Genetics and Growth of Prokaryotic Cells
High-Yield Summary
- Prokaryotes reproduce asexually via binary fission, producing genetically identical daughter cells — no diversity from division itself.
- Genetic diversity comes from 3 horizontal gene transfer mechanisms: transformation (uptake of environmental DNA), conjugation (direct transfer via sex pilus), transduction (bacteriophage-mediated).
- Plasmids replicate independently and carry accessory genes (e.g. antibiotic resistance); one integrated into the chromosome is an episome.
- Conjugation: F⁺ donor transfers F factor to recipient (→ F⁺); if F factor integrates into donor chromosome, donor becomes Hfr strain (can transfer chromosomal DNA too).
- Bacterial growth curve has 4 phases: lag (metabolic prep, constant population) → log/exponential (rapid division, most antibiotic-susceptible) → stationary (division = death rate) → death/decline (population falls).
Key Terms
- Binary fission
- Asexual reproduction: chromosome attaches to membrane, cell elongates, DNA replicates and separates, cell divides into 2 identical daughters.
- Episome
- A plasmid that has integrated into the bacterial chromosome.
- F factor (fertility factor)
- Plasmid in a donor (F⁺) cell carrying genes to build a sex pilus for conjugation.
- Hfr strain
- High-frequency recombination donor — F factor integrated into its chromosome, so chromosomal DNA transfers along with it during conjugation.
- Transposon
- DNA segment that moves between genome locations; found in both prokaryotes and eukaryotes; can carry antibiotic resistance genes.
Transformation vs. Conjugation vs. Transduction
| Mechanism | What Happens |
|---|---|
| Transformation | Bacterium takes up foreign DNA fragments directly from environment (e.g. lysed cells) |
| Conjugation | Plasmid (often F factor), sometimes chromosomal DNA, transferred cell-to-cell via conjugation bridge/sex pilus |
| Transduction | Bacterial DNA accidentally packaged into a bacteriophage, injected into next host |
Conjugation Steps
- 1Donor (F⁺) cell carries the F factor plasmid, which encodes genes for a sex pilus.
- 2Sex pilus contacts recipient (F⁻) cell, forming the conjugation bridge.
- 3A copy of the F plasmid transfers to the recipient, converting it to F⁺.
- 4If F factor is integrated into donor's chromosome (Hfr strain), part of the chromosomal DNA may transfer too, raising recombination odds.
Bacterial Growth Curve (4 Phases)
- 1Lag phase — population constant; cells metabolically active, synthesizing enzymes and replicating DNA.
- 2Log (exponential) phase — rapid division, population surges; peak antibiotic susceptibility (targets actively dividing cells).
- 3Stationary phase — division rate = death rate; nutrients depleted, waste accumulates, population levels off.
- 4Death (decline) phase — hostile environment, death rate exceeds new-cell rate, population falls.
Common MCAT Trap
- Binary fission produces clones, not diversity — genetic variation comes from transformation/conjugation/transduction/transposons, not division itself.
- Conjugation vocabulary (F factor, sex pilus, F⁺, Hfr) is easy to mix up — track what's actually being transferred, not just the labels.
- Log phase = most antibiotic-susceptible (actively dividing), not stationary or death phase — a common passage-reasoning trap.
Quick Recall
Which gene transfer mechanism uses a bacteriophage?
What is an Hfr strain?
During which growth-curve phase are bacteria most susceptible to antibiotics targeting dividing cells?
What is a plasmid integrated into the bacterial chromosome called?