNA Pol RNA Pol binds Repressor blocks Structural genes (polycistronic mRNA) Operon = Promoter + Operator + Structural Genes Transcribed as a single polycistronic mRNA

How to Identify the HGT Mechanism

1
Is a phage/virus involved? If yes: Transduction. Generalized (lytic) or specialized (lysogenic).
2
Is cell-to-cell contact required? If yes: Conjugation. Look for pilus, F plasmid, or Hfr mentions.
3
Is naked/free DNA taken up from surroundings? If yes: Transformation. Look for "competent cell" or "DNA uptake."
4
What type of DNA is transferred? Plasmid only = \(F^+\) conjugation. Chromosomal genes = Hfr conjugation or transduction.
5
Check if the recipient gains \(F^+\) status. If yes: conjugation (\(F^+\) or \(F'\)). If no F factor transferred: likely Hfr or transduction.

Example 1: Transformation Experiment

A researcher lyses ampicillin-resistant bacteria and adds the cell lysate to a culture of ampicillin-sensitive bacteria. After incubation, some sensitive bacteria can now grow on ampicillin plates. What happened?

1
DNA source: lysate from dead bacteria contains free DNA fragments
2
No cell contact: donor cells are dead (lysed), so conjugation is impossible
3
No phage involved: the scenario describes lysate, not phage particles
4
Conclusion: competent cells took up free ampicillin-resistance DNA from the environment

Example 2: Conjugation Mapping

An Hfr strain is mated with an F- strain. The mating is interrupted at different times, and researchers check which genes the F- recipient acquired:

Time (min) Gene Transferred
5\(leu^+\)
10\(thr^+\)
17\(lac^+\)
25\(gal^+\)

Example 3: Transduction in Phage P1

Phage P1 infects a \(leu^+ pro^+\) donor strain. The resulting phage lysate is used to infect a \(leu^- pro^-\) recipient. After selection, some recipients become \(leu^+\) but not \(pro^+\), and vice versa. Rarely, both genes are co-transduced.

Generalized Transduction
P1 randomly packages ~1% of host DNA. Any gene can be transferred by chance.
Co-transduction
If two genes are close together on the chromosome, they may be packaged in the same phage head and transferred together.

Common Mistakes

✕ Confusing \(F^+\) and Hfr
\(F^+\) transfers only the F plasmid; Hfr transfers chromosomal genes. The F plasmid is integrated in Hfr, so the recipient rarely becomes \(F^+\).
✕ Mixing Transduction Types
Generalized = lytic cycle, any gene. Specialized = lysogenic excision, only genes near the integration site. Don't swap them.
✕ Forgetting Competence
Transformation requires the recipient to be competent. Not all bacteria naturally take up DNA. Lab methods (heat shock, CaCl2) induce competence artificially.

Question 1 of 5

A bacterium acquires a new antibiotic resistance gene after exposure to cell-free filtrate from a resistant strain. Which mechanism is responsible?

Question 2 of 5

During conjugation, an Hfr donor mates with an \(F^-\) recipient. Which outcome is most likely?

Question 3 of 5

A phage excises imprecisely from the bacterial chromosome, carrying the gal gene to a new host. This is an example of:

Question 4 of 5

In the lac operon, what happens when both lactose is present and glucose is absent?

Question 5 of 5

Which structure is directly required for bacterial conjugation to occur?